VII. JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH’S CRUST
How we obtain an Insight at the Surface, of the Arrangement of Rocks at great Depths. — Why the Height of the successive Strata in a given Region is so disproportionate to their Thickness. — Computation of the average annual Amount of subaërial Denudation. — Antagonism of Volcanic Force to the Levelling Power of running Water. — How far the Transfer of Sediment from the Land to a neighbouring Sea-bottom may affect Subterranean Movements. — Permanence of Continental and Oceanic Areas.
How we obtain an Insight at the Surface, of the Arrangement of Rocks at Great Depths.— The reader has been already informed that, in the structure of the earth’s crust, we often find proofs of the direct superposition of marine to fresh-water strata, and also evidence of the alternation of deep-sea and shallow-water formations. In order to explain how such a series of rocks could be made to form our present continents and islands, we have not only to assume that there have been alternate upward and downward movements of great vertical extent, but that the upheaval in the areas which we at present inhabit has, in later geological times, sufficiently predominated over subsidence to cause these portions of the earth’s crust to be land instead of sea. The sinking down of a delta beneath the sea-level may cause strata of fluviatile or even terrestrial origin, such as peat with trees proper to marshes, to be covered by deposits of deep-sea origin. There is also no end to the thickness of mud and sand which may accumulate in shallow water, provided that fresh sediment is brought down from the wasting land at a rate corresponding to that of the sinking of the bed of the sea. The latter, again, may sometimes sink so fast that the earthy matter, being intercepted in some new landward depression, may never reach its former resting-place, where, the water becoming clear may favour the growth of shells and corals, and calcareous rocks of organic origin may thus be superimposed on mechanical deposits.
The succession of strata here alluded to would be consistent with the occurrence of gradual downward and upward movements of the land and bed of the sea without any disturbance of the horizontality of the several formations. But the arrangement of rocks composing the earth’s crust differs materially from that which would result from a mere series of vertical movements. Had the volcanic forces been confined to such movements, and had the stratified rocks been first formed beneath the sea and then raised above it, without any lateral compression, the geologist would never have obtained an insight into the monuments of various ages, some of extremely remote antiquity.
What we have said in Chapter V of dip and strike, of the folding and inversion of strata, of anticlinal and synclinal flexures, and in Chapter VI of denudation at different periods, whether subaërial or submarine, must be understood before the student can comprehend what may at first seem to him an anomaly, but which it is his business particularly to understand. I allude to the small height above the level of the sea attained by strata often many miles in thickness, and about the chronological succession of which, in one and the same region, there is no doubt whatever. Had stratified rocks in general remained horizontal, the waves of the sea would have been enabled during oscillations of level to plane off entirely the uppermost beds as they rose or sank during the emergence or submergence of the land. But the occurrence of a series of formations of widely different ages, all remaining horizontal and in conformable stratification, is exceptional, and for this reason the total annihilation of the uppermost strata has rarely taken place. We owe, indeed, to the side way movements of lateral compression those anticlinal and synclinal curves of the beds already described (Fig. 55), which, together with denudation, subaërial and submarine, enable us to investigate the structure of the earth’s crust many miles below those points which the miner can reach. I have already shown in Fig. 56, how, at St. Abb’s Head, a series of strata of indefinite thickness may become vertical, and then denuded, so that the edges of the beds alone shall be exposed to view, the altitude of the upheaved ridges being reduced to a moderate height above the sea-level; and it may be observed that although the incumbent strata of Old Red Sandstone are in that place nearly horizontal, yet these same newer beds will in other places be found so folded as to present vertical strata, the edges of which are abruptly cut off, as in 2, 3, 4 on the right-hand side of the diagram, Fig. 55.
Why the Height of the Successive Strata in a given Region is so Disproportionate to their Thickness.—We cannot too distinctly bear in mind how dependent we are on the joint action of the volcanic and aqueous forces, the one in disturbing the original position of rocks, and the other in destroying large portions of them, for our power of consulting the different pages and volumes of those stony records of which the crust of the globe is composed. Why, it may be asked, if the ancient bed of the sea has been in many regions uplifted to the height of two or three miles, and sometimes twice that altitude, and if it can be proved that some single formations are of themselves two or three miles thick, do we so often find several important groups resting one upon the other, yet attaining only the height of a few hundred feet above the level of the sea?
The American geologists, after carefully studying the Allegheny or Appalachian mountains, have ascertained that the older fossiliferous rocks of that chain (from the Silurian to the Carboniferous inclusive) are not less than 42,000 feet thick, and if they were now superimposed on each other in the order in which they were thrown down, they ought to equal in height the Himalayas with the Alps piled upon them. Yet they rarely reach an altitude of 5000 feet, and their loftiest peaks are no more than 7000 feet high. The Carboniferous strata forming the highest member of the series, and containing beds of coal, can be shown to be of shallow-water origin, or even sometimes to have originated in swamps in the open air. But what is more surprising, the lowest part of this great Palæozoic series, instead of having been thrown down at the bottom of an abyss more than 40,000 feet deep, consists of sediment (the Potsdam sandstone), evidently spread out on the bottom of a shallow sea, on which ripple-marked sands were occasionally formed. This vast thickness of 40,000 feet is not obtained by adding together the maximum density attained by each formation in distant parts of the chain, but by measuring the successive groups as they are exposed in a very limited area, and where the denuded edges of the vertical strata forming the parallel folds alluded to at page 87 “crop out” at the surface. Our attention has been called by Mr. James Hall, Palæontologist of New York, to the fact that these Palæozoic rocks of the Appalachian chain, which are of such enormous density, where they are almost entirely of mechanical origin, thin out gradually as they are traced to the westward, where evidently the contemporaneous seas allowed organic rocks to be formed by corals, echinoderms, and encrinites in clearer water, and where, although the same successive periods are represented, the total mass of strata from the Silurian to the Carboniferous, instead of being 40,000 is only 4000 feet thick.
A like phenomenon is exhibited in every mountainous country, as, for example, in the European Alps; but we need not go farther than the north of England for its illustration. Thus in Lancashire and central England the thickness of the Carboniferous formation, including the Millstone Grit and Yoredale beds, is computed to be more than 18,000 feet; to this we may add the Mountain Limestone, at least 2000 feet in thickness, and the overlying Permian and Triassic formations, 3000 or 4000 feet thick. How then does it happen that the loftiest hills of Yorkshire and Lancashire, instead of being 24,000 feet high, never rise above 3000 feet? For here, as before pointed out in the Alleghenies, all the great thicknesses are sometimes found in close approximation and in a region only a few miles in diameter. It is true that these same sets of strata do not preserve their full force when followed for indefinite distances. Thus the 18,000 feet of Carboniferous grits and shales in Lancashire, before alluded to, gradually thin out, as Mr. Hull has shown, as they extend southward, by attenuation or original deficiency of sediment, and not in consequence of subsequent denudation, so that when we have followed them for about 100 miles into Leicestershire, they have dwindled away to a thickness of only 3000 feet. In the same region the Carboniferous limestone attains so unusual a thickness—namely, more than 4000 feet—as to appear to compensate in some measure for the deficiency of contemporaneous sedimentary rock.[1]
It is admitted that when two formations are unconformable their fossil remains almost always differ considerably. The break in the continuity of the organic forms seems connected with a great lapse of time, and the same interval has allowed extensive disturbance of the strata, and removal of parts of them by denudation, to take place. The more we extend our investigations the more numerous do the proofs of these breaks become, and they extend to the most ancient rocks yet discovered. The oldest examples yet brought to light in the British Isles are on the borders of Rosshire and Sutherlandshire, and have been well described by Sir Roderick Murchison, by whom their chronological relations were admirably worked out, and proved to be very different from those which previous observers had imagined them to be. I had an opportunity in the autumn of 1869 of verifying the splendid section given in Fig. 82 by climbing in a few hours from the banks of Loch Assynt to the summit of the mountain called Queenaig, 2673 feet high.
The formations 1, 2, 3, the Laurentian, Cambrian, and Silurian, to be explained in Chapters XXV and XXVI, not only occur in succession in this one mountain, but their unconformable junctions are distinctly exposed to view.
Fig. 82: Unconformable Palæozoic stata, Sutherlandshire (Murchison).
To begin with the oldest set of rocks, No. 1; they consist chiefly of hornblendic gneiss, and in the neighbouring Hebrides form whole islands, attaining a thickness of thousands of feet, although they have suffered such contortions and denudation that they seldom rise more than a few hundred feet above the sea-level. In discordant stratification upon the edges of this gneiss reposes No. 2, a group of conglomerate and purple sandstone referable to the Cambrian (or Longmynd) formation, which can elsewhere be shown to be characterised by its peculiar organic remains. On this again rests No. 3, a lower member of the important group called Silurian, an outlier of which, 3′, caps the summit of Queenaig, attesting the removal by denudation of rocks of the same age, which once extended from the great mass 3 to 3′. Although this rock now consists of solid quartz, it is clear that in its original state it was formed of fine sand, perforated by numerous lob-worms or annelids, which left their burrows in the shape of tubular hollows Fig. 563 of Arenicolites), hundreds, nay thousands, of which I saw as I ascended the mountain.
Fig. 83: Diagrammatic section of the same groups near Queenaig (Murchison).
In Queenaig we only behold this single quartzose member of the Silurian series, but in the neighbouring country (see Fig. 83) it is seen to the eastward to be followed by limestones, 3_a_, and schists, 3_b_, presenting numerous folds, and becoming more and more metamorphic and crystalline, until at length, although very different in age and strike, they much resemble in appearance the group No. 1. It is very seldom that in the same country one continuous formation, such as the Silurian, is, as in this case, more fossiliferous and less altered by volcanic heat in its older than in its newer strata, and still more rare to find an underlying and unconformable group like the Cambrian retaining its original condition of a conglomerate and sandstone more perfectly than the overlying formation. Here also we may remark in regard to the origin of these Cambrian rocks that they were evidently produced at the expense of the underlying Laurentian, for the rounded pebbles occurring in them are identical in composition and texture with that crystalline gneiss which constitutes the contorted beds of the inferior formation No. 1. When the reader has studied the chapter on metamorphism, and has become aware how much modification by heat, pressure, and chemical action is required before the conversion of sedimentary into crystalline strata can be brought about, he will appreciate the insight which we thus gain into the date of the changes which had already been effected in the Laurentian rocks long before the Cambrian pebbles of quartz and gneiss were derived from them. The Laurentian is estimated by Sir William Logan to amount in Canada to 30,000 feet in thickness. As to the Cambrian, it is supposed by Sir Roderick Murchison that the fragment left in Sutherlandshire is about 3500 feet thick, and in Wales and the borders of Shropshire this formation may equal 10,000 feet, while the Silurian strata No. 3, difficult as it may be to measure them in their various foldings to the eastward, where they have been invaded by intrusive masses of granite, are supposed many times to surpass the Cambrian in volume and density.
But although we are dealing here with stratified rocks, each of which would be several miles in thickness, if they were fully represented, the whole of them do not attain the elevation of a single mile above the level of the sea.
Computation of the Average Annual Amount of Subaërial Denudation.—The geology of the district above alluded to may assist our imagination in conceiving the extent to which groups of ancient rocks, each of which may in their turn have formed continents and oceanic basins, have been disturbed, folded, and denuded even in the course of a few out of many of those geological periods to which our imperfect records relate. It is not easy for us to overestimate the effects which causes in every day action must produce when the multiplying power of time is taken into account.
Attempts were made by Manfredi in 1736, and afterwards by Playfair in 1802, to calculate the time which it would require to enable the rivers to deliver over the whole of the land into the basin of the ocean. The data were at first too imperfect and vague to allow them even to approximate to safe conclusions. But in our own time similar investigations have been renewed with more prospect of success, the amount brought down by many large rivers to the sea having been more accurately ascertained. Mr. Alfred Tylor, in 1850, inferred that the quantity of detritus now being distributed over the sea-bottom would, at the end of 10,000 years, cause an elevation of the sea-level to the extent of at least three inches.[2] Subsequently Mr. Croll, in 1867, and again, with more exactness, in 1868, deduced from the latest measurement of the sediment transported by European and American rivers the rate of subaërial denudation to which the surface of large continents is exposed, taking especially the hydrographical basin of the Mississippi as affording the best available measure of the average waste of the land. The conclusion arrived at in his able memoir,[3] was that the whole terrestrial surface is denuded at the rate of one foot in 6000 years and this opinion was simultaneously enforced by his fellow-labourer, Mr. Geikie, who, being jointly engaged in the same line of inquiry, published a luminous essay on the subject in 1868.
The student, by referring to my “Principles of Geology,”[4] may see that Messrs. Humphrey and Abbot, during their survey of the Mississippi, attempted to make accurate measurements of the proportion of sediment carried down annually to the sea by that river, including not only the mud held in suspension, but also the sand and gravel forced along the bottom.
It is evident that when we know the dimensions of the area which is drained, and the annual quantity of earthy matter taken from it and borne into the sea, we can affirm how much on an average has been removed from the general surface in one year, and there seems no danger of our overrating the mean rate of waste by selecting the Mississippi as our example, for that river drains a country equal to more than half the continent of Europe, extends through twenty degrees of latitude, and therefore through regions enjoying a great variety of climate, and some of its tributaries descend from mountains of great height. The Mississippi is also more likely to afford us a fair test of ordinary denudation, because, unlike the St. Lawrence and its tributaries, there are no great lakes in which the fluviatile sediment is thrown down and arrested in its way to the sea. In striking a general average we have to remember that there are large deserts in which there is scarcely any rainfall, and tracts which are as rainless as parts of Peru, and these must not be neglected as counterbalancing others, in the tropics, where the quantity of rain is in excess. If then, argues Mr. Geikie, we assume that the Mississippi is lowering the surface of the great basin which it drains at the rate of one foot in 6000 years, 10 feet in 60,000 years, 100 feet in 600,000 years, and 1000 feet in 6,000,000 years, it would not require more than about 4,500,000 years to wear away the whole of the North American continent if its mean height is correctly estimated by Humboldt at 748 feet. And if the mean height of all the land now above the sea throughout the globe is 1000 feet, as some geographers believe, it would only require six million years to subject a mass of rock equal in volume to the whole of the land to the action of subaërial denudation. It may be objected that the annual waste is partial, and not equally derived from the general surface of the country, inasmuch as plains, water-sheds, and level ground at all heights remain comparatively unaltered; but this, as Mr. Geikie has well pointed out, does not affect our estimate of the sum total of denudation. The amount remains the same, and if we allow too little for the loss from the surface of table-lands we only increase the proportion of the loss sustained by the sides and bottoms of the valleys, and vice versa.[5]
Antagonism of Volcanic Force to the Levelling Power of Running Water.—In all these estimates it is assumed that the entire quantity of land above the sea-level remains on an average undiminished in spite of annual waste. Were it otherwise the subaërial denudation would be continually lessened by the diminution of the height and dimensions of the land exposed to waste. Unfortunately we have as yet no accurate data enabling us to measure the action of that force by which the inequalities of the surface of the earth’s crust may be restored, and the height of the continents and depth of the seas made to continue unimpaired. I stated in 1830 in the “Principles of Geology,”[6] that running water and volcanic action are two antagonistic forces; the one labouring continually to reduce the whole of the land to the level of the sea, the other to restore and maintain the inequalities of the crust on which the very existence of islands and continents depends. I stated, however, that when we endeavour to form some idea of the relation of these destroying and renovating forces, we must always bear in mind that it is not simply by upheaval that subterranean movements can counteract the levelling force of running water. For whereas the transportation of sediment from the land to the ocean would raise the general sea-level, the subsidence of the sea-bottom, by increasing its capacity, would check this rise and prevent the submergence of the land. I have, indeed, endeavoured to show that unless we assume that there is, on the whole, more subsidence than upheaval, we must suppose the diameter of the planet to be always increasing, by that quantity of volcanic matter which is annually poured out in the shape of lava or ashes, whether on the land or in the bed of the sea, and which is derived from the interior of the earth. The abstraction of this matter causes, no doubt, subterranean vacuities and a corresponding giving way of the surface; if it were not so, the average density of parts of the interior would be always lessening and the size of the planet increasing.[7]
Our inability to estimate the amount or direction of the movements due to volcanic power by no means renders its efficacy as a land-preserving force in past times a mere matter of conjecture. The student will see in Chapter XXIV that we have proofs of Carboniferous forests hundreds of miles in extent which grew on the lowlands or deltas near the sea, and which subsided and gave place to other forests, until in some regions fluviatile and shallow-water strata with occasional seams of coal were piled one over the other, till they attained a thickness of many thousand feet. Such accumulations, observed in Great Britain and America on opposite sides of the Atlantic, imply the long-continued existence of land vegetation, and of rivers draining a former continent placed where there is now deep sea.
It will be also seen in Chapter XXV that we have evidence of a rich terrestrial flora, the Devonian, even more ancient than the Carboniferous; while on the other hand, the later Triassic, Oolitic, Cretaceous, and successive Tertiary periods have all supplied us with fossil plants, insects, or terrestrial mammalia; showing that, in spite of great oscillations of level and continued changes in the position of land and sea, the volcanic forces have maintained a due proportion of dry land. We may appeal also to fresh-water formations, such as the Purbeck and Wealden, to prove that in the Oolitic and Neocomian eras there were rivers draining ancient lands in Europe in times when we know that other spaces, now above water, were submerged.
How far the Transfer of Sediment from the Land to a Neighbouring Sea-bottom may affect Subterranean Movements.—Little as we understand at present the laws which govern the distribution of volcanic heat in the interior and crust of the globe, by which mountain chains, high table-lands, and the abysses of the ocean are formed, it seems clear that this heat is the prime mover on which all the grander features in the external configuration of the planet depend.
It has been suggested that the stripping off by denudation of dense masses from one part of a continent and the delivery of the same into the bed of the ocean must have a decided effect in causing changes of temperature in the earth’s crust below, or, in other words, in causing the subterranean isothermals to shift their position. If this be so, one part of the crust may be made to rise, and another to sink, by the expansion and contraction of the rocks, of which the temperature is altered.
I cannot, at present, discuss this subject, of which I have treated more fully elsewhere,[8] but may state here that I believe this transfer of sediment to play a very subordinate part in modifying those movements on which the configuration of the earth’s crust depends. In order that strata of shallow-water origin should be able to attain a thickness of several thousand feet, and so come to exert a considerable downward pressure, there must have been first some independent and antecedent causes at work which have given rise to the incipient shallow receptacle in which the sediment began to accumulate. The same causes there continuing to depress the sea-bottom, room would be made for fresh accessions of sediment, and it would only be by a long repetition of the depositing process that the new matter could acquire weight enough to affect the temperature of the rocks far below, so as to increase or diminish their volume.
Permanence of Continental and Oceanic Areas.—If the thickness of more than 40,000 feet of sedimentary strata before alluded to in the Appalachians proves a preponderance of downward movements in Palæozoic times in a district now forming the eastern border of North America, it also proves, as before hinted, the continued existence and waste of some neighbouring continent, probably formed of Laurentian rocks, and situated where the Atlantic now prevails. Such an hypothesis would be in perfect harmony with the conclusions forced upon us by the study of the present configuration of our continents, and the relation of their height to the depth of the oceanic basins; also to the considerable elevation and extent sometimes reached by drift containing shells of recent species, and still more by the fact of sedimentary strata, several thousand feet thick, as those of central Sicily, or such as flank the Alps and Apennines, containing fossil Mollusca sometimes almost wholly identical with species still living.
I have remarked elsewhere[9] that upward and downward movements of 1000 feet or more would turn much land into sea and sea into land in the continental areas and their borders, whereas oscillations of equal magnitude would have no corresponding effect in the bed of the ocean generally, believed as it is to have a mean depth of 15,000 feet, and which, whether this estimate be correct or not, is certainly of great profundity. Subaërial denudation would not of itself lessen the area of the land, but would tend to fill up with sediment seas of moderate depth adjoining the coast. The coarser matter falls to the bottom near the shore in the first still water which it reaches, and whenever the sea-bottom on which this matter has been thrown is slightly elevated, it becomes land, and an upheaval of a thousand feet causes it to attain the mean elevation of continents in general.
Suppose, therefore, we had ascertained that the triturating power of subaërial denudation might in a given time—in three, or six, or a greater number of millions of years—pulverise a volume of rock equal in dimensions to all the present land, we might yet find, could we revisit the earth at the end of such a period, that the continents occupied very much the same position which they held before; we should find the rivers employed in carrying down to the sea the very same mud, sand, and pebbles with which they had been charged in our own time, the superficial alluvial matter as well as a great thickness of sedimentary strata would inclose shells, all or a great part of which we should recognise as specifically identical with those already known to us as living. Every geologist is aware that great as have been the geographical changes in the northern hemisphere since the commencement of the Glacial Period, there having been submergence and re-emergence of land to the extent of 1000 feet vertically, and in the temperate latitudes great vicissitudes of climate, the marine mollusca have not changed, and the same drift which had been carried down to the sea at the beginning of the period is now undergoing a second transportation in the same direction.
As when we have measured a fraction of time in an hour-glass we have only to reverse the position of our chronometer and we make the same sand measure over again the duration of a second equal period, so when the volcanic force has remoulded the form of a continent and the adjoining sea-bottom, the same materials are made to do duty a second time. It is true that at each oscillation of level the solid rocks composing the original continent suffer some fresh denudation, and do not remain unimpaired like the wooden and glass framework of the hour-glass, still the wear and tear suffered by the larger area exposed to subaërial denudation consists either of loose drift or of sedimentary strata, which were thrown down in seas near the land, and subsequently upraised, the same continents and oceanic basins remaining in existence all the while.
From all that we know of the extreme slowness of the upward and downward movements which bring about even slight geographical changes, we may infer that it would require a long succession of geological periods to cause the submarine and supramarine areas to change places, even if the ascending movements in the one region and the descending in the other were continuously in one direction. But we have only to appeal to the structure of the Alps, where there are so many shallow and deep water formations of various ages crowded into a limited area, to convince ourselves that mountain chains are the result of great oscillations of level. High land is not produced simply by uniform upheaval, but by a predominance of elevatory over subsiding movements. Where the ocean is extremely deep it is because the sinking of the bottom has been in excess, in spite of interruptions by upheaval.
Yet persistent as may be the leading features of land and sea on the globe, they are not immutable. Some of the finest mud is doubtless carried to indefinite distances from the coast by marine currents, and we are taught by deep-sea dredgings that in clear water at depths equalling the height of the Alps organic beings may flourish, and their spoils slowly accumulate on the bottom. We also occasionally obtain evidence that submarine volcanoes are pouring out ashes and streams of lava in mid-ocean as well as on land (see Principles, vol. ii, p. 64), and that wherever mountains like Etna, Vesuvius, and the Canary Islands are now the site of eruptions, there are signs of accompanying upheaval, by which beds of ashes full of recent marine shells have been uplifted many hundred feet. We need not be surprised, therefore, if we learn from geology that the continents and oceans were not always placed where they now are, although the imagination may well be overpowered when it endeavours to contemplate the quantity of time required for such revolutions.
We shall have gained a great step if we can approximate to the number of millions of years in which the average aqueous denudation going on upon the land would convey seaward a quantity of matter equal to the average volume of our continents, and this might give us a gauge of the minimum of volcanic force necessary to counteract such levelling power of running water; but to discover a relation between these great agencies and the rate at which species of organic beings vary, is at present wholly beyond the reach of our computation, though perhaps it may not prove eventually to transcend the powers of man.
[1] Hull, Quart. Geol. Journ., vol. xxiv, p. 322, 1868.
[2] Tylor, Phil. Mag., 4th series, p. 268, 1850.
[3] Croll, Phil. Mag., 1868, p. 381.
[4] Vol. i, p. 442, 1867.
[5] Trans. Geol. Soc. Glasgow, vol. iii, p. 169.
[6] 1st ed., chap. x, p. 167, 1830; see also 10th ed., vol. i, chap. xv, p. 327, 1867.
[7] Principles, vol. ii, p. 237; also 1st ed., p. 447, 1830.
[8] Principles, vol. ii, p. 229, 1868.
[9] Principles, vol. i, p. 265, 1867.
VIII. CHRONOLOGICAL CLASSIFICATION OF ROCKS
Aqueous, Plutonic, volcanic, and metamorphic Rocks considered chronologically. — Terms Primary, Secondary, and Tertiary; Palæozoic, Mesozoic, and Cainozoic explained. — On the different Ages of the aqueous Rocks. — Three principal Tests of relative Age: Superposition, Mineral Character, and Fossils. — Change of Mineral Character and Fossils in the same continuous Formation. — Proofs that distinct Species of Animals and Plants have lived at successive Periods. — Distinct Provinces of indigenous Species. — Great Extent of single Provinces. — Similar Laws prevailed at successive Geological Periods. — Relative Importance of mineral and palæontological Characters. — Test of Age by included Fragments. — Frequent Absence of Strata of intervening Periods. — Tabular Views of fossiliferous Strata.
Chronology of Rocks.— In the first chapter it was stated that the four great classes of rocks, the aqueous, the volcanic, the Plutonic, and the metamorphic, would each be considered not only in reference to their mineral characters, and mode of origin, but also to their relative age. In regard to the aqueous rocks, we have already seen that they are stratified, that some are calcareous, others argillaceous or siliceous, some made up of sand, others of pebbles; that some contain fresh-water, others marine fossils, and so forth; but the student has still to learn which rocks, exhibiting some or all of these characters, have originated at one period of the earth’s history, and which at another.
To determine this point in reference to the fossiliferous formations is more easy than in any other class, and it is therefore the most convenient and natural method to begin by establishing a chronology for these strata, and then to refer as far as possible to the same divisions, the several groups of Plutonic, volcanic, and metamorphic rocks. Such a system of classification is not only recommended by its greater clearness and facility of application, but is also best fitted to strike the imagination by bringing into one view the contemporaneous revolutions of the inorganic and organic creations of former times. For the sedimentary formations are most readily distinguished by the different species of fossil animals and plants which they inclose, and of which one assemblage after another has flourished and then disappeared from the earth in succession.
In the present work, therefore, the four great classes of rocks, the aqueous, Plutonic, volcanic, and metamorphic, will form four parallel, or nearly parallel, columns in one chronological table. They will be considered as four sets of monuments relating to four contemporaneous, or nearly contemporaneous, series of events. I shall endeavour, in a subsequent chapter on the Plutonic rocks, to explain the manner in which certain masses belonging to each of the four classes of rocks may have originated simultaneously at every geological period, and how the earth’s crust may have been continually remodelled, above and below, by aqueous and igneous causes, from times indefinitely remote. In the same manner as aqueous and fossiliferous strata are now formed in certain seas or lakes, while in other places volcanic rocks break out at the surface, and are connected with reservoirs of melted matter at vast depths in the bowels of the earth, so, at every era of the past, fossiliferous deposits and superficial igneous rocks were in progress contemporaneously with others of subterranean and Plutonic origin, and some sedimentary strata were exposed to heat, and made to assume a crystalline or metamorphic structure.
It can by no means be taken for granted, that during all these changes the solid crust of the earth has been increasing in thickness. It has been shown, that so far as aqueous action is concerned, the gain by fresh deposits, and the loss by denudation, must at each period have been equal (see above, Chap. VI, p. 96); and in like manner, in the inferior portion of the earth’s crust, the acquisition of new crystalline rocks, at each successive era, may merely have counterbalanced the loss sustained by the melting of materials previously consolidated. As to the relative antiquity of the crystalline foundations of the earth’s crust, when compared to the fossiliferous and volcanic rocks which they support, I have already stated, in the first chapter, that to pronounce an opinion on this matter is as difficult as at once to decide which of the two, whether the foundations or superstructure of an ancient city built on wooden piles may be the oldest. We have seen that, to answer this question, we must first be prepared to say whether the work of decay and restoration had gone on most rapidly above or below; whether the average duration of the piles has exceeded that of the buildings, or the contrary. So also in regard to the relative age of the superior and inferior portions of the earth’s crust; we cannot hazard even a conjecture on this point, until we know whether, upon an average, the power of water above, or that of heat below, is most efficacious in giving new forms to solid matter.
The early geologists gave to all the crystalline and non-fossiliferous rocks the name of Primitive or Primary, under the idea that they were formed anterior to the appearance of life upon the earth, while the aqueous or fossiliferous strata were termed Secondary, and alluviums or other superficial deposits, Tertiary. The meaning of these terms, has, however, been gradually modified with advancing knowledge, and they are now used to designate three great chronological divisions under which all geological formations can be classed, each of them being characterised by the presence of distinctive groups of organic remains rather than by any mechanical peculiarities of the strata themselves. If, therefore, we retain the term “primary,” it must not be held to designate a set of crystalline rocks some of which have been proved to be even of Tertiary age, but must be applied to all rocks older than the secondary formations. Some geologists, to avoid misapprehension, have introduced the term Palæozoic for primary, from palaion, “ancient,” and zoon, “an organic being,” still retaining the terms secondary and tertiary; Mr. Phillips, for the sake of uniformity, has proposed Mesozoic, for secondary, from mesos, “middle,” etc.; and Cainozoic, for tertiary, from kainos, “recent,” etc.; but the terms primary, secondary, and tertiary have the claim of priority in their favour, and are of corresponding value.
It may perhaps be suggested that some metamorphic strata, and some granites, may be anterior in date to the oldest of the primary fossiliferous rocks. This opinion is doubtless true, and will be discussed in future chapters; but I may here observe, that when we arrange the four classes of rocks in four parallel columns in one table of chronology, it is by no means assumed that these columns are all of equal length; one may begin at an earlier period than the rest, and another may come down to a later point of time, and we may not be yet acquainted with the most ancient of the primary fossiliferous beds, or with the newest of the hypogene.
For reasons already stated, I proceed first to treat of the aqueous or fossiliferous formations considered in chronological order or in relation to the different periods at which they have been deposited.
There are three principal tests by which we determine the age of a given set of strata; first, superposition; secondly, mineral character; and, thirdly, organic remains. Some aid can occasionally be derived from a fourth kind of proof, namely, the fact of one deposit including in it fragments of a pre-existing rock, by which the relative ages of the two may, even in the absence of all other evidence, be determined.
Superposition.—The first and principal test of the age of one aqueous deposit, as compared to another, is relative position. It has been already stated, that, where strata are horizontal, the bed which lies uppermost is the newest of the whole, and that which lies at the bottom the most ancient. So, of a series of sedimentary formations, they are like volumes of history, in which each writer has recorded the annals of his own times, and then laid down the book, with the last written page uppermost, upon the volume in which the events of the era immediately preceding were commemorated. In this manner a lofty pile of chronicles is at length accumulated; and they are so arranged as to indicate, by their position alone, the order in which the events recorded in them have occurred.
In regard to the crust of the earth, however, there are some regions where, as the student has already been informed, the beds have been disturbed, and sometimes extensively thrown over and turned upside down. (See p. 73, p. 87.) But an experienced geologist can rarely be deceived by these exceptional cases. When he finds that the strata are fractured, curved, inclined, or vertical, he knows that the original order of superposition must be doubtful, and he then endeavours to find sections in some neighbouring district where the strata are horizontal, or only slightly inclined. Here, the true order of sequence of the entire series of deposits being ascertained, a key is furnished for settling the chronology of those strata where the displacement is extreme.
Mineral Character.—The same rocks may often be observed to retain for miles, or even hundreds of miles, the same mineral peculiarities, if we follow the planes of stratification, or trace the beds, if they be undisturbed, in a horizontal direction. But if we pursue them vertically, or in any direction transverse to the planes of stratification, this uniformity ceases almost immediately. In that case we can scarcely ever penetrate a stratified mass for a few hundred yards without beholding a succession of extremely dissimilar rocks, some of fine, others of coarse grain, some of mechanical, others of chemical origin; some calcareous, others argillaceous, and others siliceous. These phenomena lead to the conclusion that rivers and currents have dispersed the same sediment over wide areas at one period, but at successive periods have been charged, in the same region, with very different kinds of matter. The first observers were so astonished at the vast spaces over which they were able to follow the same homogeneous rocks in a horizontal direction, that they came hastily to the opinion, that the whole globe had been environed by a succession of distinct aqueous formations, disposed round the nucleus of the planet, like the concentric coats of an onion. But, although, in fact, some formations may be continuous over districts as large as half of Europe, or even more, yet most of them either terminate wholly within narrower limits, or soon change their lithological character. Sometimes they thin out gradually, as if the supply of sediment had failed in that direction, or they come abruptly to an end, as if we had arrived at the borders of the ancient sea or lake which served as their receptacle. It no less frequently happens that they vary in mineral aspect and composition, as we pursue them horizontally. For example, we trace a limestone for a hundred miles, until it becomes more arenaceous, and finally passes into sand, or sandstone. We may then follow this sandstone, already proved by its continuity to be of the same age, throughout another district a hundred miles or more in length.
Organic Remains.—This character must be used as a criterion of the age of a formation, or of the contemporaneous origin of two deposits in distant places, under very much the same restrictions as the test of mineral composition.
First, the same fossils may be traced over wide regions, if we examine strata in the direction of their planes, although by no means for indefinite distances. Secondly, while the same fossils prevail in a particular set of strata for hundreds of miles in a horizontal direction, we seldom meet with the same remains for many fathoms, and very rarely for several hundred yards, in a vertical line, or a line transverse to the strata. This fact has now been verified in almost all parts of the globe, and has led to a conviction that at successive periods of the past, the same area of land and water has been inhabited by species of animals and plants even more distinct than those which now people the antipodes, or which now co-exist in the arctic, temperate, and tropical zones. It appears that from the remotest periods there has been ever a coming in of new organic forms, and an extinction of those which pre-existed on the earth; some species having endured for a longer, others for a shorter, time; while none have ever reappeared after once dying out. The law which has governed the succession of species, whether we adopt or reject the theory of transmutation, seems to be expressed in the verse of the poet:—
Natura il fece, e poi ruppe la stampa. ARIOSTO. Nature made him, and then broke the die.
And this circumstance it is, which confers on fossils their highest value as chronological tests, giving to each of them, in the eyes of the geologist, that authority which belongs to contemporary medals in history.
The same cannot be said of each peculiar variety of rock; for some of these, as red marl and red sandstone, for example, may occur at once at the top, bottom, and middle of the entire sedimentary series; exhibiting in each position so perfect an identity of mineral aspect as to be undistinguishable. Such exact repetitions, however, of the same mixtures of sediment have not often been produced, at distant periods, in precisely the same parts of the globe; and even where this has happened, we are seldom in any danger of confounding together the monuments of remote eras, when we have studied their imbedded fossils and their relative position.
Zoological Provinces.—It was remarked that the same species of organic remains cannot be traced horizontally, or in the direction of the planes of stratifications for indefinite distances. This might have been expected from analogy; for when we inquire into the present distribution of living beings, we find that the habitable surface of the sea and land may be divided into a considerable number of distinct provinces, each peopled by a peculiar assemblage of animals and plants. In the “Principles of Geology,” I have endeavoured to point out the extent and probable origin of these separate divisions; and it was shown that climate is only one of many causes on which they depend, and that difference of longitude as well as latitude is generally accompanied by a dissimilarity of indigenous species.
As different seas, therefore, and lakes are inhabited, at the same period, by different aquatic animals and plants, and as the lands adjoining these may be peopled by distinct terrestrial species, it follows that distinct fossils will be imbedded in contemporaneous deposits. If it were otherwise—if the same species abounded in every climate, or in every part of the globe where, so far as we can discover, a corresponding temperature and other conditions favourable to their existence are found—the identification of mineral masses of the same age, by means of their included organic contents, would be a matter of still greater certainty.
Nevertheless, the extent of some single zoological provinces, especially those of marine animals, is very great; and our geological researches have proved that the same laws prevailed at remote periods; for the fossils are often identical throughout wide spaces, and in detached deposits, consisting of rocks varying entirely in their mineral nature.
The doctrine here laid down will be more readily understood, if we reflect on what is now going on in the Mediterranean. That entire sea may be considered as one zoological province; for although certain species of testacea and zoophytes may be very local, and each region has probably some species peculiar to it, still a considerable number are common to the whole Mediterranean. If, therefore, at some future period, the bed of this inland sea should be converted into land, the geologist might be enabled, by reference to organic remains, to prove the contemporaneous origin of various mineral masses scattered over a space equal in area to half of Europe.
Deposits, for example, are well known to be now in progress in this sea in the deltas of the Po, Rhone, Nile, and other rivers, which differ as greatly from each other in the nature of their sediment as does the composition of the mountains which their drain. There are also other quarters of the Mediterranean, as off the coast of Campania, or near the base of Etna, in Sicily, or in the Grecian Archipelago, where another class of rocks is now forming; where showers of volcanic ashes occasionally fall into the sea, and streams of lava overflow its bottom; and where, in the intervals between volcanic eruptions, beds of sand and clay are frequently derived from the waste of cliffs, or the turbid waters of rivers. Limestones, moreover, such as the Italian travertins, are here and there precipitated from the waters of mineral springs, some of which rise up from the bottom of the sea. In all these detached formations, so diversified in their lithological characters, the remains of the same shells, corals, crustacea, and fish are becoming inclosed; or, at least, a sufficient number must be common to the different localities to enable the zoologist to refer them all to one contemporaneous assemblage of species.
There are, however, certain combinations of geographical circumstances which cause distinct provinces of animals and plants to be separated from each other by very narrow limits; and hence it must happen that strata will be sometimes formed in contiguous regions, differing widely both in mineral contents and organic remains. Thus, for example, the testacea, zoophytes, and fish of the Red Sea are, as a group, extremely distinct from those inhabiting the adjoining parts of the Mediterranean, although the two seas are separated only by the narrow isthmus of Suez. Calcareous formations have accumulated on a great scale in the Red Sea in modern times, and fossil shells of existing species are well preserved therein; and we know that at the mouth of the Nile large deposits of mud are amassed, including the remains of Mediterranean species. It follows, therefore, that if at some future period the bed of the Red Sea should be laid dry, the geologist might experience great difficulties in endeavouring to ascertain the relative age of these formations, which, although dissimilar both in organic and mineral characters, were of synchronous origin.
But, on the other hand, we must not forget that the north-western shores of the Arabian Gulf, the plains of Egypt, and the Isthmus of Suez, are all parts of one province of terrestrial species. Small streams, therefore, occasional land- floods, and those winds which drift clouds of sand along the deserts, might carry down into the Red Sea the same shells of fluviatile and land testacea which the Nile is sweeping into its delta, together with some remains of terrestrial plants and the bones of quadrupeds, whereby the groups of strata before alluded to might, notwithstanding the discrepancy of their mineral composition and marine organic fossils, be shown to have belonged to the same epoch.
Yet, while rivers may thus carry down the same fluviatile and terrestrial spoils into two or more seas inhabited by different marine species, it will much more frequently happen that the coexistence of terrestrial species of distinct zoological and botanical provinces will be proved by the identity of the marine beings which inhabited the intervening space. Thus, for example, the land quadrupeds and shells of the valley of the Mississippi, of central America, and of the West India islands differ very considerably, yet their remains are all washed down by rivers flowing from these three zoological provinces into the Gulf of Mexico.
In some parts of the globe, at the present period, the line of demarkation between distinct provinces of animals and plants is not very strongly marked, especially where the change is determined by temperature, as it is in seas extending from the temperate to the tropical zone, or from the temperate to the arctic regions. Here a gradual passage takes place from one set of species to another. In like manner the geologist, in studying particular formations of remote periods, has sometimes been able to trace the gradation from one ancient province to another, by observing carefully the fossils of all the intermediate places. His success in thus acquiring a knowledge of the zoological or botanical geography of very distant eras has been mainly owing to this circumstance, that the mineral character has no tendency to be affected by climate. A large river may convey yellow or red mud into some part of the ocean, where it may be dispersed by a current over an area several hundred leagues in length, so as to pass from the tropics into the temperate zone. If the bottom of the sea be afterwards upraised, the organic remains imbedded in such yellow or red strata may indicate the different animals or plants which once inhabited at the same time the temperate and equatorial regions.
It may be true, as a general rule, that groups of the same species of animals and plants may extend over wider areas than deposits of homogeneous composition; and if so, palæontological characters will be of more importance in geological classification than the test of mineral composition; but it is idle to discuss the relative value of these tests, as the aid of both is indispensable, and it fortunately happens, that where the one criterion fails, we can often avail ourselves of the other.
Test by included Fragments of older Rocks.—It was stated, that proof may sometimes be obtained of the relative date of two formations by fragments of an older rock being included in a newer one. This evidence may sometimes be of great use, where a geologist is at a loss to determine the relative age of two formations from want of clear sections exhibiting their true order of position, or because the strata of each group are vertical. In such cases we sometimes discover that the more modern rock has been in part derived from the degradation of the older. Thus, for example, we may find chalk in one part of a country, and in another strata of clay, sand, and pebbles. If some of these pebbles consist of that peculiar flint, of which layers more or less continuous are characteristic of the chalk, and which include fossil shells, sponges, and foraminifera of cretaceous species, we may confidently infer that the chalk was the oldest of the two formations.
Chronological Groups.—The number of groups into which the fossiliferous strata may be separated are more or less numerous, according to the views of classification which different geologists entertain; but when we have adopted a certain system of arrangement, we immediately find that a few only of the entire series of groups occur one upon the other in any single section or district.
The thinning out of individual strata was before described (p. 42).But let the diagram (Fig. 84) represent seven fossiliferous groups, instead of as many strata. It will then be seen that in the middle all the superimposed formations are present; but in consequence of some of them thinning out, No. 2 and No. 5 are absent at one extremity of the section, and No. 4 at the other.
Fig. 84: Seven fossiliferous groups.
In another diagram (Fig. 85), a real section of the geological formations in the neighbourhood of Bristol and the Mendip Hills is presented to the reader, as laid down on a true scale by Professor Ramsay, where the newer groups 1, 2, 3, 4 rest unconformably on the formations 5, 6, 7 and 8. At the southern end of the line of section we meet with the beds No. 3 (the New Red Sandstone) resting immediately on Nos. 7 and 8, while farther north as at Dundry Hill in Somersetshire, we behold eight groups superimposed one upon the other, comprising all the strata from the inferior Oolite, No. 1, to the coal and carboniferous limestone. The limited horizontal extension of the groups 1 and 2 is owing to denudation, as these formations end abruptly, and have left outlying patches to attest the fact of their having originally covered a much wider area.
Section South of Bristol.
In order, therefore, to establish a chronological succession of fossiliferous groups, a geologist must begin with a single section in which several sets of strata lie one upon the other. He must then trace these formations, by attention to their mineral character and fossils, continuously, as far as possible, from the starting-point. As often as he meets with new groups, he must ascertain by superposition their age relatively to those first examined, and thus learn how to intercalate them in a tabular arrangement of the whole.
By this means the German, French, and English geologists have determined the succession of strata throughout a great part of Europe, and have adopted pretty generally the following groups, almost all of which have their representatives in the British Islands.
Abridged General Table of Fossiliferous Strata.
TABULAR VIEW OF THE FOSSILIFEROUS STRATA, SHOWING THE ORDER OF SUPERPOSITION OR CHRONOLOGICAL SUCCESSION OF THE PRINCIPAL GROUPS DESCRIBED IN THIS WORK.
POST-TERTIARY EXAMPLES
POST-TERTIARY
EXAMPLES
POST-
TERTIARY 1.
RECENT
Shells and mammals, all of living species. British
Clyde marine strata, with canoes (p. 146).
Foreign
Danish kitchen middens (p. 146).
Lacustrine mud, with remains of Swiss lake-dwellings (p. 148).
Marine strata inclosing Temple of Serapis, at Puzzuoli (p. 146). 2.
POST-
PLIOCENE.
Shells, recent mammalia in part extinct. British
Loam of Brixham cave, with flint implements and bones of extinct and living quadrupeds (p. 157)
Drift near Salisbury, with bones of mammoth, Spermophilus, and stone implements (p. 161).
Glacial drift of Scotland, with marine shells and remains of mammoth (p. 176.
Erratics of Pagham and Selsey Bill (p. 182).
Glacial drift of Wales, with marine fossil shells, about 1400 feet high, on Moel Tryfaen (p. 181).
Foreign
Dordogne caves of the reindeer period (p. 150).
Older valley-gravels of Amiens, with flint implements and bones of extinct mammalia (p. 152).
Loess of Rhine (p. 154).
Ancient Nile-mud forming river-terraces (p. 154).
Loam and breccia of Liege caverns, with human remains (pp. 156, 157).
Australian cave breccias, with bones of extinct marsupials (p. 158).
Glacial drift of Northern Europe (p. 166, p. 174).
TERTIARY OR CAINOZOIC
PLIOCENE 3. NEWER PLIOCENE. The shells almost all of living species. British Bridlington beds, marine Arctic fauna (p. 189). Glacial boulder formation of Norfolk cliffs (p. 190). Forest-bed of Norfolk cliffs, with bones of Elephas meridionalis, etc. (p. 191). Chillesford and Aldeby beds, with marine shells, chiefly Arctic (p. 192). Norwich crag (p. 193). Foreign Eastern base of Mount Etna, with marine shells (p. 204). Sicilian calcareous and tufaceous strata (p. 205, 206). Lacustrine strata of Upper Val d’Arno (p. 207). Madeira leaf-bed and land-shells (p. 532). 4. OLDER PLIOCENE. Extinct species of shells forming a large minority. British Red crag of Suffolk, marine shells, some of northern forms (p. 194, 195). White or coralline crag of Suffolk (p. 197). Foreign Antwerp crag (p. 204). Subapennine marls and sands (p. 208).
EXAMPLES
MIOCENE 5. UPPER MIOCENE. Majority of the shells extinct. British Wanting. Foreign Faluns of Touraine (p. 211). Faluns, proper, of Bordeaux (p. 214). Fresh-water strata of Gers (p. 215). Swiss Oeningen beds, rich in plants and insects (pp. 215-23). Marine Molasse, Switzerland (p. 223). Bolderberg beds of Belgium (p. 224). Vienna basin (p. 224). Beds of the Superga, near Turin (p. 226). Deposit at Pikermé, near Athens (p. 226). Strata of the Siwâlik hills, India (p. 226). Marine strata of the Atlantic border in the United States (p. 227). Volcanic tuff and limestone of Madeira, the Canaries, and the Azores (). 6. LOWER MIOCENE. Nearly all the shells extinct. British Hempstead beds, marine and fresh-water strata (p. 244). Lignites and clays of Bovey Tracey (p. 245). Isle of Mull leaf-bed, volcanic tuff (p. 247). Foreign Calcaire de la Beauce, etc. (p. 230). Grès de Fontainebleau (p. 230). Lacustrine strata of the Limagne d’Auvergne, and the Cantal (p. 233). Mayence basin (p. 242). Radaboj beds of Croatia (p. 242). Brown coal of Germany (p. 244). Lower Molasse of Switzerland, fresh-water and brackish (p. 235-9). Rupelmonde, Kleynspawen, and Tongrian beds of Belgium (p. 241, 242). Nebraska beds, United States (p. 248). Lower Miocene beds of Italy (p. 244). Miocene flora of North Greenland (p. 239). EOCENE 7. UPPER EOCENE. British Bembridge fluvio-marine strata (p. 252). Osborne or St. Helen’s series (p. 255). Headon series, with marine and fresh-water shells (p. 255). Barton sands and clays (p. 258). Foreign Gypsum of Montmartre, fresh-water with Palæotherium (p. 270). Calcaire silicieux, or Travertin inférieur (p. 273), Grès de Beauchamp, or Sables moyens (p. 273). 8. MIDDLE EOCENE. British Bracklesham beds and Bagshot sands (p. 259). White clays of Alum Bay and Bournemouth (p. 262). Foreign Calcaire grossier, miliolitic limestone (p. 274). Soissonnais sands, or Lits coquilliers, with Nummulites planulata (p. 275). Claiborne beds of the United States, with Orbitoides and Zeuglodon (p. 279). Nummulitic formation of Europe, Asia, etc. (p. 277). 9. LOWER EOCENE. British London clay proper (p. 263). Woolwich and Reading series, fluvio-marine (p. 267). Thanet sands (p. 269). Foreign Argile de Londres, near Dunkirk (p. 252). Argile plastique (p. 276). Sables de Bracheux (p. 276).
SECONDARY OR MESOZOIC.
CRETACEOUS 10. UPPER CRETACEOUS. British Upper white chalk, with flints (p. 290). Lower white chalk, without flints (p. 298). Chalk marl (p. 298). Chloritic series (or Upper Greensand), fire-stone of Surrey (p. 298). Gault (p. 300). Blackdown beds (p. 301).
EXAMPLES
CRETACEOUS 10. UPPER CRETACEOUS. Foreign Maetricht beds and Faxoe chalk (p. 233). Pisolitic limestone of France (p. 285). White chalk of France, Sweden, and Russia (p. 286, 287). Planer-kalk of Saxony (p. 293). Sands and clays of Aix-la-Chapelle (p. 302). Hippurite limestone of South of France (p. 305). New Jersey, U.S., sands and marls (p. 307). 11. LOWER CRETACEOUS or NEOCOMIAN. British Sands of Folkestone, Sandgate, and Hythe (p. 308). Atherfield clay, with Perna mulleti (p. 309). Punfield marine beds, with Vicarya lujana (p. 318). Speeton clay of Flamborough Head and Tealby (p. 311). Weald clay of Surrey, Kent, and Sussex, fresh-water, with Cypris (p. 313-5). Hastings sands (p. 316-8). Foreign Neocomian of Neufchatel, and Hils conglomerate of North Germany (p. 312). Wealden beds of Hanover (p. 319). OOLITE 12. UPPER OOLITE. British Upper Purbeck beds, fresh-water (p. 323). Middle Purbeck, with numerous marsupial quadrupeds, etc. (p. 324). Lower Purbeck, fresh-water, with intercalated dirt-bed (p. 330). Portland stone and sand. (p. 334). Kimmeridge clay (p. 335). Foreign Marnes à gryphées virgules of Argonne (p. 336). Lithographic-stone of Solenhofen, with Archæopteryx (p. 337). 13. MIDDLE OOLITE. British Coral rag of Berkshire, Wilts, and Yorkshire (p. 339). Oxford clay, with belemnites and Ammonite (p. 340). Kelloway rock of Wilts and Yorkshire (p. 341). Foreign Nerinæan limestone of the Jura (p. 339). 14. LOWER OOLITE. British Cornbrash and forest marble (p. 341). Great or Bath oolite of Bradford (p. 342). Stonesfield slate, with marsupials and Araucaria (p. 345). Fuller’s earth of Bath (p. 348). Inferior oolite (p. 349). LIAS 15. LIAS. Upper Lias, argillaceous, with Ammonites striatulus (p. 353). Shale and limestone, with Ammonites bifrons (p. 353). Middle Lias or Marlstone series, with zones containing characteristic Ammonites (p. 353). Lower Lias, also with zones characterised by peculiar Ammonites (p. 356). TRIAS 16. UPPER TRIAS. British Rhætic, Penarth or Avicula contorta beds (beds of passage) (p. 366). Keuper or Upper New Red sandstone, etc. (p. 369). Red shales of Cheshire and Lancashire, with rock-salt (p. 371). Dolomite conglomerate of Bristol (p. 373). Foreign Keuper beds of Germany (p. 375). St. Cassian or Hallstadt beds, with rich marine fauna (p. 376). Coal-field of Richmond, Virginia (p. 382). Chatham coal-field, North Carolina (p. 383). 17. MIDDLE TRIAS. British Wanting. Foreign Muschelkalk of Germany (p. 378). 18. LOWER TRIAS. British Bunter or Lower New Red sandstone of Lancashire and Cheshire (p. 372). Foreign Bunter-sandstein of Germany (p. 380). Red sandstone of Connecticut Valley, with footprints of birds and reptiles (p. 381).
PRIMARY OR PALÆOZOIC EXAMPLES
PERMIAN 19. PERMIAN. British Upper Permian of St. Bees’ Head, Cumberland (p. 386). Middle Permian, magnesian limestone, and marl-slate of Durham and Yorkshire, with Protosaurus (p. 387). Lower Permian sandstones and breccias of Penrith and Dumfriesshire, intercalated (p. 390). Foreign Dark-coloured shales of Thuringia (p. 392). Zechstein or Dolomitic limestone (p. 392). Mergel-schiefer or Kupfer-schiefer (p. 392). Rothliegendes of Thuringia, with Psaronius (p. 392). Magnesian limestones, etc., of Russia (p. 393). CARBONIFEROUS 20. UPPER CARBONIFEROUS. British Coal-measures of South Wales, with underclays inclosing Stigmaria (p. 397). Coal-measures of north and central England (p. 395). Millstone grit (p. 395). Yoredale series of Yorkshire (p. 395). Coal-field of Kilkenny with Labyrinthodont (p. 407). Foreign Coal-field of Saarbruck, with Archegosaurus (p. 406). Carboniferous strata of South Joggins, Nova Scotia (p. 409). Pennsylvania coal-field (p. 403). 21. LOWER CARBONIFEROUS. British Mountain limestone of Wales and South of England (p. 430). Same in Ireland (p. 437437). Carboniferous limestone of Scotland alternating with coal-bearing sandstones (p. 396). Erect trees in volcanic ash in the Island of Arran (p. 546). Foreign Mountain limestone of Belgium (p. 436). DEVONIAN or OLD RED SANDSTONE 22. UPPER DEVONIAN. British Yellow sandstone of Dura Den, with Holoptychius, etc. (p. 440); and of Ireland with Anodon Jukesii (p. 441). Sandstones of Forfarshire and Perthshire, with Holoptychius, etc. (p. 442). Pilton group of North Devon (p. 449). Petherwyn group of Cornwall, with Clymenia and Cypridina (p. 451). Foreign Clymenien-kalk and Cypridinen-schiefer of Germany (p. 450) 23. MIDDLE DEVONIAN. British Bituminous schists of Gamrie, Caithness, etc., with numerous fish (p. 443). Ilfracombe beds with peculiar trilobites and corals (p. 450). Limestones of Torquay, with broad-winged Spirifers (p. 451). Foreign Eifel limestone, with underlying schists containing Calceola (p. 453). Devonian strata of Russia (p. 454). 24. LOWER DEVONIAN. British Arbroath paving-stones, with Cephalaspis and Pterygotus (p. 446). Lower sandstones of Forfarshire, with Pterygotus (p. 446). Sandstones and slates of the Foreland and Linton (p. 454). Foreign Oriskany sandstone of Western Canada and New York (p. 456). Sandstones of Gaspe, with Cephalaspis (p. 455 ).
EXAMPLES
SILURIAN 25. UPPER SILURIAN British Upper Ludlow formation, Downton sandstone, with bone-bed (p. 459). Lower Ludlow formation, with oldest known fish remains (p. 461). Wenlock limestone and shale (p. 465). Woolhope limestone and grit (p. 467). Tarannon shales (p. 468). Beds of passage between Upper and Lower Silurian: Upper Llandovery, or May-hill sandstone, with Pentamerus oblongus, etc. (p. 468). Lower Llandovery slates (p. 469). Foreign Niagara limestone, with Calymene, Homalonotus, etc. (p. 479). Clinton group of America, with Pentamerus oblongus, etc. (p. 479). Silurian strata of Russia, with Pentamerus (p. 477). 26. LOWER SILURIAN. British Bala and Caradoc beds (p. 470). Llandeilo flags (p. 473). Arenig or Stiper-stones group (Lower Llandeilo of Murchison) (p. 475). Foreign Ungulite or Obolus grit of Russia (p. 477). Trenton limestone, and other Lower Silurian groups of North America (p. 479). Lower Silurian of Sweden (p. 477). CAMBRIAN 27. UPPER CAMBRIAN. British Tremadoc slates (p. 483). Lingula flags, with Lingula Davisii (p. 484). Foreign “Primordial” zone of Bohemia in part, with trilobites of the genera Paradoxides, etc. (p. 487). Alum schists of Sweden and Norway (p. 489). Potsdam sandstone, with Dikelocephalus and Obolella (p. 489). 28. LOWER CAMBRIAN. British Menevian beds of Wales, with Paradoxides Davidis, etc. (p. 484). Longmynd group, comprising the Harlech grits and Llanberis slates (p. 485). Foreign Lower portion of Barrande’s “Primordial” zone in Bohemia (p. 486). Fucoid sandstones of Sweden (p. 489). Huronian series of Canada? (p. 490). LAURENTIAN 29. UPPER LAURENTIAN. British Fundamental gneiss of the Hebrides? (p. 493). Hypersthene rocks of Skye? (p. 491). Foreign Labradorite series north of the river St. Lawrence in Canada (p. 491). Adirondack mountains of New York (p. 491). 30. LOWER LAURENTIAN. British Wanting? Foreign Beds of gneiss and quartzite, with interstratified limestones, in one of which, 1000 feet thick, occurs a foraminifer, Eozoon Canadense, the oldest known fossil (p. 491).
IX. CLASSIFICATION OF TERTIARY FORMATIONS
Order of Succession of Sedimentary Formations. — Frequent Unconformability of Strata. — Imperfection of the Record. — Defectiveness of the Monuments greater in Proportion to their Antiquity. — Reasons for studying the newer Groups first. — Nomenclature of Formations. — Detached Tertiary Formations scattered over Europe. — Value of the Shell-bearing Mollusca in Classification. — Classification of Tertiary Strata. — Eocene, Miocene, and Pliocene Terms explained.
By reference to the tables given at the end of the last chapter the reader will see that when the fossiliferous rocks are arranged chronologically, we have first to consider the Post-tertiary and then the Tertiary or Cainozoic formations, and afterwards to pass on to those of older date.
Fig. 86: Order of Superposition of Deposits
Order of Superposition.—The diagram (Fig. 86) will show the order of superposition of these deposits, assuming them all to be visible in one continuous section. In nature, as before hinted (p. 107), we have never an opportunity of seeing the whole of them so displayed in a single region; first, because sedimentary deposition is confined, during any one geological period, to limited areas; and secondly, because strata, after they have been formed, are liable to be utterly annihilated over wide areas by denudation. But wherever certain members of the series are present, they overlie one another in the order indicated in the diagram, though not always in the exact manner there represented, because some of them repose occasionally in unconformable stratification on others. This mode of superposition has been already explained (p. 94, p. 111), where I pointed out that the discordance which implies a considerable lapse of time between two formations in juxtaposition is almost invariably accompanied by a great dissimilarity in the species of organic remains.
Frequent Unconformability of Strata.—Where the widest gaps appear in the sequence of the fossil forms, as between the Permian and Triassic rocks, or between the Cretaceous and Eocene, examples of such unconformability are very frequent. But they are also met with in some part or other of the world at the junction of almost all the other principal formations, and sometimes the subordinate divisions of any one of the leading groups may be found lying unconformably on another subordinate member of the same—the Upper, for example, on the Lower Silurian, or the superior division of the Old Red Sandstone on a lower member of the same, and so forth. Instances of such irregularities in the mode of succession of the strata are the more intelligible the more we extend our survey of the fossiliferous formations, for we are continually bringing to light deposits of intermediate date, which have to be intercalated between those previously known, and which reveal to us a long series of events, of which antecedently to such discoveries we had no knowledge.
But while unconformability invariably bears testimony to a lapse of unrepresented time, the conformability of two sets of strata in contact by no means implies that the newer formation immediately succeeded the older one. It simply implies that the ancient rocks were subjected to no movements of such a nature as to tilt, bend, or break them before the more modern formation was superimposed. It does not show that the earth’s crust was motionless in the region in question, for there may have been a gradual sinking or rising, extending uniformly over a large surface, and yet, during such movement, the stratified rocks may have retained their original horizontality of position. There may have been a conversion of a wide area from sea into land and from land into sea, and during these changes of level some strata may have been slowly removed by aqueous action, and after this new strata may be superimposed, differing perhaps in date by thousands of years or centuries, and yet resting conformably on the older set. There may even be a blending of the materials constituting the older deposit with those of the newer, so as to give rise to a passage in the mineral character of the one rock into the other as if there had been no break or interruption in the depositing process.
Imperfection of the Record.—Although by the frequent discovery of new sets of intermediate strata the transition from one type of organic remains to another is becoming less and less abrupt, yet the entire series of records appears to the geologists now living far more fragmentary and defective than it seemed to their predecessors half a century ago. The earlier inquirers, as often as they encountered a break in the regular sequence of formations, connected it theoretically with a sudden and violent catastrophe, which had put an end to the regular course of events that had been going on uninterruptedly for ages, annihilating at the same time all or nearly all the organic beings which had previously flourished, after which, order being re-established, a new series of events was initiated. In proportion as our faith in these views grows weaker, and the phenomena of the organic or inorganic world presented to us by geology seem explicable on the hypothesis of gradual and insensible changes, varied only by occasional convulsions, on a scale comparable to that witnessed in historical times; and in proportion as it is thought possible that former fluctuations in the organic world may be due to the indefinite modifiability of species without the necessity of assuming new and independent acts of creation, the number and magnitude of the gaps which still remain, or the extreme imperfection of the record, become more and more striking, and what we possess of the ancient annals of the earth’s history appears as nothing when contrasted with that which has been lost.
When we examine a large area such as Europe, the average as well as the extreme height above the sea attained by the older formations is usually found to exceed that reached by the more modern ones, the primary or palaeozoic rising higher than the secondary, and these in their turn than the tertiary; while in reference to the three divisions of the tertiary, the lowest or Eocene group attains a higher summit-level than the Miocene, and these again a greater height than the Pliocene formations. Lastly, the post-tertiary deposits, such, at least, as are of marine origin, are most commonly restricted to much more moderate elevations above the sea-level than the tertiary strata.
It is also observed that strata, in proportion as they are of newer date, bear the nearest resemblance in mineral character to those which are now in the progress of formation in seas or lakes, the newest of all consisting principally of soft mud or loose sand, in some places full of shells, corals, or other organic bodies, animal or vegetable, in others wholly devoid of such remains. The farther we recede from the present time, and the higher the antiquity of the formations which we examine, the greater are the changes which the sedimentary deposits have undergone. Time, as I have explained in Chapters V, VI, and VII, has multiplied the effects of condensation by pressure and cementation, and the modification produced by heat, fracture, contortion, upheaval, and denudation. The organic remains also have sometimes been obliterated entirely, or the mineral matter of which they were composed has been removed and replaced by other substances.
Why newer Groups should be studied first.—We likewise observe that the older the rocks the more widely do their organic remains depart from the types of the living creation. First, we find in the newer tertiary rocks a few species which no longer exist, mixed with many living ones, and then, as we go farther back, many genera and families at present unknown make their appearance, until we come to strata in which the fossil relics of existing species are nowhere to be detected, except a few of the lowest forms of invertebrate, while some orders of animals and plants wholly unrepresented in the living world begin to be conspicuous.
When we study, therefore, the geological records of the earth and its inhabitants, we find, as in human history, the defectiveness and obscurity of the monuments always increasing the remoter the era to which we refer, and the difficulty of determining the true chronological relations of rocks is more and more enhanced, especially when we are comparing those which were formed simultaneously in very distant regions of the globe. Hence we advance with securer steps when we begin with the study of the geological records of later times, proceeding from the newer to the older, or from the more to the less known.
In thus inverting what might at first seem to be the more natural order of historical research, we must bear in mind that each of the periods above enumerated, even the shortest, such as the Post-tertiary, or the Pliocene, Miocene, or Eocene, embrace a succession of events of vast extent, so that to give a satisfactory account of what we already know of any one of them would require many volumes. When, therefore, we approach one of the newer groups before endeavouring to decipher the monuments of an older one, it is like endeavouring to master the history of our own country and that of some contemporary nations, before we enter upon Roman History, or like investigating the annals of Ancient Italy and Greece before we approach those of Egypt and Assyria.
Nomenclature.—The origin of the terms Primary and Secondary, and the synonymous terms Palaeozoic, and Mesozoic, were explained in Chapter VIII, p. 123.
The Tertiary or Cainozoic strata (see p. 123) were so called because they were all posterior in date to the Secondary series, of which last the Chalk of Cretaceous, No. 9, Fig. 86, constitutes the newest group. The whole of them were at first confounded with the superficial alluviums of Europe; and it was long before their real extent and thickness, and the various ages to which they belong, were fully recognised. They were observed to occur in patches, some of fresh-water, others of marine origin, their geographical area being usually small as compared to the secondary formations, and their position often suggesting the idea of their having been deposited in different bays, lakes, estuaries, or inland seas, after a large portion of the space now occupied by Europe had already been converted into dry land.
The first deposits of this class, of which the characters were accurately determined, were those occurring in the neighbourhood of Paris, described in 1810 by MM. Cuvier and Brongniart. They were ascertained to consist of successive sets of strata, some of marine, others of fresh-water origin, lying one upon the other. The fossil shells and corals were perceived to be almost all of unknown species, and to have in general a near affinity to those now inhabiting warmer seas. The bones and skeletons of land animals, some of them of large size, and belonging to more than forty distinct species, were examined by Cuvier, and declared by him not to agree specifically, nor most of them even generically, with any hitherto observed in the living creation.
Strata were soon afterwards brought to light in the vicinity of London, and in Hampshire, which, although dissimilar in mineral composition, were justly inferred by Mr. T. Webster to be of the same age as those of Paris, because the greater number of the fossil shells were specifically identical. For the same reason, rocks found on the Gironde, in the South of France, and at certain points in the North of Italy, were suspected to be of contemporaneous origin.
Another important discovery was soon afterwards made by Brocchi in Italy, who investigated the argillaceous and sandy deposits, replete with shells, which form a low range of hills, flanking the Apennines on both sides, from the plains of the Po to Calabria. These lower hills were called by him the Subapennines, and were formed of strata chiefly marine, and newer than those of Paris and London.
Another tertiary group occurring in the neighbourhood of Bordeaux and Dax, in the South of France, was examined by M. de Basterot in 1825, who described and figured several hundred species of shells, which differed for the most part both from the Parisian series and those of the Subapennine hills. It was soon, therefore, suspected that this fauna might belong to a period intermediate between that of the Parisian and Subapennine strata, and it was not long before the evidence of superposition was brought to bear in support of this opinion; for other strata, contemporaneous with those of Bordeaux, were observed in one district (the Valley of the Loire), to overlie the Parisian formation, and in another (in Piedmont) to underlie the Subapennine beds. The first example of these was pointed out in 1829 by M. Desnoyers, who ascertained that the sand and marl of marine origin called faluns, near Tours, in the basin of the Loire, full of sea-shells and corals, rested upon a lacustrine formation, which constitutes the uppermost subdivision of the Parisian group, extending continuously throughout a great table-land intervening between the basin of the Seine and that of the Loire. The other example occurs in Italy, where strata containing many fossils similar to those of Bordeaux were observed by Bonelli and others in the environs of Turin, subjacent to strata belonging to the Subapennine group of Brocchi.
Value of Testacean Fossils in Classification.—It will be observed that in the foregoing allusions to organic remains, the testacea or the shell-bearing mollusca are selected as the most useful and convenient class for the purposes of general classification. In the first place, they are more universally distributed through strata of every age than any other organic bodies. Those families of fossils which are of rare and casual occurrence are absolutely of no avail in establishing a chronological arrangement. If we have plants alone in one group of strata and the bones of mammalia in another, we can draw no conclusion respecting the affinity or discordance of the organic beings of the two epochs compared; and the same may be said if we have plants and vertebrated animals in one series and only shells in another. Although corals are more abundant, in a fossil state, than plants, reptiles, or fish, they are still rare when contrasted with shells, because they are more dependent for their well-being on the constant clearness of the water, and are, therefore, less likely to be included in rocks which endure in consequence of their thickness and the copiousness of sediment which prevailed when they originated. The utility of the testacea is, moreover, enhanced by the circumstance that some forms are proper to the sea, others to the land, and others to fresh water. Rivers scarcely ever fail to carry down into their deltas some land-shells, together with species which are at once fluviatile and lacustrine. By this means we learn what terrestrial, fresh-water, and marine species coexisted at particular eras of the past: and having thus identified strata formed in seas with others which originated contemporaneously in inland lakes, we are then enabled to advance a step farther, and show that certain quadrupeds or aquatic plants, found fossil in lacustrine formations, inhabited the globe at the same period when certain fish, reptiles, and zoophytes lived in the ocean.
Among other characters of the molluscous animals, which render them extremely valuable in settling chronological questions in geology, may be mentioned, first, the wide geographical range of many species; and, secondly, what is probably a consequence of the former, the great duration of species in this class, for they appear to have surpassed in longevity the greater number of the mammalia and fish. Had each species inhabited a very limited space, it could never, when imbedded in strata, have enabled the geologist to identify deposits at distant points; or had they each lasted but for a brief period, they could have thrown no light on the connection of rocks placed far from each other in the chronological, or, as it is often termed, vertical series.
Classification of Tertiary Strata.—Many authors have divided the European Tertiary strata into three groups—lower, middle, and upper; the lower comprising the oldest formations of Paris and London before mentioned; the middle those of Bordeaux and Touraine; and the upper all those newer than the middle group.
In the first edition of the Principles of Geology, I divided the whole of the Tertiary formations into four groups, characterised by the percentage of recent shells which they contained. The lower tertiary strata of London and Paris were thought by M. Deshayes to contain only 3½ per cent of recent species, and were termed Eocene. The middle tertiary of the Loire and Gironde had, according to the specific determinations of the same conchologist, 17 per cent, and formed the Miocene division. The Subapennine beds contained 35 to 50 per cent, and were termed Older Pliocene, while still more recent beds in Sicily, which had from 90 to 95 per cent of species identical with those now living, were called Newer Pliocene. The first of the above terms, Eocene, is derived from eos, dawn, and cainos, recent, because the fossil shells of this period contain an extremely small proportion of living species, which may be looked upon as indicating the dawn of the existing state of the testaceous fauna, no recent species having been detected in the older or secondary rocks.
The term Miocene (from meion, less, and cainos, recent) is intended to express a minor proportion of recent species (of testacea), the term Pliocene (from pleion, more, and cainos, recent) a comparative plurality of the same. It may assist the memory of students to remind them, that the _Mi_ocene contain a _mi_nor proportion, and _ Pl_iocene a comparative _pl_urality of recent species; and that the greater number of recent species always implies the more modern origin of the strata.
It has sometimes been objected to this nomenclature that certain species of infusoria found in the chalk are still existing, and, on the other hand, the Miocene and Older Pliocene deposits often contain the remains of mammalia, reptiles, and fish, exclusively of extinct species. But the reader must bear in mind that the terms Eocene, Miocene, and Pliocene were originally invented with reference purely to conchological data, and in that sense have always been and are still used by me.
Since the year 1830 the number of known shells, both recent and fossil, has largely increased, and their identification has been more accurately determined. Hence some modifications have been required in the classifications founded on less perfect materials. The Eocene, Miocene, and Pliocene periods have been made to comprehend certain sets of strata of which the fossils do not always conform strictly in the proportion of recent to extinct species with the definitions first given by me, or which are implied in the etymology of those terms.
X. RECENT AND POST-PLIOCENE PERIODS
Recent and Post-pliocene Periods. — Terms defined. — Formations of the Recent Period. — Modern littoral Deposits containing Works of Art near Naples. — Danish Peat and Shell-mounds. — Swiss Lake-dwellings. — Periods of Stone, Bronze, and Iron. — Post-pliocene Formations. — Coexistence of Man with extinct Mammalia. — Reindeer Period of South of France. — Alluvial Deposits of Paleolithic Age. — Higher and Lower-level Valley-gravels. — Loess or Inundation-mud of the Nile, Rhine, etc. — Origin of Caverns. — Remains of Man and extinct Quadrupeds in Cavern Deposits. — Cave of Kirkdale. — Australian Cave-breccias. — Geographical Relationship of the Provinces of living Vertebrata and those of extinct Post-pliocene Species. — Extinct struthious Birds of New Zealand. — Climate of the Post-pliocene Period. — Comparative Longevity of Species in the Mammalia and Testacea. — Teeth of Recent and Post-pliocene Mammalia.
We have seen in the last chapter that the uppermost or newest strata are called Post-tertiary, as being more modern than the Tertiary. It will also be observed that the Post-tertiary formations are divided into two subordinate groups: the Recent, and Post-pliocene. In the former, or the Recent, the mammalia as well as the shells are identical with species now living: whereas in the Post-pliocene, the shells being all of living forms, a part, and often a considerable part, of the mammalia belonged to extinct species. To this nomenclature it may be objected that the term Post-pliocene should in strictness include all geological monuments posterior in date to the Pliocene; but when I have occasion to speak of the whole collectively, I shall call them Post-tertiary, and reserve the term Post-pliocene for the older Post-tertiary formations, while the Upper or newer ones will be called “Recent.”
Cases will occur where it may be scarcely possible to draw the boundary line between the Recent and Post-pliocene deposits; and we must expect these difficulties to increase rather than diminish with every advance in our knowledge, and in proportion as gaps are filled up in the series of records.
RECENT PERIOD
It was stated in the sixth chapter, when I treated of denudation, that the dry land, or that part of the earth’s surface which is not covered by the waters of lakes or seas, is generally wasting away by the incessant action of rain and rivers, and in some cases by the undermining and removing power of waves and tides on the sea-coast. But the rate of waste is very unequal, since the level and gently sloping lands, where they are protected by a continuous covering of vegetation, escape nearly all wear and tear, so that they may remain for ages in a stationary condition, while the removal of matter is constantly widening and deepening the intervening ravines and valleys.
The materials, both fine and coarse, carried down annually by rivers from the higher regions to the lower, and deposited in successive strata in the basins of seas and lakes, must be of enormous volume. We are always liable to underrate their magnitude, because the accumulation of strata is going on out of sight.
There are, however, causes at work which, in the course of centuries, tend to render visible these modern formations, whether of marine or lacustrine origin. For a large portion of the earth’s crust is always undergoing a change of level, some areas rising and others sinking at the rate of a few inches, or a few feet, perhaps sometimes yards, in a century; so that spaces which were once subaqueous are gradually converted into land, and others which were high and dry become submerged. In consequence of such movements we find in certain regions, as in Cashmere, for example, where the mountains are often shaken by earthquakes, deposits which were formed in lakes in the historical period, but through which rivers have now cut deep and wide channels. In lacustrine strata thus intersected, works of art and fresh-water shells are seen. In other districts on the borders of the sea, usually at very moderate elevations above its level, raised beaches occur, or marine littoral deposits, such as those in which, on the borders of the Bay of Baiæ, near Naples, the well-known temple of Serapis was imbedded. In that case the date of the monument buried in the marine strata is ascertainable, but in many other instances the exact age of the remains of human workmanship is uncertain, as in the estuary of the Clyde at Glasgow, where many canoes have been exhumed, with other works of art, all assignable to some part of the Recent Period.
Danish Peat and Shell-mounds or Kitchen-middens.—Sometimes we obtain evidence, without the aid of a change of level, of events which took place in pre-historic times. The combined labours, for example, of the antiquary, zoologist, and botanist have brought to light many monuments of the early inhabitants buried in peat-mosses in Denmark. Their geological age is determined by the fact that, not only the contemporaneous fresh-water and land shells, but all the quadrupeds, found in the peat, agree specifically with those now inhabiting the same districts, or which are known to have been indigenous in Denmark within the memory of man. In the lower beds of peat (a deposit varying from 20 to 30 feet in thickness), weapons of stone accompany trunks of the Scotch fir, Pinus sylvestris. This peat may be referred to that part of the stone period for which Sir John Lubbock proposed the name of “Neolithic”[1] in contradistinction to a still older era, termed by him “Paleolithic,” and which will be described in the sequel. In the higher portions of the same Danish bogs, bronze implements are associated with trunks and acorns of the common oak. It appears that the pine has never been a native of Denmark in historical times, and it seems to have given place to the oak about the time when articles and instruments of bronze superseded those of stone. It also appears that, at a still later period, the oak itself became scarce, and was nearly supplanted by the beech, a tree which now flourishes luxuriantly in Denmark. Again, at the still later epoch when the beech-tree abounded, tools of iron were introduced, and were gradually substituted for those of bronze.
On the coasts of the Danish islands in the Baltic, certain mounds, called in those countries “Kjökken-mödding,” or “kitchen-middens,” occur, consisting chiefly of the castaway shells of the oyster, cockle, periwinkle, and other eatable kinds of molluscs. The mounds are from three to ten feet high, and from 100 to 1000 feet in their longest diameter. They greatly resemble heaps of shells formed by the Red Indians of North America along the eastern shores of the United States. In the old refuse-heaps, recently studied by the Danish antiquaries and naturalists with great skill and diligence, no implements of metal have ever been detected. All the knives, hatchets, and other tools, are of stone, horn, bone, or wood. With them are often intermixed fragments of rude pottery, charcoal and cinders, and the bones of quadrupeds on which the rude people fed. These bones belong to wild species still living in Europe, though some of them, like the beaver, have long been extirpated in Denmark. The only animal which they seem to have domesticated was the dog.
As there is an entire absence of metallic tools, these refuse-heaps are referred to the Neolithic division of the age of stone, which immediately preceded in Denmark the age of bronze. It appears that a race more advanced in civilisation, armed with weapons of that mixed metal, invaded Scandinavia, and ousted the aborigines.
Lacustrine Habitations of Switzerland.—In Switzerland a different class of monuments, illustrating the successive ages of stone, bronze, and iron, has been of late years investigated with great success, and especially since 1854, in which year Dr. F. Keller explored near the shore at Meilen, in the bottom of the lake of Zurich, the ruins of an old village, originally built on numerous wooden piles, driven, at some unknown period, into the muddy bed of the lake. Since then a great many other localities, more than a hundred and fifty in all, have been detected of similar pile-dwellings, situated near the borders of the Swiss lakes, at points where the depth of water does not exceed 15 feet.[2] The superficial mud in such cases is filled with various articles, many hundreds of them being often dredged up from a very limited area. Thousands of piles, decayed at their upper extremities, are often met with still firmly fixed in the mud.
As the ages of stone, bronze, and iron merely indicate successive stages of civilisation, they may all have coexisted at once in different parts of the globe, and even in contiguous regions, among nations having little intercourse with each other. To make out, therefore, a distinct chronological series of monuments is only possible when our observations are confined to a limited district, such as Switzerland.
The relative antiquity of the pile-dwellings, which belong respectively to the ages of stone and bronze, is clearly illustrated by the associations of the tools with certain groups of animal remains. Where the tools are of stone, the castaway bones which served for the food of the ancient people are those of deer, the wild boar, and wild ox, which abounded when society was in the hunter state. But the bones of the later or bronze epoch were chiefly those of the domestic ox, goat, and pig, indicating progress in civilisation. Some villages of the stone age are of later date than others, and exhibit signs of an improved state of the arts. Among their relics are discovered carbonised grains of wheat and barley, and pieces of bread, proving that the cultivation of cereals had begun. In the same settlements, also, cloth, made of woven flax and straw, has been detected.
The pottery of the bronze age in Switzerland is of a finer texture, and more elegant in form, than that of the age of stone. At Nidau, on the lake of Bienne, articles of iron have also been discovered, so that this settlement was evidently not abandoned till that metal had come into use.
At La Thène, in the northern angle of the lake of Neufchâtel, a great many articles of iron have been obtained, which in form and ornamentation are entirely different both from those of the bronze period and from those used by the Romans. Gaulish and Celtic coins have also been found there by MM. Schwab and Desor. They agree in character with remains, including many iron swords, which have been found at Tiefenau, near Berne, in ground supposed to have been a battle-field; and their date appears to have been anterior to the great Roman invasion of Northern Europe, though perhaps not long before that event.[3] Coins, which sometimes occur in deposits of the age of iron, have never yet been found in formations of the ages of bronze or stone.
The period of bronze must have been one of foreign commerce, as tin, which enters into this metallic mixture in the proportion of about ten per cent to the copper, was obtained by the ancients chiefly from Cornwall.[4] Very few human bones of the bronze period have been met with in the Danish peat, or in the Swiss lake-dwellings, and this scarcity is generally attributed by archæologists to the custom of burning the dead, which prevailed in the age of bronze.
POST-PLIOCENE PERIOD
From the foregoing observations we may infer that the ages of iron and bronze in Northern and Central Europe were preceded by a stone age, the Neolithic, referable to that division of the post-tertiary epoch which I have called Recent, when the mammalia as well as the other organic remains accompanying the stone implements were of living species. But memorials have of late been brought to light of a still older age of stone, for which, as above stated, the name Paleolithic has been proposed, when man was contemporary in Europe with the elephant and rhinoceros, and various other animals, of which many of the most conspicuous have long since died out.
Reindeer Period in South of France.—In the larger number of the caves of Europe, as for example in those of England, Belgium, Germany, and many parts of France, the animal remains agree specifically with the fauna of this oldest division of the age of stone, or that to which belongs the drift of Amiens and Abbeville presently to be mentioned, containing flint implements of a very antique type. But there are some caves in the departments of Dordogne, Aude, and other parts of the south of France, which are believed by M. Lartet to be of intermediate date between the Paleolithic and Neolithic periods. To this intermediate era M. Lartet gave, in 1863, the name of the “reindeer period,” because vast quantities of the bones and horns of that deer have been met with in the French caverns. In some cases separate plates of molars of the mammoth, and several teeth of the great Irish deer, Cervus megaceros, and of the cave-lion, Felis spelæa, have been found mixed up with cut and carved bones of reindeer. On one of these sculptured bones in the cave of Perigord, a rude representation of the mammoth, with its long curved tusks and covering of wool, occurs, which is regarded by M. Lartet as placing beyond all doubt the fact that the early inhabitants of these caves must have seen this species of elephant still living in France. The presence of the marmot, as well as the reindeer and some other northern animals, in these caverns seems to imply a colder climate than that of the Swiss lake-dwellings, in which no remains of reindeer have as yet been discovered. The absence of this last in the old lacustrine habitations of Switzerland is the more significant, because in a cave in the neighbourhood of the lake of Geneva, namely, that of Mont Saleve, bones of the reindeer occur with flint implements similar to those of the caverns of Dordogne and Perigord.
The state of the arts, as exemplified by the instruments found in these caverns of the reindeer period, is somewhat more advanced than that which characterises the tools of the Amiens drift, but is nevertheless more rude than that of the Swiss lake-dwellings. No metallic articles occur, and the stone hatchets are not ground after the fashion of celts; the needles of bone are shaped in a workmanlike style, having their eyes drilled with consummate skill.
The formations above alluded to, which are as yet but imperfectly known, may be classed as belonging to the close of the Paleolithic era, of the monuments of which I am now about to treat.
Alluvial Deposits of the Paleolithic Age.—The alluvial and marine deposits of the Paleolithic age, the earliest to which any vestiges of man have yet been traced back, belong to a time when the physical geography of Europe differed in a marked degree from that now prevailing. In the Neolithic period, the valleys and rivers coincided almost entirely with those by which the present drainage of the land is effected, and the peat-mosses were the same as those now growing. The situation of the shell-mounds and lake-dwellings above alluded to is such as to imply that the topography of the districts where they are observed has not subsequently undergone any material alteration. Whereas we no sooner examine the Post-pliocene formations, in which the remains of so many extinct mammalia are found, than we at once perceive a more decided discrepancy between the former and present outline of the surface. Since those deposits originated, changes of considerable magnitude have been effected in the depth and width of many valleys, as also in the direction of the superficial and subterranean drainage, and, as is manifest near the sea-coast, in the relative position of land and water. In Fig. 87 an ideal section is given, illustrating the different position which the Recent and Post-pliocene alluvial deposits occupy in many European valleys.
Fig. 87: Recent and Post-pliocene alluvial deposits.
The peat, No. 1, has been formed in a low part of the modern alluvial plain, in parts of which gravel No. 2 of the recent period is seen. Over this gravel the loam or fine sediment 2′ has in many places been deposited by the river during floods which covered nearly the whole alluvial plain.
No. 3 represents an older alluvium, composed of sand and gravel, formed before the valley had been excavated to its present depth. It contains the remains of fluviatile shells of living species associated with the bones of mammalia, in part of recent, and in part of extinct species. Among the latter, the mammoth (E. primigenius) and the Siberian rhinoceros (R. tichorhinus) are the most common in Europe. No. 3′ is a remnant of the loam or brick-earth by which No. 3 was overspread. No. 4 is a still older and more elevated terrace, similar in its composition and organic remains to No. 3, and covered in like manner with its inundation-mud, 4′. Sometimes the valley-gravels of older date are entirely missing, or there is only one, and occasionally there are more than two, marking as many successive stages in the excavation of the valley. They usually occur at heights varying from 10 to 100 feet, sometimes on the right and sometimes on the left side of the existing river-plain, but rarely in great strength on exactly opposite sides of the valley.
Among the genera of extinct quadrupeds most frequently met with in England, France, Germany, and other parts of Europe, are the elephant, rhinoceros, hippopotamus, horse, great Irish deer, bear, tiger, and hyæna. In the peat, No. 1 (Fig. 87), and in the more modern gravel and silt (No. 2), works of art of the ages of iron and bronze, and of the later or Neolithic stone period, already described, are met with. In the more ancient or Paleolithic gravels, 3 and 4, there have been found of late years in several valleys in France and England—as, for example, in those of the Seine and Somme, and of the Thames and Ouse, near Bedford—stone implements of a rude type, showing that man coexisted in those districts with the mammoth and other extinct quadrupeds of the genera above enumerated. In 1847, M. Boucher de Perthes observed in an ancient alluvium at Abbeville, in Picardy, the bones of extinct mammalia associated in such a manner with flint implements of a rude type as to lead him to infer that both the organic remains and the works of art were referable to one and the same period. This inference was soon after confirmed by Mr. Prestwich, who found in 1859 a flint tool in situ in the same stratum at Amiens that contained the remains of extinct mammalia.
The flint implements found at Abbeville and Amiens are most of them considered to be hatchets and spear-heads, and are different from those commonly called “celts.” These celts, so often found in the recent formations, have a more regular oblong shape, the result of grinding, by which also a sharp edge has been given to them. The Abbeville tools found in gravel at different levels, as in Nos. 3 and 4, Fig. 87, in which bones of the elephant, rhinoceros, and other extinct mammalia occur, are always unground, having evidently been brought into their present form simply by the chipping off of fragments of flint by repeated blows, such as could be given by a stone hammer.
Some of them are oval, others of a spear-headed form, no two exactly alike, and yet the greater number of each kind are obviously fashioned after the same general pattern. Their outer surface is often white, the original black flint having been discoloured and bleached by exposure to the air, or by the action of acids, as they lay in the gravel. They are most commonly stained of the same ochreous colour as the flints of the gravel in which they are imbedded. Occasionally their antiquity is indicated not only by their colour but by superficial incrustations of carbonate of lime, or by dendrites formed of oxide of iron and manganese. The edges also of most of them are worn, sometimes by having been used as tools, or sometimes by having been rolled in the old river’s bed. They are met with not only in the lower-level gravels, as in No. 3, Fig. 87, but also in No. 4, or the higher gravels, as at St. Acheul, in the suburbs of Amiens, where the old alluvium lies at an elevation of about 100 feet above the level of the river Somme. At both levels fluviatile and land-shells are met with in the loam as well as in the gravel, but there are no marine shells associated, except at Abbeville, in the lowest part of the gravel, near the sea, and a few feet only above the present high-water mark. Here with fossil shells of living species are mingled the bones of Elephas primigenius and E. antiquus, Rhinoceros tichorhinus, Hippopotamus, Felis spelæa, Hyæna spelæa, reindeer, and many others, the bones accompanying the flint implements in such a manner as to show that both were buried in the old alluvium at the same period.
Nearly the entire skeleton of a rhinoceros was found at one point, namely, in the Menchecourt drift at Abbeville, the bones being in such juxtaposition as to show that the cartilage must have held them together at the time of their inhumation.
The general absence here and elsewhere of human bones from gravel and sand in which flint tools are discovered, may in some degree be due to the present limited extent of our researches. But it may also be presumed that when a hunter population, always scanty in numbers, ranged over this region, they were too wary to allow themselves to be overtaken by the floods which swept away many herbivorous animals from the low river-plains where they may have been pasturing or sleeping. Beasts of prey prowling about the same alluvial flats in search of food may also have been surprised more readily than the human tenant of the same region, to whom the signs of a coming tempest were better known.
Inundation-mud of Rivers.—Brick-earth.—Fluviatile Loam, or Loess.—As a general rule, the fluviatile alluvia of different ages (Nos. 2, 3, 4, Fig. 87) are severally made up of coarse materials in their lower portions, and of fine silt or loam in their upper parts. For rivers are constantly shifting their position in the valley-plain, encroaching gradually on one bank, near which there is deep water, and deserting the other or opposite side, where the channel is growing shallower, being destined eventually to be converted into land. Where the current runs strongest, coarse gravel is swept along, and where its velocity is slackened, first sand, and then only the finest mud, is thrown down. A thin film of this fine sediment is spread, during floods, over a wide area, on one, or sometimes on both sides, of the main stream, often reaching as far as the base of the bluffs or higher grounds which bound the valley. Of such a description are the well-known annual deposits of the Nile, to which Egypt owes its fertility. So thin are they, that the aggregate amount accumulated in a century is said rarely to exceed five inches, although in the course of thousands of years it has attained a vast thickness, the bottom not having been reached by borings extending to a depth of 60 feet towards the central parts of the valley. Everywhere it consists of the same homogeneous mud, destitute of stratification—the only signs of successive accumulation being where the Nile has silted up its channel, or where the blown sands of the Libyan desert have invaded the plain, and give rise to alternate layers of sand and mud.
In European river-loams we occasionally observe isolated pebbles and angular pieces of stone which have been floated by ice to the places where they now occur; but no such coarse materials are met with in the plains of Egypt.
In some parts of the valley of the Rhine the accumulation of similar loam, called in Germany “loess,” has taken place on an enormous scale. Its colour is yellowish-grey, and very homogeneous; and Professor Bischoff has ascertained, by analysis, that it agrees in composition with the mud of the Nile. Although for the most part unstratified, it betrays in some places marks of stratification, especially where it contains calcareous concretions, or in its lower part where it rests on subjacent gravel and sand which alternate with each other near the junction. About a sixth part of the whole mass is composed of carbonate of lime, and there is usually an intermixture of fine quartzose and micaceous sand.
Although this loam of the Rhine is unsolidified, it usually terminates where it has been undermined by running water in a vertical cliff, from the face of which shells of terrestrial, fresh-water and amphibious mollusks project in relief. These shells do not imply the permanent sojourn of a body of fresh water on the spot, for the most aquatic of them, the Succinea, inhabits marshes and wet grassy meadows. The Succinea elongata (or S. oblongata), Fig. 88, is very characteristic both of the loess of the Rhine and of some other European river-loams.
Fig. 88: Succinea elongata; Fig. 89: Pupa muscorum (Linn.); Fig. 90: Helix hispida (Linn.) (plebia).
Among the land-shells of the Rhenish loess, Helix hispida, Fig. 90, and Pupa muscorum, Fig. 89, are very common. Both the terrestrial and aquatic shells are of most fragile and delicate structure, and yet they are almost invariably perfect and uninjured. They must have been broken to pieces had they been swept along by a violent inundation. Even the colour of some of the land-shells, as that of Helix nemoralis, is occasionally preserved.
In parts of the valley of the Rhine, between Bingen and Basle, the fluviatile loam or loess now under consideration is several hundred feet thick, and contains here and there throughout that thickness land and amphibious shells. As it is seen in masses fringing both sides of the great plain, and as occasionally remnants of it occur in the centre of the valley, forming hills several hundred feet in height, it seems necessary to suppose, first, a time when it slowly accumulated; and secondly, a later period, when large portions of it were removed, or when the original valley, which had been partially filled up with it, was re-excavated.
Such changes may have been brought about by a great movement of oscillation, consisting first of a general depression of the land, and then of a gradual re-elevation of the same. The amount of continental depression which first took place in the interior, must be imagined to have exceeded that of the region near the sea, in which case the higher part of the great valley would have its alluvial plain gradually raised by an accumulation of sediment, which would only cease when the subsidence of the land was at an end. If the direction of the movement was then reversed, and, during the re-elevation of the continent, the inland region nearest the mountains should rise more rapidly than that near the coast, the river would acquire a denuding power sufficient to enable it to sweep away gradually nearly all the loam and gravel with which parts of its basin had been filled up. Terraces and hillocks of mud and sand would then alone remain to attest the various levels at which the river had thrown down and afterwards removed alluvial matter.
Cavern Deposits containing Human Remains and Bones of Extinct Animals.—In England, and in almost all countries where limestone rocks abound, caverns are found, usually consisting of cavities of large dimensions, connected together by low, narrow, and sometimes torturous galleries or tunnels. These subterranean vaults are usually filled in part with mud, pebbles, and breccia, in which bones occur belonging to the same assemblage of animals as those characterising the Post-pliocene alluvia above described. Some of these bones are referable to extinct and others to living species, and they are occasionally intermingled, as in the valley-gravels, with implements of one or other of the great divisions of the stone age, and these are not unfrequently accompanied by human bones, which are much more common in cavern deposits than in valley-alluvium.
Each suite of caverns, and the passages by which they communicate the one with the other, afford memorials to the geologist of successive phases through which they must have passed. First, there was a period when the carbonate of lime was carried out gradually by springs; secondly, an era when engulfed rivers or occasional floods swept organic and inorganic debris into the subterranean hollows previously formed; and thirdly, there were such changes in the configuration of the region as caused the engulfed rivers to be turned into new channels, and springs to be dried up, after which the cave-mud, breccia, gravel, and fossil bones would bear the same kind of relation to the existing drainage of the country as the older valley-drifts with their extinct mammalian remains and works of art bear to the present rivers and alluvial plains.
The quarrying away of large masses of Carboniferous and Devonian limestone, near Liege, in Belgium, has afforded the geologist magnificent sections of some of these caverns, and the former communication of cavities in the interior of the rocks with the old surface of the country by means of vertical or oblique fissures, has been demonstrated in places where it would not otherwise have been suspected, so completely have the upper extremities of these fissures been concealed by superficial drift, while their lower ends, which extended into the roofs of the caves, are masked by stalactitic incrustations.
The origin of the stalactite is thus explained by the eminent chemist Liebig. Mould or humus, being acted on by moisture and air, evolves carbonic acid, which is dissolved by rain. The rain-water, thus impregnated, permeates the porous limestone, dissolves a portion of it, and afterwards, when the excess of carbonic acid evaporates in the caverns, parts with the calcareous matter, and forms stalactite. Even while caverns are still liable to be occasionally flooded such calcareous incrustations accumulate, but it is generally when they are no longer in the line of drainage that a solid floor of hard stalagmite is formed on the bottom.
The late Dr. Schmerling examined forty caves near Liege, and found in all of them the remains of the same fauna, comprising the mammoth, tichorhine rhinoceros, cave-bear, cave-hyæna, cave-lion, and many others, some of extinct and some of living species, and in all of them flint implements. In four or five caves only parts of human skeletons were met with, comprising sometimes skulls with a few other bones, sometimes nearly every part of the skeleton except the skull. In one of the caves, that of Engihoul, where Schmerling had found the remains of at least three human individuals, they were mingled in such a manner with bones of extinct mammalia, as to leave no doubt on his mind (in 1833) of man having co-existed with them.
In 1860, Professor Malaise, of Liege, explored with me this same cave of Engihoul, and beneath a hard floor of stalagmite we found mud full of bones of extinct and recent animals, such as Schmerling had described, and my companion, persevering in his researches after I had returned to England, extracted from the same deposit two human lower jaw-bones retaining their teeth. The skulls from these Belgian caverns display no marked deviation from the normal European type of the present day.
The careful investigations carried on by Dr. Falconer, Mr. Pengelly, and others, in the Brixham cave near Torquay, in 1858, demonstrated that flint knives were there imbedded in such a manner in loam underlying a floor of stalagmite as to prove that man had been an inhabitant of that region when the cave-bear and other members of the ancient post-pliocene fauna were also in existence.
The absence of gnawed bones had led Dr. Schmerling to infer that none of the Belgian caves which he explored had served as the dens of wild beasts; but there are many caves in Germany and England which have certainly been so inhabited, especially by the extinct hyæna and bear.
A fine example of a hyæna’s den was afforded by the cave of Kirkdale, so well described by the late Dr. Buckland in his Reliquiæ Diluvianæ. In that cave, above twenty-five miles north-north-east of York, the remains of about 300 hyænas, belonging to individuals of every age, were detected. The species (Hyæna spelæa) has been considered by palæontologists as extinct; it was larger than the fierce Hyæna crocuta of South Africa, which it closely resembled, and of which it is regarded by Mr. Boyd Dawkins as a variety. Dr. Buckland, after carefully examining the spot, proved that the hyænas must have lived there; a fact attested by the quantity of their dung, which, as in the case of the living hyæna, is of nearly the same composition as bone, and almost as durable. In the cave were found the remains of the ox, young elephant, hippopotamus, rhinoceros, horse, bear, wolf, hare, water-rat, and several birds. All the bones have the appearance of having been broken and gnawed by the teeth of the hyænas; and they occur confusedly mixed in loam or mud, or dispersed through a crust of stalagmite which covers it. In these and many other cases it is supposed that portions of herbivorous quadrupeds have been dragged into caverns by beasts of prey, and have served as their food—an opinion quite consistent with the known habits of the living hyæna.
Australian Cave-breccias.—Ossiferous breccias are not confined to Europe, but occur in all parts of the globe; and those discovered in fissures and caverns in Australia correspond closely in character with what has been called the bony breccia of the Mediterranean, in which the fragments of bone and rock are firmly bound together by a red ochreous cement.
Fig. 91: Part of a lower jaw of Macropus atlas.
Some of these caves were examined by the late Sir T. Mitchell in the Wellington Valley, about 210 miles west of Sidney, on the river Bell, one of the principal sources of the Macquarie, and on the Macquarie itself. The caverns often branch off in different directions through the rock, widening and contracting their dimensions, and the roofs and floors are covered with stalactite. The bones are often broken, but do not seem to be water-worn. In some places they lie imbedded in loose earth, but they are usually included in a breccia.
The remains belong to marsupial animals. Among the most abundant are those of the kangaroo, of which there are four species, while others belong to the genera Phascolomys, the wombat; Dasyurus), the ursine opossum; Phalangista, the vulpine opossum; and Hypsiprymnus, the kangaroo-rat.
Fig. 92: Lower jaw of largest living species of kangaroo.
In the fossils above enumerated, several species are larger than the largest living ones of the same genera now known in Australia. Fig. 91 of the right side of a lower jaw of a kangaroo (Macropus atlas, Owen) will at once be seen to exceed in magnitude the corresponding part of the largest living kangaroo, which is represented in Fig. 92. In both these specimens part of the substance of the jaw has been broken open, so as to show the permanent false molar (a, Fig. 91), concealed in the socket. From the fact of this molar not having been cut, we learn that the individual was young, and had not shed its first teeth.
The reader will observe that all these extinct quadrupeds of Australia belong to the marsupial family, or, in other words, that they are referable to the same peculiar type of organisation which now distinguishes the Australian mammalia from those of other parts of the globe. This fact is one of many pointing to a general law deducible from the fossil vertebrate and invertebrate animals of times immediately antecedent to our own, namely, that the present geographical distribution of organic forms dates back to a period anterior to the origin of existing species; in other words, the limitation of particular genera or families of quadrupeds, mollusca, etc., to certain existing provinces of land and sea, began before the larger part of the species now contemporary with man had been introduced into the earth.
Professor Owen, in his excellent “History of British Fossil Mammals,” has called attention to this law, remarking that the fossil quadrupeds of Europe and Asia differ from those of Australia or South America. We do not find, for example, in the Europæo-Asiatic province fossil kangaroos, or armadillos, but the elephant, rhinoceros, horse, bear, hyæna, beaver, hare, mole, and others, which still characterise the same continent.
In like manner, in the Pampas of South America the skeletons of Megatherium, Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct forms, are analogous to the living sloth, armadillo, cavy, capybara, and llama. The fossil quadrumana, also associated with some of these forms in the Brazilian caves, belong to the Platyrrhine family of monkeys, now peculiar to South America. That the extinct fauna of Buenos Ayres and Brazil was very modern has been shown by its relation to deposits of marine shells, agreeing with those now inhabiting the Atlantic.
The law of geographical relationship above alluded to, between the living vertebrata of every great zoological province and the fossils of the period immediately antecedent, even where the fossil species are extinct, is by no means confined to the mammalia. New Zealand, when first examined by Europeans, was found to contain no indigenous land quadrupeds, no kangaroos, or opossums, like Australia; but a wingless bird abounded there, the smallest living representative of the ostrich family, called the Kiwi by the natives (Apteryx). In the fossils of the Post-pliocene period in this same island, there is the like absence of kangaroos, opossums, wombats, and the rest; but in their place a prodigious number of well-preserved specimens of gigantic birds of the struthious order, called by Owen Dinornis and Palapteryx, which are entombed in superficial deposits. These genera comprehended many species, some of which were four, some seven, others nine, and others eleven feet in height! It seems doubtful whether any contemporary mammalia shared the land with this population of gigantic feathered bipeds.
Mr. Darwin, when describing the recent and fossil mammalia of South America, has dwelt much on the wonderful relationship of the extinct to the living types in that part of the world, inferring from such geographical phenomena that the existing species are all related to the extinct ones which preceded them by a bond of common descent.
Climate of the Post-pliocene Period.—The evidence as to the climate of Europe during this epoch is somewhat conflicting. The fluviatile and land-shells are all of existing species, but their geographical range has not always been the same as at present. Some, for example, which then lived in Britain are now only found in Norway and Finland, probably implying that the Post-pliocene climate of Britain was colder, especially in the winter. So also the reindeer and the musk-ox (Ovibos moschatus), now inhabitants of the Arctic regions, occur fossil in the valleys of the Thames and Avon, and also in France and Germany, accompanied in most places by the mammoth and the woolly rhinoceros. At Grays in Essex, on the other hand, another species both of elephant and rhinoceros occurs, together with a hippopotamus and the Cyrena fluminalis, a shell now extinct in Europe but still an inhabitant of the Nile and some Asiatic rivers. With it occurs the Unio littoralis, now living in the Seine and Loire. In the valley of the Somme flint tools have been found associated with Hippopotamus major and Cyrena fluminalis in the lower-level Post-pliocene gravels; while in the higher-level (and more ancient) gravels similar tools are more abundant, and are associated with the bones of the mammoth and other Post-pliocene quadrupeds indicative of a colder climate.
It is possible that we may here have evidence of summer and winter migrations rather than of a general change of temperature. Instead of imagining that the hippopotamus lived all the year round with the musk-ox and lemming, we may rather suppose that the apparently conflicting evidence may be due to the place of our observations being near the boundary line of a northern and southern fauna, either of which may have advanced or receded during comparatively slight and temporary fluctuations of climate. There may then have been a continuous land communication between England and the North of Siberia, as well as in an opposite direction with Africa, then united to Southern Europe.
In drift at Fisherton, near Salisbury, thirty feet above the river Wiley, the Greenland lemming and a new species of the Arctic genus Spermophilus have been found, along with the mammoth, reindeer, cave-hyæna, and other mammalia suited to a cold climate. A flint implement was taken out from beneath the bones of the mammoth. In a higher and older deposit in the vicinity, flint tools like those of Amiens have been discovered. Nearly all the known Post-pliocene quadrupeds have now been found accompanying flint knives or hatchets in such a way as to imply their coexistence with man; and we have thus the concurrent testimony of several classes of geological facts to the vast antiquity of the human race. In the first place, the disappearance of a great variety of species of wild animals from every part of a wide continent must have required a vast period for its accomplishment; yet this took place while man existed upon the earth, and was completed before that early period when the Danish shell-mounds were formed or the oldest of the Swiss lake-dwellings constructed. Secondly, the deepening and widening of valleys, indicated by the position of the river gravels at various heights, implies an amount of change of which that which has occurred during the historical period forms a scarcely perceptible part. Thirdly, the change in the course of rivers which once flowed through caves now removed from any line of drainage, and the formation of solid floors of stalagmite, must have required a great lapse of time. Lastly, ages must have been required to change the climate of wide regions to such an extent as completely to alter the geographical distribution of many mammalia as well as land and fresh-water shells. The 3000 or 4000 years of the historical period does not furnish us with any appreciable measure for calculating the number of centuries which would suffice for such a series of changes, which are by no means of a local character, but have operated over a considerable part of Europe.
Relative Longevity of Species in the Mammalia and Testacea.—I called attention in 1830[5] to the fact, which had not at that time attracted notice, that the association in the Post-pliocene deposits of shells, exclusively of living species, with many extinct quadrupeds betokened a longevity of species in the testacea far exceeding that in the mammalia. Subsequent researches seem to show that this greater duration of the same specific forms in the class mollusca is dependent on a still more general law, namely, that the lower the grade of animals, or the greater the simplicity of their structure, the more persistent are they in general in their specific characters throughout vast periods of time. Not only have the invertebrata, as shown by geological data, altered at a less rapid rate than the vertebrata, but if we take one of the classes of the former, as for example the mollusca, we find those of more simple structure to have varied at a slower rate than those of a higher and more complex organisation; the Brachiopoda, for example, more slowly than the lamellibranchiate bivalves, while the latter have been more persistent than the univalves, whether gasteropoda or cephalopoda. In like manner the specific identity of the characters of the foraminifera, which are among the lowest types of the invertebrata, has outlasted that of the mollusca in an equally decided manner.
Teeth of Post-pliocene Mammalia.—To those who have never studied comparative anatomy, it may seem scarcely credible that a single bone taken from any part of the skeleton may enable a skilful osteologist to distinguish, in many cases, the genus, and sometimes the species, of quadrupeds to which it belonged. Although few geologists can aspire to such knowledge, which must be the result of long practice and study, they will nevertheless derive great advantage from learning, what is comparatively an easy task, to distinguish the principal divisions of the mammalia by the forms and characters of their teeth.
Fig. 93: Elephas primigenius (or Mammoth) molar of upper jaw, right side. Post-pliocene; Fig. 94: Elephas antiquus, Falconer. Penultimate molar. Post-pliocene and Pliocene.
Figures 93 through 105 represent the teeth of some of the more common species and genera found in alluvial and cavern deposits.
Figs. 95 to 100: Teeth of extinct mammalia.
Figs. 101 to 105: Teeth of extinct mammalia.
On comparing the grinding surfaces of the corresponding molars of the three species of elephants, Figs. 93, 94, 95 it will be seen that the folds of enamel are most numerous in the mammoth, fewer and wider, or more open, in E. antiquus; and most open and fewest in E. meridionalis. It will be also seen that the enamel in the molar of the Rhinoceros tichorhinus (Fig. 97), is much thicker than in that of the Rhinoceros leptorhinus (Fig. 96).
[1] Sir John Lubbock, Pre-historic Times, p. 3, 1865.
[2] Bulletin de la Sociétié Vaudoise des Sci. Nat., tome vi, Lausanne 1860; and Antiquity of Man, by the author, chap. ii.
[3] Sir J. Lubbock’s Lecture, Royal Institution, Feb. 27th, 1863.
[4] Diodorus, v, 21, 22 and Sir H. James Note on Block of Tin dredged up in Falmouth Harbour. Royal Institution of Cornwall, 1863.
[5] Principles of Geology, 1st ed., vol. iii, p. 140.
XI. POST-PLIOCENE PERIOD, continued—GLACIAL CONDITIONS.[1]
Geographical Distribution, Form, and Characters of Glacial Drift. — Fundamental Rocks, polished, grooved, and scratched. — Abrading and striating Action of Glaciers. — Moraines, Erratic Blocks, and “Roches Moutonnees.” Alpine Blocks on the Jura. — Continental Ice of Greenland. — Ancient Centres of the Dispersion of Erratics. — Transportation of Drift by floating Icebergs. — Bed of the Sea furrowed and polished by the running aground of floating Ice-islands.
Character and Distribution of Glacial Drift.—In speaking of the loose transported matter commonly found on the surface of the land in all parts of the globe, I alluded to the exceptional character of what has been called the boulder formation in the temperate and Arctic latitudes of the northern hemisphere. The peculiarity of its form in Europe north of the 50th, and in North America north of the 40th parallel of latitude, is now universally attributed to the action of ice, and the difference of opinion respecting it is now chiefly restricted to the question whether land-ice or floating icebergs have played the chief part in its distribution. It is wanting in the warmer and equatorial regions, and reappears when we examine the lands which lie south of the 40th and 50th parallels in the southern hemisphere, as, for example, in Patagonia, Tierra del Fuego, and New Zealand. It consists of sand and clay, sometimes stratified, but often wholly devoid of stratification for a depth of 50, 100, or even a greater number of feet. To this unstratified form of the deposit the name of till has long been applied in Scotland. It generally contains a mixture of angular and rounded fragments of rock, some of large size, having occasionally one or more of their sides flattened and smoothed, or even highly polished. The smoothed surfaces usually exhibit many scratches parallel to each other, one set of which often crosses an older set. The till is almost everywhere wholly devoid of organic remains, except those washed into it from older formations, though in some places it contains marine shells, usually of northern or Arctic species, and frequently in a fragmentary state. The bulk of the till has usually been derived from the grinding down into mud of rocks in the immediate neighbourhood, so that it is red in a region of Red Sandstone, as in Strathmore in Forfarshire; grey or black in a district of coal and bituminous shale, as around Edinburgh; and white in a chalk country, as in parts of Norfolk and Denmark. The stony fragments dispersed irregularly through the till usually belong, especially in mountainous countries, to rocks found in some part of the same hydrographical basin; but there are regions where the whole of the boulder clay has come from a distance, and huge blocks, or “erratics,” as they have been called, many feet in diameter, have not unfrequently travelled hundreds of miles from their point of departure, or from the parent rocks from which they have evidently been detached. These are commonly angular, and have often one or more of their sides polished and furrowed.
The rock on which the boulder formation reposes, if it consists of granite, gneiss, marble, or other hard stone, capable of permanently retaining any superficial markings which may have been imprinted upon it, is usually smoothed or polished, like the erratics above described, and exhibits parallel striæ and furrows having a determinate direction. This direction, both in Europe and North America, agrees generally in a marked manner with the course taken by the erratic blocks in the same district. The boulder clay, when it was first studied, seemed in many of its characters so singular and anomalous, that geologists despaired of ever being able to interpret the phenomena by reference to causes now in action. In those exceptional cases where marine shells of the same date as the boulder clay were found, nearly all of them were recognised as living species—a fact conspiring with the superficial position of the drift to indicate a comparatively modern origin.
The term “diluvium” was for a time the most popular name of the boulder formation, because it was referred by many to the deluge of Noah, while others retained the name as expressive of their opinion that a series of diluvial waves raised by hurricanes and storms, or by earthquakes, or by the sudden upheaval of land from the bed of the sea, had swept over the continents, carrying with them vast masses of mud and heavy stones, and forcing these stones over rocky surfaces so as to polish and imprint upon them long furrows and striæ. But geologists were not long in seeing that the boulder formation was characteristic of high latitudes, and that on the whole the size and number of erratic blocks increases as we travel towards the Arctic regions. They could not fail to be struck with the contrast which the countries bordering the Baltic presented when compared with those surrounding the Mediterranean. The multitude of travelled blocks and striated rocks in the one region, and the absence of such appearances in the other, were too obvious to be overlooked. Even the great development of the boulder formation, with large erratics so far south as the Alps, offered an exception to the general rule favourable to the hypothesis that there was some intimate connection between it and accumulations of snow and ice.
Fig. 106: Limestone, polished, furrowed, and scratched by the glacier of Rosenlau in Switzerland.
Transporting and abrading Power of Glaciers.—I have described elsewhere (“Principles” vol. i, chap. xvi, 1867) the manner in which the snow of the Alpine heights is prevented from accumulating indefinitely in thickness by the constant descent of a large portion of it by gravitation. Becoming converted into ice it forms what are termed glaciers, which glide down the principal valleys. On their surface are seen mounds of rubbish or large heaps of sand and mud, with angular fragments of rock which fall from the steep slopes or precipices bounding the glaciers. When a glacier, thus laden, descends so far as to reach a region about 3500 feet above the level of the sea, the warmth of the air is such that it melts rapidly in summer, and all the mud, sand, and pieces of rock are slowly deposited at its lower end, forming a confused heap of unstratified rubbish called a moraine, and resembling the till before described (p. 166).
Besides the blocks thus carried down on the top of the glacier, many fall through fissures in the ice to the bottom, where some of them become firmly frozen into the mass, and are pushed along the base of the glacier, abrading, polishing, and grooving the rocky floor below, as a diamond cuts glass, or as emery-powder polishes steel. The striæ which are made, and the deep grooves which are scooped out by this action, are rectilinear and parallel to an extent never seen in those produced on loose stones or rocks, where shingle is hurried along by a torrent, or by the waves on a sea-beach. In addition to these polished, striated, and grooved surfaces of rock, another mark of the former action of a glacier is the “roche moutonnee.” Projecting eminences of rock so called have been smoothed and worn into the shape of flattened domes by the glacier as it passed over them. They have been traced in the Alps to great heights above the present glaciers, and to great horizontal distances beyond them.
Alpine Blocks on the Jura.—The moraines, erratics, polished surfaces, domes, and striæ, above described, are observed in the great valley of Switzerland, fifty miles broad; and almost everywhere on the Jura, a chain which lies to the north of this valley. The average height of the Jura is about one-third that of the Alps, and it is now entirely destitute of glaciers; yet it presents almost everywhere similar moraines, and the same polished and grooved surfaces. The erratics, moreover, which cover it, present a phenomenon which has astonished and perplexed the geologist for more than half a century. No conclusion can be more incontestable than that these angular blocks of granite, gneiss, and other crystalline formations came from the Alps, and that they have been brought for a distance of fifty miles and upward across one of the widest and deepest valleys in the world; so that they are now lodged on a chain composed of limestone and other formations, altogether distinct from those of the Alps. Their great size and angularity, after a journey of so many leagues, has justly excited wonder; for hundreds of them are as large as cottages; and one in particular, composed of gneiss, celebrated under the name of Pierre à Bot, rests on the side of a hill about 900 feet above the lake of Neufchâtel, and is no less than 40 feet in diameter.
In the year 1821, M. Venetz first announced his opinion that the Alpine glaciers must formerly have extended far beyond their present limits, and the proofs appealed to by him in confirmation of this doctrine were acknowledged by all subsequent observers, and greatly strengthened by new observations and arguments. M. Charpentier supposed that when the glaciers extended continuously from the Alps to the Jura, the former mountains were 2000 or 3000 feet higher than at present. Other writers, on the contrary, conjectured that the whole country had been submerged, and the moraines and erratic blocks transported on floating icebergs; but a careful study of the distribution of the travelled masses, and the total absence of marine shells from the old glacial drift of Switzerland, have entirely disproved this last hypothesis. In addition to the many evidences of the action of ice in the northern parts of Europe which we have already mentioned, there occur here and there in some of these countries, what are wanting in Switzerland, deposits of marine fossil shells, which exhibit so arctic a character that they must have led the geologist to infer the former prevalence of a much colder climate, even had he not encountered so many accompanying signs of ice-action. The same marine shells demonstrate the submergence of large areas in Scandinavia and the British Isles, during the glacial cold.
A characteristic feature of the deposits under consideration in all these countries is the occurrence of large erratic blocks, and sometimes of moraine matter, in situations remote from lofty mountains, and separated from the nearest points where the parent rocks appear at the surface by great intervening valleys, or arms of the sea. We also often observe striæ and furrows, as in Norway, Sweden, and Scotland, which deviate from the direction which they ought to follow if they had been connected with the present line of drainage, and they, therefore, imply the prevalence of a very distinct condition of things at the time when the cold was most intense. The actual state of North Greenland seems to afford the best explanation of such abnormal glacial markings.
Greenland Continental Ice.—Greenland is a vast unexplored continent, buried under one continuous and colossal mass of ice that is always moving seaward, a very small part of it in an easterly direction, and all the rest westward, or towards Baffin’s Bay. All the minor ridges and valleys are levelled and concealed under a general covering of snow, but here and there some steep mountains protrude abruptly from the icy slope, and a few superficial lines of stones or moraines are visible at certain seasons, when no snow has fallen for many months, and when evaporation, promoted by the wind and sun, has caused much of the upper snow to disappear. The height of this continent is unknown, but it must be very great, as the most elevated lands of the outskirts, which are described as comparatively low, attain altitudes of 4000 to 6000 feet. The icy slope gradually lowers itself towards the outskirts, and then terminates abruptly in a mass about 2000 feet in thickness, the great discharge of ice taking place through certain large friths, which, at their upper ends, are usually about four miles across. Down these friths the ice is protruded in huge masses, several miles wide, which continue their course—grating along the rocky bottom like ordinary glaciers long after they have reached the salt water. When at last they arrive at parts of Baffin’s Bay deep enough to buoy up icebergs from 1000 to 1500 feet in vertical thickness, broken masses of them float off, carrying with them on their surface not only fine mud and sand but large stones. These fragments of rock are often polished and scored on one or more sides, and as the ice melts, they drop down to the bottom of the sea, where large quantities of mud are deposited, and this muddy bottom is inhabited by many mollusca.
Although the direction of the ice-streams in Greenland may coincide in the main with that which separate glaciers would take if there were no more ice than there is now in the Swiss Alps, yet the striation of the surface of the rocks on an ice-clad continent would, on the whole, vary considerably in its minor details from that which would be imprinted on rocks constituting a region of separate glaciers. For where there is a universal covering of ice there will be a general outward movement from the higher and more central regions towards the circumference and lower country, and this movement will be, to a certain extent, independent of the minor inequalities of hill and valley, when these are all reduced to one level by the snow. The moving ice may sometimes cross even at right angles deep narrow ravines, or the crests of buried ridges, on which last it may afterwards seem strange to detect glacial striæ and polishing after the liquefaction of the snow and ice has taken place.
Rink mentions that in North Greenland powerful springs of clayey water escape in winter from under the ice, where it descends to “the outskirts,” and where, as already stated, it is often 2000 feet thick—a fact showing how much grinding action is going on upon the surface of the subjacent rocks. I also learn from Dr. Torell that there are large areas in the outskirts, now no longer covered with permanent snow or glaciers, which exhibit on their surface unmistakable signs of ancient ice-action, so that, vast as is the power now exerted by ice in Greenland, it must once have operated on a still grander scale. The land, though now very elevated, may perhaps have been formerly much higher. It is well-known that the south coast of Greenland, from latitude 60° to about 70° N., has for the last four centuries been sinking at the rate of several feet in a century. By this means a surface of rock, well scored and polished by ice, is now slowly subsiding beneath the sea, and is becoming strewed over, as the icebergs melt, with impalpable mud and smoothed and scratched stones. It is not precisely known how far north this downward movement extends.
Drift carried by Icebergs.—An account was given so long ago as the year 1822, by Scoresby, of icebergs seen by him in the Arctic seas drifting along in latitudes 69° and 70° N., which rose above the surface from 100 to 200 feet, and some of which measured a mile in circumference. Many of them were loaded with beds of earth and rock, of such thickness that the weight was conjectured to be from 50,000 to 100,000 tons. A similar transportation of rocks is known to be in progress in the southern hemisphere, where boulders included in ice are far more frequent than in the north. One of these icebergs was encountered in 1839, in mid-ocean, in the antarctic regions, many hundred miles from any known land, sailing northward, with a large erratic block firmly frozen into it. Many of them, carefully measured by the officers of the French exploring expedition of the Astrolabe, were between 100 and 225 feet high above water, and from two to five miles in length. Captain d’Urville ascertained one of them which he saw floating in the Southern Ocean to be 13 miles long and 100 feet high, with walls perfectly vertical. The submerged portions of such islands must, according to the weight of ice relatively to sea-water, be from six to eight times more considerable than the part which is visible, so that when they are once fairly set in motion, the mechanical force which they might exert against any obstacle standing in their way would be prodigious.
We learn, therefore, from a study both of the arctic and antarctic regions, that a great extent of land may be entirely covered throughout the whole year by snow and ice, from the summits of the loftiest mountains to the sea-coast, and may yet send down angular erratics to the ocean. We may also conclude that such land will become in the course of ages almost everywhere scored and polished like the rocks which underlie a glacier. The discharge of ice into the surrounding sea will take place principally through the main valleys, although these are hidden from our sight. Erratic blocks and moraine matter will be dispersed somewhat irregularly after reaching the sea, for not only will prevailing winds and marine currents govern the distribution of the drift, but the shape of the submerged area will have its influence; inasmuch as floating ice, laden with stones, will pass freely through deep water, while it will run a ground where there are reefs and shallows. Some icebergs in Baffin’s Bay have been seen stranded on a bottom 1000 or even 1500 feet deep. In the course of ages such a sea-bed may become densely covered with transported matter, from which some of the adjoining greater depths may be free. If, as in West Greenland, the land is slowly sinking, a large extent of the bottom of the ocean will consist of rock polished and striated by land-ice, and then overspread by mud and boulders detached from melting bergs.
The mud, sand, and boulders thus let fall in still water must be exactly like the moraines of terrestrial glaciers, devoid of stratification and organic remains. But occasionally, on the outer side of such packs of stranded bergs, the waves and currents may cause the detached earthy and stony materials to be sorted according to size and weight before they reach the bottom, and to acquire a stratified arrangement.
I have already alluded (p. 172) to the large quantity of ice, containing great blocks of stone, which is sometimes seen floating far from land, in the southern or Antarctic seas. After the emergence, therefore, of such a submarine area, the superficial detritus will have no necessary relation to the hills, valleys, and river-plains over which it will be scattered. Many a water-shed may intervene between the starting-point of each erratic or pebble and its final resting-place, and the only means of discovering the country from which it took its departure will consist in a careful comparison of its mineral or fossil contents with those of the parent rocks.
[1] As to the former excess of cold, whether brought about by modifications in the height and distribution of the land or by altered astronomical conditions, see Principles, vol. i, (10th ed., 1867), chaps. xii and xiii, “Vicissitudes of Climate.”
XII. POST-PLIOCENE PERIOD, continued.—GLACIAL CONDITIONS, concluded
Glaciation of Scandinavia and Russia. — Glaciation of Scotland. — Mammoth in Scotch Till. — Marine Shells in Scotch Glacial Drift. — Their Arctic Character. — Rarity of Organic Remains in Glacial Deposits. — Contorted Strata in Drift. — Glaciation of Wales, England, and Ireland. — Marine Shells of Moel Tryfaen. — Erratics near Chichester. — Glacial Formations of North America. — Many Species of Testacea and Quadrupeds survived the Glacial Cold. — Connection of the Predominance of Lakes with Glacial Action. — Action of Ice in preventing the silting up of Lake-basins. — Absence of Lakes in the Caucasus. — Equatorial Lakes of Africa.
Glaciation of Scandinavia and Russia.—In large tracts of Norway and Sweden, where there have been no glaciers in historical times, the signs of ice-action have been traced as high as 6000 feet above the level of the sea. These signs consist chiefly of polished and furrowed rock-surfaces, of moraines and erratic blocks. The direction of the erratics, like that of the furrows, has usually been conformable to the course of the principal valleys; but the lines of both sometimes radiate outward in all directions from the highest land, in a manner which is only explicable by the hypothesis above alluded to of a general envelope of continental ice, like that of Greenland (page 170). Some of the far-transported blocks have been carried from the central parts of Scandinavia towards the Polar regions; others southward to Denmark; some south-westward, to the coast of Norfolk in England; others south-eastward, to Germany, Poland, and Russia.
In the immediate neighbourhood of Upsala, in Sweden, I had observed, in 1834, a ridge of stratified sand and gravel, in the midst of which occurs a layer of marl, evidently formed originally at the bottom of the Baltic, by the slow growth of the mussel, cockle, and other marine shells of living species, intermixed with some proper to fresh water. The marine shells are all of dwarfish size, like those now inhabiting the brackish waters of the Baltic; and the marl, in which many of them are imbedded, is now raised more than 100 feet above the level of the Gulf of Bothnia. Upon the top of this ridge repose several huge erratics, consisting of gneiss for the most part unrounded, from nine to sixteen feet in diameter, and which must have been brought into their present position since the time when the neighbouring gulf was already characterised by its peculiar fauna. Here, therefore, we have proof that the transport of erratics continued to take place, not merely when the sea was inhabited by the existing testacea, but when the north of Europe had already assumed that remarkable feature of its physical geography which separates the Baltic from the North Sea, and causes the Gulf of Bothnia to have only one-fourth of the saltness belonging to the ocean. In Denmark, also, recent shells have been found in stratified beds, closely associated with the boulder clay.
Glaciation of Scotland.—Mr. T. F. Jamieson, in 1858, adduced a great body of facts to prove that the Grampians once sent down glaciers from the central regions in all directions towards the sea. “The glacial grooves,” he observed, “radiate outward from the central heights towards all points of the compass, though they do not always strictly conform to the actual shape and contour of the minor valleys and ridges.”
These facts and other characteristics of the Scotch drift lead us to the inference that when the glacial cold first set in, Scotland stood higher above the sea than at present, and was covered for the most part with snow and ice, as Greenland is now. This sheet of land-ice sliding down to lower levels, ground down and polished the subjacent rocks, sweeping off nearly all superficial deposits of older date, and leaving only till and boulders in their place. To this continental state succeeded a period of depression and partial submergence. The sea advanced over the lower lands, and Scotland was converted into an archipelago, some marine sand with shells being spread over the bottom of the sea. On this sand a great mass of boulder clay usually quite devoid of fossils was accumulated. Lastly, the land re-emerged from the water, and, reaching a level somewhat above its present height, became connected with the continent of Europe, glaciers being formed once more in the higher regions, though the ice probably never regained its former extension.[1] After all these changes, there were some minor oscillations in the level of the land, on which, although they have had important geographical consequences, separating Ireland from England, for example, and England from the Continent, we need not here enlarge.
Mammoth in Scotch Till.—Almost all remains of the terrestrial fauna of the Continent which preceded the period of submergence have been lost; but a few patches of estuarine and fresh-water formations escaped denudation by submergence. To these belong the peaty clay from which several mammoths’ tusks and horns of reindeer were obtained at Kilmaurs, in Ayrshire as long ago as 1816. Mr. Bryce in 1865 ascertained that the fresh-water formation containing these fossils rests on carboniferous sandstone, and is covered, first by a bed of marine sand with arctic shells, and then with a great mass of till with glaciated boulders.[2] Still more recent explorations in the neighbourhood of Kilmaurs have shown that the fresh-water formation contains the seed of the pond-weed Potamogeton and the aquatic Ranunculus; and Mr. Young of the Glasgow Museum washed the mud adhering to the reindeer horns of Kilmaurs and that which filled the cracks of the associated elephants’ tusks, and detected in these fossils (which had been in the Glasgow Museum for half a century) abundance of the same seeds.
All doubts, therefore, as to the true position of the remains of the mammoth, a fossil so rare in Scotland, have been set at rest, and it serves to prove that part of the ancient continent sank beneath the sea at a period of great cold, as the shells of the overlying sand attest. The incumbent till or boulder clay is about 40 feet thick, but it often attains much greater thickness in the same part of Scotland.
Figs. 107-112: Northern shells common in the drift of the Clyde, in Scotland.
Marine Shells of Scotch Drift.—The greatest height to which marine shells have yet been traced in this boulder clay is at Airdie, in Lanarkshire, ten miles east of Glasgow, 524 feet above the level of the sea. At that spot they were found imbedded in stratified clays with till above and below them. There appears no doubt that the overlying deposit was true glacial till, as some boulders of granite were observed in it, which must have come from distances of sixty miles at the least.
Fig. 113: Leda truncata; Fig. 114: Tellina calcarea, Chem.
The shells figured in Figs. 107 to 112 are only a few out of a large assemblage of living species, which, taken as a whole, bear testimony to conditions far more arctic than those now prevailing in the Scottish seas. But a group of marine shells, indicating a still greater excess of cold, has been brought to light since 1860 by the Reverend Thomas Brown, from glacial drift or clay on the borders of the estuaries of the Forth and Tay. This clay occurs at Elie, in Fife, and at Errol, in Perthshire; and has already afforded about 35 shells, all of living species, and now inhabitants of arctic regions, such as Leda truncata, Tellina proxima (see Figs. 113 and 114), Pecten Grœnlandicus, Crenella lævigata, Crenella nigra, and others, some of them first brought by Captain Sir E. Parry from the coast of Melville Island, latitude 76° N. These were all identified in 1863 by Dr. Torell, who had just returned from a survey of the seas around Spitzbergen, where he had collected no less than 150 species of mollusca, living chiefly on a bottom of fine mud derived from the moraines of melting glaciers which there protrude into the sea. He informed me that the fossil fauna of this Scotch glacial deposit exhibits not only the species but also the peculiar varieties of mollusca now characteristic of very high latitudes. Their large size implies that they formerly enjoyed a colder, or, what was to them a more genial climate, than that now prevailing in the latitude where the fossils occur. Marine shells have also been found in the glacial drift of Caithness and Aberdeenshire at heights of 250 feet, and in Banff of 350 feet, and stratified drift continuous with the above ascends to heights of 500 feet. Already 75 species are enumerated from Caithness, and the same number from Aberdeenshire and Banff, and in both cases all but six are arctic species.
I formerly suggested that the absence of all signs of organic life in the Scotch drift might be connected with the severity of the cold, and also in some places with the depth of the sea during the period of extreme submergence; but my faith in such an hypothesis has been shaken by modern investigations, an exuberance of life having been observed both in arctic and antarctic seas of great depth, and where floating ice abounds. The difficulty, moreover, of accounting for the entire dearth of marine shells in till is removed when once we have adopted the theory of this boulder clay being the product of land-ice. For glaciers coming down from a continental ice-sheet like that which covers Greenland may fill friths many hundred feet below the sea-level, and even invade parts of a bay a thousand feet deep, before they find water enough to float off their terminal portions in the form of icebergs. In such a case till without marine shells may first accumulate, and then, if the climate becomes warmer and the ice melts, a marine deposit may be superimposed on the till without any change of level being required.
Another curious phenomenon bearing on this subject was styled by the late Hugh Miller the “striated pavements” of the boulder clay. Where portions of the till have been removed by the sea on the shores of the Forth, or in the interior by railway cuttings, the boulders imbedded in what remains of the drift are seen to have been all subjected to a process of abrasion and striation, the striæ and furrows being parallel and persistent across them all, exactly as if a glacier or iceberg had passed over them and scored them in a manner similar to that so often undergone by the solid rocks below the glacial drift. It is possible, as Mr. Geikie conjectures, that this second striation of the boulders may be referable to floating ice.[3]
Contorted Strata in Drift.—In Scotland the till is often covered with stratified gravel, sand, and clay, the beds of which are sometimes horizontal and sometimes contorted for a thickness of several feet. Such contortions are not uncommon in Forfarshire, where I observed them, among other places, in a vertical cutting made in 1840 near the left bank of the South Esk, east of the bridge of Cortachie. The convolutions of the beds of fine and coarse sand, gravel, and loam, extend through a thickness of no less than 25 feet vertical, or from b to c, Fig. 115, the horizontal stratification being resumed very abruptly at a short distance, as to the right of f, g. The overlying coarse gravel and sand, a, is in some places horizontal, in others it exhibits cross bedding, and does not partake of the disturbances which the strata b, c, have undergone. The underlying till is exposed for a depth of about 20 feet; and we may infer from sections in the neighbourhood that it is considerably thicker.
Fig. 15: Section of contorted drift overlying till, seen on left bank of South Esk, near Cortachie, in 1840.
In some cases I have seen fragments of stratified clays and sands, bent in like manner, in the middle of a great mass of till. Mr. Trimmer has suggested, in explanation of such phenomena, the intercalation in the glacial period of large irregular masses of snow or ice between layers of sand and gravel. Some of the cliffs near Behring’s Straits, in which the remains of elephants occur, consist of ice mixed with mud and stones; and Middendorf describes the occurrence in Siberia of masses of ice, found at various depths from the surface after digging through drift. Whenever the intercalation of snow and ice with drift, whether stratified or unstratified, has taken place, the melting of the ice will cause such a failure of support as may give rise to flexures, and sometimes to the most complicated foldings. But in many cases the strata may have been bent and deranged by the mechanical pressure of an advancing glacier, or by the sideway thrust of huge islands of ice running aground against sandbanks; in which case, the position of the beds forming the foundation of the banks may not be at all disturbed by the shock.
There are indeed many signs in Scotland of the action of floating ice, as might have been expected where proofs of submergence in the Glacial Period are not wanting. Among these are the occurrence of large erratic blocks, frequently in clusters at or near the tops of hills or ridges, places which may have formed islets or shallows in the sea where floating ice would mostly ground and discharge its cargo on melting. Glaciers or land-ice would, on the contrary, chiefly discharge their cargoes at the bottom of valleys. Traces of an earlier and independent glaciation have also been observed in some regions where the striation, apparently produced by ice proceeding from the north-west, is not explicable by the radiation of land-ice from a central mountainous region.[4]
Glaciation of Wales and England.—The mountains of North Wales were recognised, in 1842, by Dr. Buckland, as having been an independent centre of the dispersion of erratics—great glaciers, long since extinct, having radiated from the Snowdonian heights in Carnarvonshire, through seven principal valleys towards as many points of the compass, carrying with them large stony fragments, and grooving the subjacent rocks in as many directions.
Besides this evidence of land-glaciers, Mr. Trimmer had previously, in 1831, detected the signs of a great submergence in Wales in the Post-pliocene period. He had observed stratified drift, from which he obtained about a dozen species of marine shells, near the summit of Moel Tryfaen, a hill 1400 feet high, on the south side of the Menai Straits. I had an opportunity of examining in the summer of 1863, together with the Reverend W. S. Symonds, a long and deep cutting made through this drift by the Alexandra Mining Company in search of slates. At the top of the hill above-mentioned we saw a stratified mass of incoherent sand and gravel 35 feet thick, from which no less than 54 species of mollusca, besides three characteristic arctic varieties—in all 57 forms—have been obtained by Mr. Darbishire. They belong without exception to species still living in British or more northern seas; eleven of them being exclusively arctic, four common to the arctic and British seas, and a large proportion of the remainder having a northward range, or, if found at all in the southern seas of Britain, being comparatively less abundant. In the lowest beds of the drift were large heavy boulders of far-transported rocks, glacially polished and scratched on more than one side. Underneath the whole we saw the edges of vertical slates exposed to view, which here, like the rocks in other parts of Wales, both at greater and less elevations, exhibit beneath the drift unequivocal marks of prolonged glaciation. The whole deposit has much the appearance of an accumulation in shallow water or on a beach, and it probably acquired its thickness during the gradual subsidence of the coast—an hypothesis which would require us to ascribe to it a high antiquity, since we must allow time, first for its sinking, and then for its re-elevation.
The height reached by these fossil shells on Moel Tryfaen is no less than 1300 feet—a most important fact when we consider how very few instances we have on record beyond the limits of Wales, whether in Europe or North America, of marine shells having been found in glacial drift at half the height above indicated. A marine molluscous fauna, however, agreeing in character with that of Moel Tryfaen, and comprising as many species, has been found in drift at Macclesfield and other places in central England, sometimes reaching an elevation of 1200 feet.
Professor Ramsay[5] estimated the probable amount of submergence during some part of the glacial period at about 2300 feet; for he was unable to distinguish the superficial sands and gravel which reached that high elevation from the drift which, at Moel Tryfaen and at lower points, contains shells of living species. The evidence of the marine origin of the highest drift is no doubt inconclusive in the absence of shells, so great is the resemblance of the gravel and sand of a sea beach and of a river’s bed, when organic remains are wanting; but, on the other hand, when we consider the general rarity of shells in drift which we know to be of marine origin, we cannot suppose that, in the shelly sands of Moel Tryfaen, we have hit upon the exact uppermost limit of marine deposition, or, in other words, a precise measure of the submergence of the land beneath the sea since the glacial period.
We are gradually obtaining proofs of the larger part of England, north of a line drawn from the mouth of the Thames to the Bristol Channel, having been under the sea and traversed by floating ice since the commencement of the glacial epoch. Among recent observations illustrative of this point, I may allude to the discovery, by Mr. J. F. Bateman, near Blackpool, in Lancashire, fifty miles from the sea, and at the height of 568 feet above its level, of till containing rounded and angular stones and marine shells, such as Turritella communis, Purpura lapillus, Cardium edule, and others, among which Trophon clathratum (=Fusus Bamffius), though still surviving in North British seas, indicates a cold climate.
Erratics near Chichester.—The most southern memorials of ice-action and of a Post-pliocene fauna in Great Britain is on the coast of the county of Sussex, about 25 miles west of Brighton, and 15 south of Chichester. A marine deposit exposed between high and low tide occurs on both sides of the promontory called Selsea Bill, in which Mr. Godwin-Austen found thirty-eight species of shells, and the number has since been raised to seventy.
This assemblage is interesting because on the whole, while all the species are recent, they have a somewhat more southern aspect than those of the present British Channel. It is true that about forty of them range from British to high northern latitudes; but several of them, as, for example, Lutraria rugosa and Pecten polymorphous, which are abundant, are not known at present to range farther north than the coast of Portugal, and seem to indicate a warmer temperature than now prevails on the coast where we find them fossil. What renders this curious is the fact that the sandy loam in which they occur is overlaid by yellow clayey gravel with large erratic blocks which must have been drifted into their present position by ice when the climate had become much colder. These transported fragments of granite, syenite, and greenstone, as well as of Devonian and Silurian rocks, may have come from the coast of Normandy and Brittany, and are many of them of such large size that we must suppose them to have been drifted into their present site by coast-ice. I measured one of granite, at Pagham, 21 feet in circumference. In the gravel of this drift with erratics are a few littoral shells of living species, indicating an ancient coast-line.
Glacial Formations of North America.—In the western hemisphere, both in Canada and as far south as the 40th and even 38th parallel of latitude in the United States, we meet with a repetition of all the peculiarities which distinguish the European boulder formation. Fragments of rock have travelled for great distances, especially from north to south: the surface of the subjacent rock is smoothed, striated, and fluted; unstratified mud or till containing boulders is associated with strata of loam, sand, and clay, usually devoid of fossils. Where shells are present, they are of species still living in northern seas, and not a few of them identical with those belonging to European drift, including most of those already given in Figs. 107 to 112, p. 176. The fauna also of the glacial epoch in North America is less rich in species than that now inhabiting the adjacent sea, whether in the Gulf of St. Lawrence, or off the shores of Maine, or in the Bay of Massachusetts.
The extension on the American continent of the range of erratics during the Post-pliocene period to lower latitudes than they reached in Europe, agrees well with the present southward deflection of the isothermal lines, or rather the lines of equal winter temperature. It seems that formerly, as now, a more extreme climate and a more abundant supply of ice prevailed on the western side of the Atlantic. Another resemblance between the distribution of the drift fossils in Europe and North America has yet to be pointed out. In Canada and the United States, as in Europe, the marine shells are generally confined to very moderate elevations above the sea (between 100 and 700 feet), while the erratic blocks and the grooved and polished surfaces of rock extend to elevations of several thousand feet.
I have already mentioned that in Europe several quadrupeds of living, as well as extinct, species were common to pre-glacial and post-glacial times. In like manner there is reason to suppose that in North America much of the ancient mammalian fauna, together with nearly all the invertebrata, lived through the ages of intense cold. That in the United States the Mastodon giganteus was very abundant after the drift period, is evident from the fact that entire skeletons of this animal are met with in bogs and lacustrine deposits occupying hollows in the glacial drift. They sometimes occur in the bottom even of small ponds recently drained by the agriculturist for the sake of the shell-marl. In 1845 no less than six skeletons of the same species of Mastodon were found in Warren county, New Jersey, six feet below the surface, by a farmer who was digging out the rich mud from a small pond which he had drained. Five of these skeletons were lying together, and a large part of the bones crumbled to pieces as soon as they were exposed to the air.
It would be rash, however, to infer from such data that these quadrupeds were mired in modern times, unless we use that term strictly in a geological sense. I have shown that there is a fluviatile deposit in the valley of the Niagara, containing shells of the genera Melania, Lymnea, Planorbis, Velvata, Cyclaz, Unio, Helix, etc., all of recent species, from which the bones of the great Mastodon have been taken in a very perfect state. Yet the whole excavation of the ravine, for many miles below the Falls, has been slowly effected since that fluviatile deposit was thrown down. Other extinct animals accompany the Mastodon giganteus in the post-glacial deposits of the United States, and this, taken with the fact that so few of the mollusca, even of the commencement of the cold period, differ from species now living is important, as refuting the hypothesis, for which some have contended, that the intensity of the glacial cold annihilated all the species in temperate and arctic latitudes.
Connection of the Predominance of Lakes with Glacial Action.—It was first pointed out by Professor Ramsay in 1862, that lakes are exceedingly numerous in those countries where erratics, striated blocks, and other signs of ice-action abound; and that they are comparatively rare in tropical and sub-tropical regions. Generally in countries where the winter cold is intense, such as Canada, Scandinavia, and Finland, even the plains and lowlands are thickly strewn with innumerable ponds and small lakes, together with some others of a larger size; while in more temperate regions, such as Great Britain, Central and Southern Europe, the United States, and New Zealand, lake districts occur in all such mountainous tracts as can be proved to have been glaciated in times comparatively modern or since the geographical configuration of the surface bore a considerable resemblance to that now prevailing. In the same countries, beyond the glaciated regions, lakes abruptly cease, and in warmer and tropical countries are either entirely absent, or consist, as in equatorial Africa, of large sheets of water unaccompanied so far as we yet know by numerous smaller ponds and tarns.
The southern limits of the lake districts of the Northern Hemisphere are found at about 40° N. latitude on the American continent, and about 50° in Europe, or where the Alps intervene four degrees farther south. A large proportion of the smaller lakes are dammed up by barriers of unstratified drift, having the exact character of the moraines of glaciers, and are termed by geologists “morainic,” but some of them are true rock-basins, and would hold water even if all the loose drift now resting on their margins were removed.
In a paper read before the Geological Society of London in 1862, Professor Ramsay maintained that the first formation of most existing lakes took place during the glacial epoch, and was due, not to elevation or subsidence, but to actual erosion of their basins by glaciers. M. Mortillet in the same year advanced the theory that after the Alpine lake-basins had been filled up with loose fluviatile deposits, they were re-excavated by the great glaciers which passed down the valleys at the time of the greatest cold, a doctrine which would attribute to moving ice almost as great a capacity of erosion as that which assumed that the original basins were scooped out of solid rock by glaciers. It is impossible to deny that the mere geographical distribution of lakes points to the intimate connection of their origin with the abundance of ice during a former excess of cold, but how far the erosive action of moving ice has been the sole or even the principal cause of lake-basins, is a question still open to discussion.
The lakes of Switzerland and the north of Italy are some of them twenty and thirty miles in length, and so deep that their bottoms are in some cases from 1000 to 2000 feet beneath the level of the sea. It is admitted on all hands that they were once filled with ice, and as the existing glaciers polish and grind down, as before stated, the surface of the rocks, we are prepared to find that every lake-basin in countries once covered by ice should bear the marks of superficial glaciation, and also that the ice during its advance and retreat should have left behind it much transported matter as well as some evidence of its having enlarged the pre-existing cavity. But much more than this is demanded by the advocates of glacial erosion. They suggest that as the old extinct glaciers were several thousand feet thick, they were able in some places gradually to scoop out of the solid rock cavities twenty or thirty miles in length, and as in the case of Lago Maggiore from a thousand to two thousand six hundred feet below the previous level of the river-channel, and also that the ice had the power to remove from the cavity formed by its grinding action all the materials of the missing rocks. A constant supply, it is argued, of fine mud issues from the termination of every glacier in the stream which is produced by the melting of the ice, and this result of friction is exhibited both during winter and summer, affording evidence of the continual deepening and widening of the valleys through which glaciers pass. As the fine mud is carried away by a river from the deep pool which is formed from the base of every cataract, so it seems to be imagined that lake-basins may be gradually emptied of the mud formed by abrasion during the glacial period.
I am by no means disposed to object to this theory on the ground of the insufficiency of the time during which the extreme cold endured, but we must carefully consider whether that same time is not so vast as to make it probable that other forces, besides the motion of glaciers, must have cooperated in converting some parts of the ancient valley courses into lake-basins. They who have formed the most exalted conceptions of the erosive energy of moving ice do not deny that during the period termed “Glacial” there have been movements of the earth’s crust sufficient to produce oscillations of level in Europe amounting to 1000 feet or more in both directions. M. Charpentier, indeed, attributed some of the principal changes of climate in Switzerland, during the glacial period, to a depression of the central Alps to the extent of 3000 feet, and Swiss geologists have long been accustomed to attribute their lake basins, in part, to those convulsions by which the shape and course of the valleys may have been modified. Our experience, in the lifetime of the present generation, of the changes of level witnessed in New Zealand during great earthquakes is entirely opposed to the notion that the movements, whether upward or downward, are uniform in amount or direction throughout areas of indefinite extent. On the contrary, the land has been permanently raised in one region several feet or yards, and the rise has been found gradually to die out, so as to be imperceptible at a distance of twenty miles, and in some areas is even exchanged for a simultaneous downward movement of several feet.
But, it is asked, if such inequality of movement can have contributed towards the production of lake basins, does it not leave unexplained the comparative rarity of lakes in tropical and subtropical countries. In reply to this question it may be observed that in our endeavour to estimate the effects of subterranean movements in modifying the superficial geography of a country we must remember that each convulsion effects a very slight change. If it interferes with the drainage, whether by raising the lower or sinking the higher portion of a hydrographical basin, the upheaval or depression will only amount to a few feet at a time, and there may be an interval of years or centuries before any further movement takes place in the same region. In the mean time an incipient lake if produced may be filled up with sediment, and the recently-formed barrier will then be cut through by the river, whereas in a country where glacial conditions prevail no such obliteration of the temporary lake-basin would take place; for however deep it became by repeated sinking of the upper or rising of the lower extremity, being always filled with ice it might remain, throughout the greater part of its extent, free from sediment or drift until the ice melted at the close of the glacial period.
One of the most serious objections to the exclusive origin by ice-erosion of wide and deep lake-basins arises from their capricious distribution, as for example in Piedmont, both to the eastward and westward of Turin, where great lakes are wanting,[6] although some of the largest extinct glaciers descending from Mont Blanc and Monte Rosa came down from the Alps, leaving their gigantic moraines in the low country. Here, therefore, we might have expected to find lakes of the first magnitude rivalling the contiguous Lago Maggiore in importance.
A still more striking illustration of the same absence of lakes where large glaciers abound is afforded by the Caucasus, a chain more than 300 miles long, and the loftiest peaks of which attain heights from 16,000 to 18,000 feet. This greatest altitude is reached by Elbruz, a mountain in lat. 43° N. three degrees south of Mont Blanc, but on the other hand 3000 feet higher. The present Caucasian glaciers are equal or superior in dimensions to those of Switzerland, and like them give rise occasionally to temporary lakes by obstructing the course of rivers, and causing great floods when the icy barriers give way. Mr. Freshfield, a careful observer, writing in 1869, says:[7] “A total absence of lakes on both sides of the chains is the most marked feature. Not only are there no great subalpine sheets of water, like Como or Geneva, but mountain tarns, such as the Dauben See on the Gemmi, or the Klonthal See near Glarus, are equally wanting.” The same author states on the authority of the eminent Swiss geologist, Mons. E. Favre, who also explored the Caucasus in 1868, that moraines of great height and huge erratics of granite and other rocks “justify the assertion that the present glaciers of the Caucasus, like those of the Alps, are only the shadows of their former selves.”
It seems safe to assume that the chain of lakes, of which the Albert Nyanza forms one in equatorial Africa, was due to causes other than glacial. Yet if we could imagine a glacial period to visit that region filling the lakes with ice and scoring the rocks which form their sides and bottoms, we should be unable to decide how much the capacity of the basins had been enlarged and the surface modified by glacial erosion. The same may be true of the Lago Maggiore and Lake Superior, although the present basins of both of them afford abundant superficial markings due to ice-action.
But to whatever combination of causes we attribute the great Alpine lakes one thing is clear, namely, that they are, geologically speaking, of modern origin. Every one must admit that the upper valley of the Rhone has been chiefly caused by fluviatile denudation, and it is obvious that the quantity of matter removed from that valley previous to the glacial period would have been amply sufficient to fill up with sediment the basin of the Lake of Geneva, supposing it to have been in existence, even if its capacity had been many times greater than it is now.[8]
On the whole, it appears to me, in accordance with the views of Professor Ramsay, M. Mortillet, Mr. Geikie, and others, that the abrading action of ice has formed some mountain tarns and many morainic lakes; but when it is a question of the origin of larger and deeper lakes, like those of Switzerland or the north of Italy, or inland fresh-water seas, like those of Canada, it will probably be found that ice has played a subordinate part in comparison with those movements by which changes of level in the earth’s crust are gradually brought about.
[1] Jamieson, Quart. Geol. Journ., 1860, vol. xvi, p. 370.
[2] Bryce, Quart. Geol. Journ., vol. xxi, p. 217, 1865.
[3] Geikie, Trans. Geol. Soc. Glasgow, vol. i, part ii, p. 68, 1863.
[4] Milne Home, Trans. Royal Soc. Edinburgh, vol. xxv, 1868-9.
[5] Quart. Geol. Journ., 1852, vol. viii, p. 372.
[6] Antiquity of Man, p. 313.
[7] Travels in Central Caucasus, 1869, p. 452.
[8] See Principles, vol. i, p. 420, 10th ed., 1867.
TERTIARY OR CAINOZOIC PERIOD