Popular physical geology

Front matter

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POPULAR HISTORY

Preface

branch of Geology may hereafter be treated of by one who is a thorough master of that portion of the subject.

My excuse for putting forth an elementary work on Geology at all, must be, that there can hardly be too many elementary books which are the results, more or less com¬ pletely, of actual experience, and not the mere compilation and compression of other books. I recollect, when leaving school for college, being advised by an eminent scholar to read as many different grammars as possible. Not that it was supposed that any of them would contain any large amount of matter that was not to be found in all the rest, but that, by the different order and arrangement of the dif¬ ferent writers, the matter wTas looked at on all sides and in every light. The mere variations in the turns of expression, or the terms of description, as well as in the collocation of parts, may make one thing more intelligible to one person and another to another.

There is one feature, however, on which I may be allowed perhaps to speak rather more boldly ; and that is the illus-

PREFACE.

Vll tration of this little work. It was the expressed wish and desire of the publisher to have illustrations that should be of artistic merit as well as of accurate illustrative character. For this purpose I had recourse to the talents of my friend and colleague on the Irish Survey, Mr. G. Y. D unoyer, who unites the skill of the artist with the knowledge of the geologist. I think I may confidently appeal to the eyes of the Reader for the goodness of the result.

ERRATA .

Page 50, line \l,for carburet, read carbonate.

Page 63, line 2, for the mountain-like walls, read themountaiu, like walls. Page 94, Table, in analysis of Analcime, read, in Silica column, 5 5 '2.

Page 208, line 3 from bottom, “ many” should be at the beginning of the line instead of at the end.

Page 281, line 1 ,for inland, read inclined.

Page 301, line 2, for rage, read large.

DESCRIPTION OF THE PLATES.

Plates I., II.

I. Falls, on the River Exploits, Newfoundland (Frontispiece).

II. Buchan’s Island, on the River Exploits.

These two Plates taken together are intended to illustrate the sawing or grooving action of rivers in deepening their own beds. The present Falls are about a mile above the island, which is an isolated rock in the middle of the river ; in the intermediate space the river flows in a narrow ravine, of the same depth as the height of the island. It is remarkable that the Falls are now divided into two by an island, which, as they cut back the rocks and recede up the river, will probably be left standing as an isolated pillar, like Buchan’s Island. From my own sketches. See page 10.

Plate III.

Baginbun Head, on the coast of Wexford, was selected as a pictu¬ resque example of a worn and indented coast, evidently pro-

X

DESCRIPTION OF TIIE PLATES.

duced by the erosive action of the sea-breakers on rocks of unequal hardness, or placed in varying positions, so as to oppose an unequal resistance to their action. See page 15.

Plate IY.

Likewise illustrates the erosive action of the sea, a kind of ap¬ proximate date being afforded by the nearly entire destruction of a human work. The top of the cliff, in the centre of the picture, is occupied by the remains of a double entrenchment, which could never have been thrown up for the purpose of defending the few feet of ground now included between it and the vertical cliff. The rock is hard gritstone and slate, yet we see that the action of the breakers has destroyed and re¬ moved a considerable block of land thus composed, since the time when this old embankment was thrown up. It is called Doonsorske Path, — rath being the name for the common circular embankments so often seen in Ireland. It is at Carrigadda Bay, between Cork Harbour and Kinsale. See page 15.

Plate Y.

Is a composition by Mr. Dunoyer, from sketches and recollec¬ tions of the Mitchelstown caves (county Cork) to illustrate the formation of stalactites and stalagmites. See page 29.

Plate YI.

Sketch of a gravel-pit, to illustrate the way in which fine and

DESCRIPTION OF THE PLATES.

XI coarse gravel are interstratified, and the occasional regu¬ larity of the beds and the high angle at which these coarse materials are thrown down. The two latter characters were more than unusually prominent in the locality where the sketch was taken. See page 3G.

Plate VII.

A sketch of a cliff in the Old lied Sandstone at Ballyhack, county Wexford, on the east side of the Waterford estuary. It shows the usual characters of a sandstone and conglomerate, and, in conjunction with the last Plate, the varying manner in which sand and pebbles are accumulated. See pages 36 and 39.

Plate VIII.

Sketch of a quarry in the Oolite at Minchinhampton, in Glou¬ cestershire, drawn by Mr. Dunoyer, from a rough sketch of my own. It illustrates what is called “false bedding” or “oblique lamination,” the varying inclinations and direc¬ tions namely assumed by the minor layers in the beds or strata of some kinds of rock. See page 41.

Plate IX.

This very remarkable example was sketched by Mr. Dunoyer in a quarry near Mitchelstown, county Cork. The Mountain Limestone there contains layers of chert nodules. Its beds xii DESCRIPTION OF THE PLATES.

have been bent by disturbing forces into regular arches, sub¬ sequently to which movement they have been affected by “cleavage” (represented by the fine lines crossing the face of the rock at a high angle), and this cleavage has so acted on the chert nodules as slightly to alter their shape, and 'pro¬ duce them along its direction. See pages 51, 132, and 160.

Plate X.

Sketch of a cutting in the side of the Dublin and Drogheda railway, near Malahide ; it illustrates both the regular strati¬ fication of the Limestone, represented by the parallel lines which traverse the rocks at an angle inclining gently from the spectator ; and the regularity of the “ joints,” which are found in all rocks, and which in this case produce the square corners and angles which the outline of the cliff is seen to present. See page 54.

Plate XT.

Carnivan Head, county Wexford, shows the way in which re¬ gularly bedded rocks are often made up of alternating strata of different materials, — in this case conglomerate, sandstone, and shale. The hard sandstone beds are seen to project, like mouldings in relief, on the face of the cliff. See page 55.

Plate XII.

Is from a rough sketch of my own, taken in a ravine near Santa

DESCRIPTION OE THE PLATES.

Xlll

Cruz, Teneriffe. It illustrates the alternation of masses of rudely columnar basalt (old lava) with beds, which when fine-grained are singularly regular, and are composed of water- worn mate¬ rials, — pebbles and sand derived from volcanic rocks. In the drawing, both the upper bed of columnar basalt and the under one, the surface of which forms the floor of the ravine and passes under the base of the cliff, are represented as much too regularly columnar. In reality the columns were very rude and irregular for the most part. The interposed stratified material however was quite as regular as represented in the drawing. See page 64.

Plate XIII.

Is likewise from a rough sketch of my own, giving merely the outline and principal features of the little Island of St. Paul5 in the Indian Ocean, taken from the anchorage. The sketch given in the Admiralty Chart was taken by Mr. Evans from the south or left-hand corner of the drawing. It is a well-known whaling ground, and two whales were basking in the anchorage when we landed. See page 67.

Plates XIV., XV.

Are from sketches by Mr. Dunoyer on the Dundalk and Ennis¬ killen Pailway. They illustrate the way in which dykes and veins of igneous rock intrude among aqueous and stratified

XIV

DESCRIPTION OF TIIE PLATES.

rock. In this case the stratified rock is the Mountain Lime¬ stone.

In Plate XIV. the centre of the drawing is occupied by a broad vertical dyke of Greenstone, having in places a globular or spheroidal structure, which has filled up a chasm or rent formed in the Mountain Limestone beds. On the right-hand side of the picture is seen an oblique fault, or line of fracture, along which the thick-bedded limestones traversed by the dyke have slid down, so as to come opposite some much thinner-bedded rocks. This fracture was probably caused subsequently to the production of the dyke, or it would itself have had intruded into it some of the igneous rock.

In Plate XY. the three figures on the cliff to the left of the centre of the drawing, stand on a horizontal mass of Green¬ stone, which at first sight seems to be interstratified regularly wfitli the beds, but which on being traced to the right is seen to split into two, one part passing into a thin bifurcating vein, that shortly ends, while the other cuts up right across the beds, and ultimately disappears at the top of the cliff: how much higher it extended into the beds that once existed above, we have of course no means of ascertaining. The place of a few of these beds is seen to be occupied by a few feet of “ drift” gravel and sand, forming a soft, incoherent bed along the top of the cliff. See pages 62, 69, 75.

DESCRIPTION OP THE PLATES.

NV

Plate XVI.

Sketch of a small Granite Quarry in the county Carlow, showing the regularity and parallelism of the “ joints” traversing that rock, and the smoothness of the faces caused by those natural planes of division. See page 113.

Plate XVII.

Sketch of some rocks on the north side of Eingabella Pay, at the mouth of Cork Harbour. It illustrates the jointing and clea¬ vage of stratified rocks, and is described at page 131, which see.

Plate XVIII.

Sketch of some slate and gritstone rocks at Garrylucas Strand Old Head of Kinsale, county Cork. It illustrates the remark¬ ably contorted positions often seen in stratified rocks. Near the left hand of the picture is seen a small fault, bringing down the two thick bands of gritstone against some thin-bedded rocks. The cliffs are seen to be capped by a bed of drift sand and gravel, about five feet in thickness. See page 160, etc.

Plates XIX., XX.

These Plates are from two sketches by my late uncle, Mr. II. W. Jukes.

XVI

DESCRIPTION OF THE PLATES.

Plate XIX. represents Matlock, as seen from tlie escarpment of the Millstone Grit on the east of the Wirksworth road. The high ground on the left of the drawing is the Mountain Lime¬ stone, the beds of which dip to the east (or to the right of the spectator), and are cut through by the river Derwent, so as to form the High Tor and the cliffs in the centre of the drawing, just before they become buried by the soft beds of the Limestone Shale, which compose all the low ground and central valley. These soft beds are covered again by the hard rocks of the Millstone Grit, which form the crags from which the view is taken, and all the high ground on the right of the picture.

Plate XX. is a view taken in the wild scenery of Kinder Scout, in the north of Derbyshire. The Limestone Shale here lies in nearly horizontal beds, in which the deep valleys and ravines are scooped out, while the Millstone Grit, reposing on it, forms the rough crags of the mountain summits, and the tabular mass on the right of the drawing. In the course of two

copyings, the crags in the distance on the left have become transformed into trees or bushes, somewhat to the detriment of the true effect, as the tops of the hills are occupied entirely by peat-bogs and .crags of rock. See page 311.

POPULAR GEOLOGY.

?part 3L

ELEMENTARY FACTS AND PRINCIPLES.

Chapter I. How Sand, Gravel, Clay, And Mud Are Formed

I do not know that we can commence an elementary book on Geology in a better way than by asking the reader if he knows what sand is. Because any one who thoroughly understands the origin and nature of common sand, whether it be found in the gravel-pit, in the river-bed, or be “ the ribbed sea-sand” of the shore, has made no despicable com¬ mencement in the study of the science.

If the reader will examine a handful of sand by the aid of a lens, he will find that it is composed of grains or

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POPULAR- GEOLOGY.

minute irregularly-sliaped particles of a hard, shining, often semi-transparent, substance. These particles are, if not round, very much rounded, often having on the surface a rubbed appearance, as if they had been worn and ground against each other.

As to river or sea sand, it is obvious that this rubbing mast actually have taken place, because, as the moving water must frequently wash the sand about and roll it onwards in its course, the particles must be constantly exposed to friction against each other, or against whatever substance it may be that lies at the bottom of the water. It is clearly possible therefore that all river or sea sand may have been produced, or brought into the state of sand, by the action of the running or moving waters tearing away fragments of rock, breaking them up into constantly diminishing parti¬ cles, and, by perpetual friction and rolling, grinding those particles into small rounded grains.

If this mode of formation be true for all sand found now beneath or on the margin of any moving water, it is, d priori, highly probable that all sand whatever, even that of the wide deserts of Sahara, the sands of Arabia, or those of the centre of Australia, have been thus formed.

If we come to consider of it, indeed, there appears to be

ORIGIN OF SAND.

no other at all likely method by which sand could in any case be formed, unless it were originally created as sand, such as we now find it*.

Let it be taken for granted then, for the present, as the reader sees its great probability now, and will be quite con¬ vinced of its truth hereafter, that all sand was produced by the action of moving water on solid rock.

It is however by no means necessary to suppose that the water always detached the sand directly from the rock as sand, that is, in small grains. On the contrary, if we exa¬ mine the action of moving water now, whether we go to the rapids and cataracts of rivers or to the breakers of the sea battering against a rocky coast, we shall everywhere see large blocks of rock lying about, often but newly de¬ tached from their original site, with all their angles sharp

* In commencing the study of a science, it is often useful to the student to have every stumbling-block cleared out of his path, however little may be the likelihood that he should fall over it. If any one therefore chooses to start as a difficulty the supposition that sand may have been created as we now find it, I have only to remind him that, if so, ever since its creation the action of water must have been tending to wear and waste it away, and grind it into powder. Given only G000 years of this action, and there could have been no sand left, in any river or on any shore, unless the supply had been recruited by other larger pieces of rock being ground down into sand, which is precisely the origin we are claiming for all sand.

POPULAR GEOLOGY.

and the fractures fresh, the yet unhealed scar perhaps plainly visible in the cliff above. We should see also blocks having every gradation of form, from this newly broken angular fragment to smaller and smoother, well rounded boulders and pebbles, having every projecting angle ground off and all the surface worn as smooth as a billiard-ball. This has been effected by the frequent moving and rolling* of all these blocks one against the other on the pebble-beach or in the bed of the torrent, every roll removing some little corner, chipping off some little projection, each sepa¬ rated fragment being itself shortly smoothed and rolled into a pebble or shingle, and all the waste of this process being carried off by the moving water in the shape of sand.

We come then now to look upon, not only all sand as a waterworn material, but upon every pebble and every de¬ tached stone, of whatever shape and size, whether found in river, lake, or ocean, if it has at all a worn and rounded out-

* The power of water to move blocks of stone that are in it must not be measured by the power required to move them in air. Stone loses more than a third of its weight in water. Let any one standing up to his middle in water lift a large stone from the bottom, he will find that he can raise in the water a mass that, as soon as he attempts to lift it out of the water, is quite beyond his strength. A block of stone weighing 100 lbs. in the air will often only weigh 60 lbs. in the wTater.

ORIGIN OF PEBBLES.

O line, as having probably acquired that outline by the action of moving water, and as having been probably transported by that action from its parent site to the place where we now find it. But, as in the case of the sand, so in that of the pebbles and boulders — if all those found in rivers or on shores have been rounded by the action of moving water, it is, a priori, highly probable that all pebbles and boulders and round stones whatever, however high and dry they may now be on plains or hills or mountain-slopes, are in fact but waterworn fragments of older rock.

There are two mineral substances which enter more largely into the structure of all rocks than any other: these are Silica and Alumina. The most common form of silica is quartz, which is almost entirely pure silica. Bock-crystal is a common name for quartz in its crystalline form ; in this state it is quite transparent ; it however is often found in veins in the hard rocks as an opake milk-white stone, very hard and brittle. When quartz is coloured dull white or brown by the slight admixture of other substances, it is called flint. All non-crystalline quartz, and most rocks that are made of it, when broken by the hammer or in any other way, commonly split into squarish or cubical lumps, which, when acted on by moving water, soon get their corners

G

POPULAR GEOLOGY.

rounded off so as to be easily rolled or moved, either as large pebbles or as small round grains. It is partly for this reason, and partly on account of their superior hardness and unyieldingness to chemical or mechanical force, that the great majority of all pebbles and sand consist of quartz. If we re-examine with a lens our handful of sea-sand, we should find all the little glassy-looking or semi-transparent grains, and most of the opake ones, to be made of quartz, mingled perhaps with grains of a few other substances, and, in the case of sea-sand, with grains of broken shell or coral, or other sea creatures. Prom this connection between quartz and sand, it comes that all sandy or are?iaceous rocks are more or less purely quartzose.

Alumina is a substance that does not commonly occur in any pure form, as silica does in quartz, but which enters into the composition of many minerals, giving them the property of forming clay. Every one knows what clay is in its com¬ moner form. Its varieties depend on the varying proportion of the mixture of alumina with silica and other substances. Pure alumina however would not make clay, as pure silica does quartz. Some of the hardest gems, such as sapphire, corundum, and emery, are nearly pure alumina. When how¬ ever a rock formed of such minerals as feldspar or horn-

ORIGIN OF CLAY AND MUD.

blende, containing a certain proportion of alumina, becomes rotten or decomposed, the decomposed part forms wliat we call clay. Such rocks, or rather the rocks formed of such decomposed matter, are called argillaceous or clayey rocks. Now if a block of any argillaceous rock, or of any rock con¬ taining the constituents of clay, be acted on by moving water in the way we have already described, it is evident that the ultimate effect of the wearing and grinding of its particles will be to produce clay or mud rather than sand.

As however most rocks contain several minerals, some of which are more silicious, some more aluminous, the ultimate effect of their degradation, as it is called, by water, will probably be that one part is ground down into sand, and another triturated into mud. It will often happen also, from the mixture of the two substances, that the difference between mud and sand will depend rather on the minute¬ ness of the subdivision of the particles than on their mineral composition, so that that which is now sand might be readily ground down into clay and mud, or that what is now mud formerly existed as sand, and before that as pebbles. Clay or mud therefore must have just the same origin and method of formation as sand or gravel.

We will now entreat the reader to let this fact (or if he

POPULAR GEOLOGY.

pleases for the present, this possibility) sink deeply into his mind : —

All detached pieces and fragments of stone or rock, whether they be boulders of many tons weight, or pebbles, or sand, or clay, or mud, have once formed portions of large, solid, ori¬ ginally formed masses of rock, and have all (with the ex¬ ception of matters actually blown from the crater of a vol¬ cano) acquired their present form and condition through the action of running water.

There is not a shower of rain that falls, whether on the crowded street, the dusty road, the plains, the hills, or the mountain summits, that does not cause a multitude of rills and streams of muddy water to flow from higher to lower levels. The mud borne along by that water was once part of a solid rock. Even if it be but the waste of the bricks and tiles of our houses, this is still true; and it is equally true for every other case, except for those particles of it that may be the result of the decomposition of animal or vegetable matter. Even the gentlest rain that soaks silently into the most richly carpeted meadow of grass, contributes to the stock of water contained below ground, which here and there bursts forth in springs, carrying momently some grain of mineral matter to the brook, the river, and the

I '/ nJ o y\ d ei StWaibc m

ACTION OF RAIN.

ocean. Who has not seen the springs discoloured after heavy rain ? Who has not watched in wet weather the swollen brook or the roaring mountain-torrent, with its thick, muddy, coffee-coloured water ? Who does not know the flooded aspect of a river, with its dull, yellow, turbid eddies, so different from the limpid stream that commonly flows between its banks ? Whoever has seen these things, has seen one of the multitudinous actions of nature which are for ever and everywhere in operation, performing slowly, and in the lapse of ages, mighty works by means apparently inadequate and at first sight perhaps not especially adapted to the purpose.

There are however other agencies at work, agencies acting with greater local powrer than mere rain, in wearing away solid rocks and transporting the w'aste to other localities. We have alluded to the action of brooks and rivers ; but if we were to trace them more minutely and in detail, and follow them up to where they acquire a swifter stream, or where rapids and cataracts occur in them, wre should esti¬ mate still more highly their destructive power on solid rock. Rivers are in fact great natural saws or planes, for ever grooving furrows in the land. Let any one look at the bed of a mountain torrent, where it has cut a deep ravine

POPULAR GEOLOGY.

through hard rock, and he will see the amount of its force perpetually acting through uncounted ages. As a well-known example let him take the Falls of Niagara, as detailed in Sir C. LyelFs ' Principles of Geology/ and he will see somewhat of the nature of river action in deepening its own bed through the destruction and transportation of the rock composing it.

Equally good examples on a smaller scale however are to be met with almost everywhere. On the north side of Newfoundland I once met with one of these, admitting of good illustration, in the Fiver Exploits, which runs out of the desert interior to the head of Exploits Bay. About twenty miles above the mouth, the river falls over a ledge of hard, dark red gritstone and compact “ slate rock,” a height of about thirty or forty feet, the fall being divided into two by a small rocky islet covered with the same wood of stunted fir-trees that stretches all over the country*. Above the falls the river is one hundred and fifty yards wide, very shallow, its bed composed entirely of huge boulders. Below the falls it is contracted to a channel of fifty, or in some places only twTenty-five yards in width, and it runs in a narrow precipitous ravine (full of ledges and ridges of bare

* See Frontispiece.

ACTION OF RIVERS.

hard rocks striking right across it) for at least a mile, when it issues out into a wider valley in a lower piece of country. At the mouth of this ravine stands a pinnacle of rock called Buchan’s Island, just in the centre of the river, formed of the same rock, and exactly of the same height, as the banks of the ravine on either side*. This pinnacle of rock evidently marks the position where the falls once were, at which time it formed just such an island, dividing the falls into two, as we see in the existing falls at the present day. The inter¬ mediate part of the ravine, a mile long, and on an average fifty yards wide and forty feet deep has clearly been exca¬ vated by the grooving action of the river on rocks of the hardest and most intractable character.

On mountain-tops, or in high latitudes even on lower ground, frost is another great agent of degradation. Any one who ascends the mountains of our own islands for the first time, will often be surprised at the multitude of an¬ gular fragments and fallen blocks he sees scattered over their summits or piled at the foot of their precipices. Of these many, if not most, have been detached by the action of frost causing the water contained in the joints and crevices to expand and rend them asunder, just as in a cold winter's

* See Plate II.

POPULAR GEOLOGY.

night the jugs and water-bottles are apt to be burst by the frost in our bed-rooms.

If we were to visit mountains such as the Alps, where glaciers are formed, we should see still another effect of frost. A glacier is in fact a stream of ice slowly descending* the mountain-side. In its slow but mighty and irresistible progress, it exerts an enormous force in grinding and groov¬ ing the rocks on which it rests, tearing off many of the pro¬ jecting blocks it meets with, and, by undermining the cliffs at its sides, causing blocks to fall upon its surface. These blocks it constantly carries forward, till it comes to the boundary, where it melts away, when it of course deposits them. They there form a huge mound or ridge of broken fragments, called in the Alps a “ Moraine.”

Similarly in all those countries where ice forms periodically in large quantity, either on the rivers or on the sea-shores, it both rends asunder and grinds up whatever rock it comes in contact with, but more especially in these cases does it aid in carrying off to distant localities blocks already de¬ tached. It encases these in its own mass, freezes them in, and when it is itself floated off, bears them along with it, transporting them perhaps many miles, or, in the case of icebergs, even many hundred miles from the place where it

ACTION OP PROST.

found them, and, on melting, drops them at the bottom of the sea.

Of all agencies, however, the most efficient in the de¬ struction and degradation of rock, because it is both locally powerful and very widely diffused, is the action of the sea-breakers. In all climes, in all latitudes, along all shores of all seas and oceans, this action is ceaselessly at work, day and night, summer and winter, gently and imperceptibly even in calms, furiously and vigorously in storms, gradually but steadily in moderate weather, wave after wave is launched from the sea against the land, eating and tearing it away.

No one can have visited the soft cliffy shores of the east and south of England without having been almost an eye¬ witness of this action. It is nowhere perhaps better dis¬ played than on the coast of Yorkshire, near Scarborough. I well remember many years ago being struck, when at¬ tempting to walk under the cliffs from Scarborough to Eiley Bay, with the enormous slices or square pilasters of cliff, that, having been undermined by the action of the breakers at high water, had fallen forwards headlong into the sea, the empty space they had once filled in the precipice above showing its still freshly exposed and jagged surface, gaping from the wound.