Chapter V
Underground Phases of Volcanic Action. B. Materials injected or consolidated beneath the Surface--Intrusive Series: I. Vents of Eruption--i. Necks of Fragmentary Materials; ii. Necks of Lava-form Materials; iii. Distribution of Vents in relation to Geological Structure-Lines; iv. Metamorphism in and around Volcanic Cones, Solfataric Action; v. Inward Dip of Rocks towards Necks; vi. Influence of contemporaneous Denudation upon Volcanic Cones; vii. Stages in the History of old Volcanic Vents.
In our profound ignorance of the nature of the earth's interior, we know as yet nothing certain regarding the condition and distribution there of those molten materials which form the prime visible source of volcanic energy. By the study of volcanoes and their products we learn that the fused substances are not everywhere precisely the same and do not remain absolutely uniform, even in the same volcanic region. But in what manner and from what causes these variations arise is still unknown. We are further aware that the molten magma, under a centre of volcanic disturbance, manifests from time to time energetic movements which culminate in eruptions at the surface. But what may be the exciting cause of these movements, to what depth they descend, and over what extent of superficies they spread, are matters regarding which nothing better than conjecture can as yet be offered. It is true that, in some cases, a magma of fairly uniform composition has been erupted over a vast tract of the earth's surface, and must have had a correspondingly wide extent within the terrestrial crust. Thus in the case of the older Tertiary volcanic eruptions of North-Western Europe, basalt of practically the same composition was discharged from thousands of fissures and vents distributed from the south of Antrim northward beyond the Inner Hebrides, through the chain of the Faroe Islands and over the whole breadth of Iceland. Under the British Isles alone, the subterranean reservoirs of molten lavas must have been at least 40,000 square miles in united area. If they stretched continuously northwards below the Faroe Islands and Iceland, as is highly probable, that is, for 600 miles further, their total extent may have been comparable to such a region as Scandinavia.
Was this vast underground body of lava part of a universal liquid mass within the globe, or was it rather of the nature of one or more lakes or large vesicles within the crust? We can only offer speculation for answer. On the other hand, there seems to be good proof that in some districts, both now and in former geological periods, such differences exist between the materials ejected from vents not far distant from each other as to show the existence of more limited distinct reservoirs of liquid rock underneath.
Some of the questions here asked will be further dealt with in later pages in connection with such geological evidence as can be produced regarding them. But it will be found that at every step in the endeavour to ascertain the origin of volcanic phenomena difficulties present themselves which are now and may long remain insoluble.
I. Vents of Eruption
It is a general belief that the first stage in the formation of a volcano of the Vesuvian type by the efforts of subterranean energy is the rending of the terrestrial crust in a line of fissure. Some of the most remarkable groups of active volcanoes on the face of the globe are certainly placed in rows, as if they had risen along some such great rents. The actual fissure, however, is not there seen, and its existence is only a matter of probable inference. Undoubtedly the effect of successive eruptions must be to conceal the fissure, even if it ever revealed itself at the surface.
What is supposed to have marked the initial step in the formation of a great volcano is occasionally repeated in the subsequent history of the mountain. During the convulsive shocks that precede and accompany an eruption, the sides of the cone, and even sometimes part of the ground beyond, are rent open, occasionally for a distance of several miles, and on the fissures thus formed minor volcanoes are built up.
It is in Iceland, as already stated, that the phenomena of fissures are best displayed. There the great deserts of lava are from time to time dislocated by new lines of rent, which ascend up to the surface and stretch for horizontal distances of many miles. From these long narrow chasms lava flows out to either side; while cones of slag and scoriæ usually form upon them. This interesting eruptive phase will be more fully described in the chapters dealing with the Tertiary volcanic rocks of Britain.
There can be no doubt, however, that in a vast number of volcanic vents of all geological periods no trace can be discovered of their connection with any fissure in the earth's crust. Such fissures may indeed exist underneath, and may have served as passages for the ascent of lava to within a greater or less distance from the surface. But it is certain that volcanic energy has the power of blowing out an opening for itself through the upper part of the crust without the existence of any visible fissure there. What may be the limits of depth at which this mode of communication with the outer air is possible we do not yet know. They must obviously vary greatly according to the structure of the terrestrial crust on the one hand, and the amount and persistence of volcanic energy on the other. We may suppose that where a fissure terminates upward under a great depth of overlying rock, the internal magma may rise up to the end of the rent, and even be injected laterally into the surrounding parts of the crust, but may be unable to complete the formation of a volcano by opening a passage to the surface. But where the thickness of rock above the end of the fissure is not too great, the expansive energy of the vapours absorbed in the magma may overcome the resistance of that cover, and blow out an orifice by which the volcanic materials can reach the surface. In the formation of new cones within the historic period at a distance from any central volcano, the existence of an open fissure at the surface has not been generally observed. When, for example, Monte Nuovo was formed, it rose close to the shore among fields and gardens, but without the appearance of any rent from which its materials were discharged.
That in innumerable instances during the geological past, similar vents have been opened without the aid of fissures that reached the surface, will be made clear from the evidence to be drawn from the volcanic history of the British Isles. So abundant, indeed, are these instances that they may be taken as proving that, at least in the Puy type of volcanoes, the actual vents have generally been blown out by explosions rather than by the ascent of fissures to the open air.
In cases where, as in Iceland, fissures open at the surface and discharge lava there, the channel of ascent is the open space between the severed walls of the rent. Within this space the lava will eventually cool and solidify as a dyke. It is obvious that a comparatively small amount of denudation will suffice to remove all trace of the connection of such a dyke with the stream of lava that issued from it. Among the thousands of dykes belonging to the Tertiary period in the British Islands, it is probable that many may have served as lines of escape for the basalt at the surface. But it is now apparently impossible to distinguish between those which had such a communication with the outer air and those that ended upward within the crust of the earth. The structure of dykes will be subsequently discussed among the subterranean intrusions of volcanic material.
In an ordinary volcanic orifice the ground-plan is usually irregularly circular or elliptical. If that portion of the crust of the earth through which the vent is drilled should be of uniform structure, and would thus yield equally to the effects of the volcanic energy, we might anticipate that the ascent and explosion of successive globular masses of highly heated vapours would give rise to a cylindrical pipe. But in truth the rocks of the terrestrial crust vary greatly in structure; while the direction and force of volcanic explosions are liable to change. Hence considerable irregularities of ground-plan are to be looked for among vents.
Some of these irregularities are depicted in Fig. 22, which represents the ground plan of some vents from the Carboniferous volcanic districts of Scotland. They are all drawn on the same scale. Other examples will be cited in later chapters from the same and other parts of the British Isles.
Some of the most marked departures from the normal and simple type of vent occur where two orifices have been opened close to each other, or where the same vent has shifted its position (Figs. 29, 125, 205, and 214). Curiously irregular or elongated forms may thus arise in the resultant "necks" now visible at the surface. Many striking examples of these features may be seen among the Carboniferous and Permian volcanoes to be afterwards described. Occasionally where an open fissure has served as a vent it has given rise to a long dyke-like mass (No. 1 in Fig. 22).
The size of a volcanic vent may vary indefinitely from a diameter of not more than a yard or two up to one or two or more miles. As a rule, the smaller the vents the more numerously are they crowded together. In the case of large central volcanoes like Etna, where many subsidiary vents, some of them forming not inconsiderable hills, may spring up along the sides of the parent cone, denudation will ultimately remove all the material that was heaped up on the surface, and leave the stumps or necks of the parasitic vents in groups around the central funnel.
Each volcanic chimney, by which vapours, ashes or lava are discharged at the surface, may be conceived to descend in a more or less nearly vertical direction until it reaches the surface of the lava whence the eruptions proceed. After the cessation of volcanic activity, this pipe will be left filled up with the last material discharged, which will usually take the form of a rudely cylindrical column reaching from the bottom of the crater down to the lava-reservoir. It will be obvious that no matter how great may be the denudation of the volcano, or how extensive may be the removal of the various materials discharged over the surrounding ground, the pipe or funnel with its column of solid rock must still remain. No amount of waste of the surface of the land can efface that column. Successively lower and yet lower levels may be laid bare in it, but the column itself goes still further down. It will continue to make its appearance at the surface until its roots are laid bare in the lava of the subterranean magma. Hence, of all the relics of volcanic action, the filled-up chimney of the eruptive vent is the most enduring. Save where it may have been of the less deep-seated nature of a "hornito" upon a lava-stream, we may regard it as practically permanent. The full meaning of these statements will be best understood from a consideration of the numerous illustrations to be afterwards given.
The stumps of volcanic columns of this nature, after prolonged denudation, generally project above the surrounding ground as rounded or conical eminences known as "Necks" (Fig. 23. See also Figs. 52, 82, 102, 109, 123, 133, 144, 178, 192, 195, 203, 204, 209, 294, 298, 306 and 310). Their outlines, however, vary with the nature of their component materials. The softer rocks, such as tuffs and agglomerates, are apt to assume the form of smooth domes or cones, while the harder and especially the crystalline rocks rise into irregular, craggy hills. Occasionally, indeed, it may happen that a neck makes no prominence on the surface of the ground, and its existence may only be discoverable by a careful examination of the geological structure of the locality. Now and then an old vent will be found not to form a hill, but to sink into a hollow. Such variations, however, have little or no reference to original volcanic contours in the history of the localities which display them. They arise mainly from the differing hardness and structure of the materials that have filled the vents, and the consequent diversity in the amount of resistance which they have offered to the progress of denudation.
The materials now found in volcanic funnels are of two kinds: 1st, Fragmentary, derived from volcanic explosions; and 2nd, Lava-form, arising from the ascent and consolidation of molten rock within the funnel.
i. Necks of Fragmentary Materials
By far the most satisfactory evidence of a former volcanic orifice is furnished by a neck of fragmentary materials. Where "bosses" of crystalline rock rise to the surface and assume the outward form of necks, we cannot always be certain that they may not have been produced by subterranean intrusions that never effected any connection with the surface. In other words, such bosses may not mark volcanic orifices at all, though they may have been part of the underground protrusions of volcanoes in their neighbourhood. But where the chimney has been filled with debris, there can be no doubt that it truly marks the site of a once active volcano. The fragmentary material is an eloquent memorial of the volcanic explosions that drilled the vent, kept it open, and finally filled it up. These explosions could not have taken place unless the elastic vapours which caused them had found an escape from the pressure under which they lay within the crust of the earth. Now and then, indeed, where the outpouring of lava or some other cause has left cavernous spaces within the crust, there may conceivably be some feeble explosion there, and some trifling accumulation of fragmentary materials. But we may regard it as practically certain that the mass of tumultuous detritus now found in volcanic necks could not have been formed unless where a free passage had been opened from the molten magma underneath to the outer surface of the planet.
Considerable diversity may be observed in the nature and arrangement of the fragmentary materials in volcanic necks. The chief varieties may be arranged in four groups: (1) Necks of non-volcanic detritus; (2) Necks of volcanic agglomerate or tuff; (3) Necks of agglomerate or tuff with a central plug of lava; and (4) Necks of agglomerate or tuff with veins, dykes or some lateral irregular mass of lava.
(1) Necks of non-volcanic Detritus.--During the first convulsive efforts of a volcanic focus to find a vent at the surface, the explosions that eventually form the orifice do so by blowing out in fragments the solid rocks of the exterior of the terrestrial crust. Of the detritus thus produced, shot up the funnel and discharged into the air, part may gather round the mouth of the opening and build up there a cone with an enclosed crater, while part will fall back into the chimney, either to accumulate there, should the explosions cease, or to be thrown out again, should they continue. In the feeblest or most transient kinds of volcanic energy, the explosive vapours may escape without any accompanying ascent of the molten magma to the surface, and even without any sensible discharge of volcanic "ashes" from that magma. In such cases, as I have already pointed out, the detritus of the non-volcanic rocks, whatever they may be, through which volcanic energy has made an opening, accumulate in the pipe and eventually consolidate there. Examples of this nature will be adduced in later chapters from the volcanic districts of Britain.
Where only non-volcanic materials fill up a vent we may reasonably infer that the eruptions were comparatively feeble, never advancing beyond the initial stage when elastic vapours made their escape with explosive violence, but did not lead to the outflow of lava or the discharge of ashes. In the great majority of necks, however, traces of the earliest eruptions have been destroyed by subsequent explosions, and the uprise of thoroughly volcanic fragments. Yet even among these fragments, occasional blocks may be detected which have been detached from the rocks forming the walls of the funnel.
The general name of Agglomerate, as already stated, is given to all accumulations of coarse, usually unstratified, detritus in volcanic funnels, irrespective of the lithological nature of the materials. For further and more precise designation, when an agglomerate is mainly made up of fragments of one particular rock, the name of that rock may be prefixed as sandstone-agglomerate, granite-agglomerate, basalt-agglomerate, trachyte-agglomerate. Volcanic agglomerate is a useful general term that may include all the coarser detritus ejected by volcanic action.
Where volcanic explosions have been of sufficient violence or long continuance, the upper part of the funnel may be left empty, and on the cessation of volcanic activity, may be filled with water and become a lake. The ejected detritus left round the edge of the orifice sometimes hardly forms any wall, the crater-bottom being but little below the level of the surrounding ground. Explosion-lakes are not infrequent in Central France and the Eifel (Maare). A more gigantic illustration is afforded by the perfectly circular crater of Coon Butte in Arizona, about 4000 feet in diameter and 600 feet deep. It has been blown out in limestone, the debris of which forms a rampart 200 feet high around it. Examples will afterwards be cited from the Tertiary volcanic plateaux of North-Western Europe. Vents may also be formed by an engulphment or subsidence of the material, like that which has taken place at the great lava cauldron of Hawaii, still an active volcano. The picturesque Crater Lake of Oregon is an admirable instance of this structure.
(2) Necks of Agglomerate or Tuff.--In the vast majority of cases, the explosions that clear out a funnel through the rocks of the upper part of the crust do not end by merely blowing out these rocks in fragments. The elastic vapours that escape from the molten lava underneath are usually followed by an uprise of the lava within the pipe. Relieved from the enormous pressure under which it had before lain, the lava as it ascends is kept in ebullition, or may be torn into bombs which are sent whirling up into the air, or may even be blown into the finest dust by the sudden expansion of the imprisoned steam. If its ascent is arrested within the vent, and a crust is formed on the upper surface of the lava-column, this congealed crust may be disrupted and thrown out in scattered pieces by successive explosions, but may re-form again and again.
In many vents, both in recent and in ancient times, volcanic progress has never advanced beyond this early stage of the ejection of stones and dust. The column of lava, though rising near enough to the surface to supply by its ebullition abundant pyroclastic detritus, coarse and fine, has not flowed out above ground, nor even ascended to the top of the funnel. It may have formed, at the surface, cones of stones and cinders with enclosed craters. But thereafter the eruptions have ceased. The vents, filled up with the fragmentary ejected material, have given passage only to hot vapours and gases. As these gradually ceased, the volcanoes have become finally extinct. Denudation has attacked their sides and crests. If submerged in the sea or a lake, the cones have been washed down, and their materials have been strewn over the bottom of the water. If standing on the land, they have been gradually levelled, until perhaps only the projecting knob or neck of solidified rubbish in each funnel has remained to mark its site. The buried column of compacted fragmentary material will survive as the only memorial of the eruptions (Fig. 24. For views of necks formed of agglomerate or tuff see Figs. 23, 82, 102, 123, 144, 178, 192, 203, 204, 209, 210, 212, 216).
The volcanic agglomerates of such vents sometimes include, among their non-volcanic materials, pieces of rock which bear evidence of having been subjected to considerable heat (see vol. ii. p. 78). Carbonaceous shales, for instance, have had their volatile constituents driven off, limestones have been converted into marble, and a general induration or "baking" may be perceptible. In other cases, however, the fragments exhibit no sensible alteration. Fossiliferous limestones and shales often retain their organic remains so unchanged that specimens taken out of the agglomerate cannot be distinguished from those gathered from the strata lying in situ outside. Some stones have evidently been derived from a deeper part of the chimney, where they have been exposed to a higher temperature than others, or they may have been lain longer within the influence of hot ascending vapours.
The volcanic materials in agglomerate range in size from the finest dust to blocks several yards in length, with occasionally even much larger masses. The proportions of dust to stones vary indefinitely, the finer material sometimes merely filling in the interstices between the stones, at other times forming a considerable part of the whole mass.
The stones of an agglomerate may be angular or subangular, but are more usually somewhat rounded. Many of them are obviously pieces that have been broken from already solid rock and have had their edges rounded by attrition, probably by knocking against each other and the walls of the chimney as they were hurled up and fell back again. Their frequently angular shapes negative the supposition that they could have been produced by the discharge of spurts of still liquid lava. As already stated, they have probably been in large measure derived from the violent disruption of the solidified cake or crust on the top of the column of lava in the pipe. Many of them may have been broken off from the layer of congealed lava that partially coated the rough walls of the funnel after successive uprises of the molten material. Among them may be observed many large and small blocks that appear to have been derived from the disruption of true lava-streams, as if beds of lava had been pierced in the formation of the vent, or as if those that congealed on the slopes of the cone had been broken up by subsequent explosions. These fragments of lava are sometimes strongly amygdaloidal. A characteristic feature, indeed, of the blocks of volcanic material in the agglomerates is their frequent cellular structure. Many of them may be described as rough slags or scoriæ. These have generally come from the spongy crust or upper part of the lava where the imprisoned steam, relieved from pressure, is able to expand and gather into vesicles.
Less frequently evidence is obtainable that the blocks were partially or wholly molten at the time of expulsion. Sometimes, for example, a mass which presents on one side such a broken face as to indicate that it came from already solidified material, will show on the other that its steam-vesicles have been pulled out in such a way as to conform to the rounded surface of the block. This elongation could only take place in lava that was not yet wholly consolidated. It seems to indicate that such blocks were derived from a thin hardened crust lying upon still molten material, and that they carried up parts of that material with them. As each stone went whirling up the funnel into the open air, its melted part would be drawn round the gyrating mass, and would rapidly cool there.
In other cases, we encounter true volcanic bombs, that is, rounded or bomb-shaped blocks of lava, with their vesicles elongated all round them and conforming to their spherical shape. Sometimes such blocks are singularly vesicular in the centre, with a more close-grained crust on the outside. Their rapid centrifugal motion during flight would allow of the greater expansion of the dissolved steam in the central part of each mass, while the outer parts would be quickly chilled, and would assume a more compact texture. Bombs of this kind are met with among ancient volcanic products, and, like those of modern volcanoes, have obviously been produced by the ejection of spurts or gobbets of lava from the surface of a mass in a state of violent ebullition. Occasionally they are hollow inside, the rotation in these cases having probably been exceptionally rapid.
Passing from the larger blocks to the smaller fragments, we notice the great abundance of nut-like subangular or rounded pieces of lava in the agglomerates. These include lumps of fine grain not specially vesicular, and probably derived from the disruption of solidified rock. But in many agglomerates, especially those associated with the outpouring of basalts or other basic lavas (as those of Carboniferous and Tertiary age described in later chapters), they comprise also vast numbers of very finely cellular material or pumice. These pumiceous lapilli have been already alluded to as ingredients of the stratified tuffs. But they are still more characteristic of the necks, and reach there a larger size, ranging from the finest grains up to lumps as large as a hen's egg, or even larger.
The peculiar distinctions of this ejected pumice are the extreme minuteness of its vesicles, their remarkable abundance, their prevalent spherical forms, and the thinness of the walls which separate them. In these respects they present a marked contrast to the large irregularly-shaped steam-cavities of the outflowing lavas, or even of the scoriæ in the agglomerates.
This characteristic minutely vesicular pumice is basic in composition. Where not too much decayed, it may be recognized as a basic glass. Thus among the remarkable agglomerates which fill up the Pliocene or Pleistocene vents of the Velay, the fragments consist of a dark very basic glass, which encloses such a multitude of minute steam-cavities that, when seen under the microscope, they are found to be separated from each other by walls so thin that the slice looks like a pattern of delicate lace.[25] In necks of earlier date, such as those of older Tertiary, and still more of Palæozoic, time, the glass has generally been altered into some palagonitic material.
[Footnote 25: M. Boule, Bull. Cart. Géol. France, No. 28, tome iv. (1892) p. 193.]
This finely pumiceous substance appears to be peculiar to the vents and to the deposits of tuff immediately derived from them. It is not found, so far as I know, among any of the superficial lavas, and, of course, would not be looked for among intrusive rocks. It was evidently a special product of the volcanic chimney, as distinguished from the mass of the magma below. We may perhaps regards it as in some way due to a process of quiet simmering within the vent, when the continual passage of ascending vapours kept the molten lava there in ebullition, and gave it its special frothy or finely pumiceous character.
The compacted dust, sand or gravelly detritus found in necks, and comprised under the general name of Tuff, consists partly of the finer particles produced during the violent disruption of already solidified rocks, partly of the detritus arising from the friction and impact of stones ascending and descending above an active vent during times of eruption, and partly of the extremely light dust or ash into which molten lava may be blown by violent volcanic explosions. In old volcanic necks, where the rocks have long been subjected to the influence of percolating meteoric water, it is not perhaps possible to discriminate, except in a rough way, the products from these three sources. The more minutely comminuted material has generally undergone considerable alteration, so that under the microscope it seldom reveals any distinctive structures. Here and there in a slide, traces may occasionally be detected of loose volcanic microlites, though more usually these can only be found in lapilli of altered glass or finely pumiceous lava.
The composition of the detritus in a neck of agglomerate or tuff has almost always a close relation to that of any lavas which may have been emitted from that vent. If the lavas have been of an acid character, such as rhyolites, felsites or obsidians, the pyroclastic materials will almost always be found to be also acid. Where, on the other hand, the lavas have been intermediate or basic, so also will be the tuffs and agglomerates. Occasionally, however, as has already been pointed out, from the same or closely adjoining vents lavas of very different chemical composition have been successively erupted. Felsites or rhyolites have alternated with diabases, basalts or andesites. In such cases, a commingling of acid and basic detritus may be observed, as, for example, among the volcanoes of the Old Red Sandstone. It has even happened sometimes that such a mixture of material has taken place when only one class of lavas has been poured out at the surface, as in the agglomerates that fill vents among the basalts of the Inner Hebrides. But we may be sure that, though not discharged at the surface, the lavas of which pieces are found in the tuffs must have risen high enough in the vents to be actually blown out in a fragmentary form. The occurrence of felsitic fragments among the otherwise basic agglomerates of Mull and Skye will be described in subsequent pages, likewise the intercalation of rhyolitic detritus between the basalts of Antrim. A similar association occurs among the modern vents of Iceland.
Among the contents of the tuffs and agglomerates that occupy old volcanic vents, some are occasionally to be observed of which the source is not easily conjectured. Detached crystals of various minerals sometimes occur abundantly which were certainly not formed in situ, but must have been ejected as loose lapilli with the other volcanic detritus. Where these crystals belong to minerals that enter into the composition of the lavas of the district in which they are found, they may be regarded as having probably been derived from the explosion of such lavas in the vents, the molten magma being blown into dust, and its already formed crystals being liberated and expelled as separate grains. But it seems to be extremely rare to find any neighbouring lava in which the minerals in question are so largely and so perfectly crystallized as they are in these loose crystals of the neck. The beautifully complete crystals of augite found in the old tuffs of Vesuvius and on the flanks of Stromboli may be paralleled among Palæozoic tuffs and agglomerates in Britain. Thus the necks belonging to the Arenig and Llandeilo volcanoes of southern Scotland are sometimes crowded with augite, varying from minute seed-like grains up to perfectly formed crystals as large as hazel nuts. The conditions under which such well-shaped idiomorphic minerals were formed were probably different from those that governed the cooling and consolidation of the ordinary lavas.
But besides the minerals that may be claimed as belonging to the volcanic series of a district, others occur not infrequently in some tuff-necks, the origin of which is extremely puzzling. Such are the large felspars, micas, garnets and the various gems that have been obtained from necks. The large size of some of these crystals and their frequently perfect crystallographic forms negative the idea that they can, as a rule, be derived from the destruction of any known rocks, though they may sometimes be conceivably the residue left after the solution of the other constituents of a rock by the underground magma, like the large residual felspars enclosed in some dykes. The crystals in question, however, seem rather to point to some chemical processes still unknown, which, in the depths of a volcanic focus, under conditions of pressure and temperature which we may speculate about but can perhaps hardly ever imitate in our laboratories, lead to the elaboration of the diamond, garnet, sahlite, smaragdite, zircon and other minerals.[26] Examples of such foreign or deep-seated crystals will be described from the probably Permian necks of Central Scotland.
[Footnote 26: For lists of the minerals found in the diamond-bearing necks of Kimberley, see M. Boutan in Frémy's Encyclopédie Chimique (1886), vol. ii. p. 168; Dr. M. Bauer's Edelsteinkunde (1895), p. 223.]
Whatsoever may be the source and nature of the fragmentary materials that fill old volcanic vents, they present, as a general rule, no definite arrangement in the necks. Blocks of all sizes are scattered promiscuously through the agglomerate, just as they fell back into the chimney and came to rest there. The larger masses are placed at all angles, or stand on end, and are sometimes especially conspicuous in the centre of a neck, though more usually dispersed through the whole. Such a thoroughly tumultuous accumulation is precisely what might be expected where explosions have taken place in still liquid and in already consolidated lavas, and where the materials, violently discharged to the surface, have fallen back and come finally to rest in the chimney of the volcano.
Nevertheless, this absence of arrangement sometimes gives place to a stratification which becomes more distinct in proportion as the material of the vent passes from coarse agglomerate into fine tuff. It is possible that the existence and development of this structure depend on the depth at which the materials accumulate in the funnel. We may conceive, for instance, that in the lower parts of the chimney, the stones and dust, tumultuously falling and rebounding from projections of the rugged walls, will hardly be likely to show much trace of arrangement, though even there, if the explosions continue to keep an open though diminishing passage in the vent, alternations of coarser and finer layers, marking varying phases of eruptivity, may be formed in the gradually heightening pile of agglomerate. Rude indications of some such alternations may sometimes be detected in what are otherwise quite unstratified necks.
In the upper part of a volcanic funnel, however, close to and even within the crater, the conditions are not so unfavourable to the production of a stratified arrangement. As the pipe is filled up, and the activity of eruption lessens, explosions may occur only from the very middle of the orifice. The debris that falls back into the vent will gather most thickly round the walls, whence it will slide down to the central, still eruptive hole. It will thus assume a stratified arrangement, the successive layers lying at the steepest angles of repose, or from 30° to 35°, and dipping down in an inverted conical disposition towards the centre. If the process should continue long enough, the crater itself may be partially or completely filled up with detritus (Fig. 25).
Of this gradual infilling of a volcanic chimney with stratified agglomerate and tuff, examples belonging to different geological periods will be cited in subsequent chapters. I may here especially allude to one of the most recently observed and best marked illustrations, which occurs on the west side of Stromö, in the Faroe Islands (see Figs. 310, 311, 312). A neck has there been filled up with coarse agglomerate, which is rudely stratified, the layers dipping steeply into the centre, where the tumultuous assemblage of large blocks no doubt points to the final choking up of the diminished orifice of explosion. The walls of the neck are nearly vertical, and consist of the bedded basaltic lavas through which the vent has been opened. They terminate upward in a conical expansion, evidently the old crater, which has subsequently been filled up by the inroads of several lava-streams from adjacent vents. It is here manifest that the bedded agglomerate belongs to the uppermost part of the volcanic funnel.
Where vents have been filled up with tuff rather than with agglomerate, the stratified structure is best developed. Alternations of coarser and finer detritus give rise to more or less definite layers, which, though inconstant and irregular, serve to impart a distinctly stratified character to the mass. Where there has been no subsequent disturbance within a vent, these layers show the same inward dip towards the centre just referred to, at the ordinary angles of repose. Now and then, where a neck with this structure has been laid bare on a beach, its denuded cross-section presents a series of concentric rings of strata from the walls towards the centre. Good illustrations of these features are supplied by the probably Permian necks of eastern Fife (Figs. 25 A and 217).[27]
[Footnote 27: See also the sections of vents on the west coast of Stromö Faroes, above referred to.]
It has frequently happened, however, that, owing to subsidence of the materials filling up the vents or to later volcanic disturbances, the compacted tuffs have been broken up and thrown into various positions, large masses being even placed on end. Among the Carboniferous and Permian necks of Central Scotland such dislocated and vertical tuffs are of common occurrence (see Figs. 145, 218). If, as is probable, we are justified in regarding the stratified parts of necks as indicative of the uppermost parts of volcanic funnels, not far from the surface, the importance of this inference will be best understood when the Carboniferous and Permian volcanoes are described.
(3) Necks with a central Lava-plug.--Some vents of agglomerate or tuff are pierced by a plug of lava, as may be instructively seen in many of the Carboniferous and Permian necks of the centre and south of Scotland (Fig. 26; compare also Figs. 148, 174, 207, and 226). Where this structure shows itself, the contrast in hardness and durability between the more destructible fragmentary material and the solid resisting lava leads to a topographical distinction in the outer forms of necks. The smooth declivities of the friable tuffs are crowned or interrupted by more craggy features, which mark the position of the harder intrusive rock.
The plug, like the pipe up which it has risen, is in general irregularly circular in ground-plan. It may be conceived to be a column of rock, descending to an unknown depth into the interior, with a casing of pyroclastic debris surrounding it. It may vary considerably in the proportion which its cross-section bears to that of the surrounding fragmental material. Sometimes it does not occupy more than a small part of the whole, often appearing in the centre. In other cases, it more than equals all the rest of the material in the vent, while instances may be noted where only occasional patches of tuff or agglomerate are visible between the lava-plug and the wall of the pipe. From these we naturally pass to the second type of vent, where no fragmentary material is to be seen, but where the chimney is now entirely filled with some massive once-molten rock.
A neck with a lava-plug probably contains the records of two stages in volcanic progress, the first of which, indicated by the tuff or agglomerate, was confined to the discharge of fragmentary materials; while the second, shown by the lava-plug, belonged to the time when, after the earlier explosions, lava ascended in the vent and solidified there, thus bringing the eruptions from that particular orifice to an end. Where a small central column of lava rises through the tuff, we may suppose that the funnel had been mainly choked up by the accumulation in it of ejected detritus, which was compacted to a solid mass adhering to the wall of the funnel, but leaving a central orifice to be kept open by the gradually waning energy of the volcano. By a final effort that impelled molten rock up that duct and allowed it to consolidate there, the operations of the vent were brought to a close.
Where, on the other hand, only occasional strips of tuff or agglomerate are to be found between the lava-plug and the wall of the pipe, the last uprise of lava may be supposed to have been preceded by more vigorous explosions which cleared the throat of the volcano, driving out the accumulated detritus and leaving only scattered patches adhering to the sides of the funnel.
There is, no doubt, some downward limit to the production of fragmentary material, and if we could lay bare successive levels in the chimney of a volcano we should find the agglomerate eventually replaced entirely by lava.
The materials of the lava-plugs vary widely in composition. Sometimes they are remarkably basic, and present rocks of the picrite or limburgite type; in other cases they are thoroughly acid rocks such as felsite and granophyre. Many intermediate varieties may be found between these extremes. It is noteworthy that, in districts where the lavas erupted to the surface have been andesitic or basaltic, the material which has finally solidified in the vents is often more acid in composition, trachytic rocks being specially frequent.
(4) Necks with Dykes, Veins, or irregular intrusions of Lava.--While the presence of a central plug of lava in a neck of fragmental material may indicate that the vent was still to some extent open, there is another structure which seems to point to the ascent of lava after the funnel has been choked up. Numerous instances have been observed where lava has been forced upward through rents in a mass of tuff or agglomerate, and has solidified there in the form of dykes or veins (Fig. 27). Illustrations of this structure abound among the Carboniferous and Permian necks of Britain. Here, again, though on a less marked scale, the contrast in the amount and character of the weathering of the two groups of rock gives rise to corresponding topographical features, which are especially observable in cliffs and coast-sections, where the dykes and veins project out of the tuffs as dark prominent walls (Figs. 135, 149, 166, 168, 219, 221, 222).
These intrusive injections are generally irregular in their forms, the lava having evidently been driven through a mass of material which, not having yet consolidated sufficiently to acquire a jointed structure, afforded few dominant lines of division along which it could ascend. Now and then, however, sharply defined dykes or veins, which at a distance look like dark ribbons, may be seen running vertically or at a high angle, and with a straight or wavy course, through the fine compacted tuff of a vent. Frequently the injected material has found its readiest line of ascent along the walls of the funnel, between the tuff and the surrounding rocks. Occasionally it has made its way into rents in these rocks, as well as into the body of the neck.
It is worthy of remark in passing that complete consolidation of the fragmentary material does not appear to be always requisite in order to allow of the formation of such fissures as are needed for the production of dykes. A singularly interesting illustration of this fact may be seen on the northern crest of the outer crater of the Puy Pariou in Auvergne. A dyke of andesite 8 or 10 feet broad may there be traced running for a distance of about 300 yards through the loose material of the cone. The rock is highly vesicular, and the vesicles have been elongated in the direction of the course of the dyke so as to impart a somewhat fissile structure to the mass.
There can be little doubt that the dykes and veins which traverse necks of agglomerate belong to one of the closing phases in the history of the vents in which they occur. They could only have been injected after the pipes had been so choked up that explosions had almost or entirely ceased, and eruptions had consequently become nearly or quite impossible. They show, however, that volcanic energy still continued to manifest itself by impelling the molten magma into these extinct funnels, while at the same time it may have been actively discharging materials from other still open vents in the same neighbourhood.
With regard to the composition of these dykes and veins, it may be remarked that in a district of acid lavas they may be expected to be felsitic or rhyolitic, sometimes granophyric. Where, on the other hand, the lavas poured out at the surface have been intermediate or basic, the veins in the necks may be andesites, basalts or other still more basic compounds. But it is observable, as in the case of the lava-plugs, that the injections into the necks may be much more acid than any of the superficial lavas. The advent of acid material in the later part of a volcano's history has been already alluded to, and many examples of it will be given in this work.
After all explosions and eruptions have ceased, heated vapours may still for a long period continue to make their way upward through the loose spongy detritus filling up the vent. The ascent of such vapours, and more particularly of steam, may induce considerable metamorphism of the agglomerate, as is more particularly noticed at p. 71.
ii. Necks of Lava-form Material
The second type of neck is that in which the volcanic pipe has been entirely filled up with some massive or crystalline rock. As already remarked, it is not always possible to be certain that bosses of rock, having the external form of necks of this kind, mark the sites of actual volcanic orifices. Eruptive material that has never reached the surface, but has been injected into the crust of the earth, has sometimes solidified there in forms which, when subsequently exposed by denudation, present a deceptive resemblance to true volcanic necks. Each example must be examined by itself, and its probable origin must be determined by a consideration of all the circumstances connected with it. Where other evidence exists of volcanic activity, such, for instance, as the presence of bedded tuffs or intercalated sheets of lava, the occurrence of neck-like eminences or bosses of felsite, andesite, dolerite, basalt or other eruptive rock, would furnish a presumption that these marked the sites of some of the active vents of the period to which the tuffs and lavas belonged.
If a neck-like eminence of this kind were found to possess a circular or elliptical ground-plan, and to descend vertically like a huge pillar into the crust of the earth; if the surrounding rocks were bent down towards it and altered in the manner which I shall afterwards describe in detail; if, moreover, the material composing the eminence were ascertained to be closely related petrographically to some parts of the surrounding volcanic series, it might with some confidence be set down as marking the place of one of the active vents from which that series was ejected.
The chief contrast in external form between this type of neck and that formed of fragmentary material arises from differences in the relative durability of their component substance. The various kinds of lava-form rock found in necks are, as a whole, much harder and more indestructible than agglomerates and tuffs. Consequently bosses of them are apt to stand out more prominently. They mount into higher points, present steeper declivities, and are scarped into more rugged crags. But essentially they are characterized by similar conical outlines, and by rising in the same solitary and abrupt way from lower ground around them (see Figs. 109, 133, and 195, 294).
Various joint-structures may be observed in these necks. In some cases there is a tendency to separate into joints parallel to the bounding walls, and occasionally this arrangement goes so far that the rock has acquired a fissile structure as if it were composed of vertical strata. In other instances, the rock shows a columnar structure, the columns diverging from the outer margin, or curving inwards, or displaying various irregular groupings. More usually, however, this jointing is so indefinite that no satisfactory connection can be traced between it and the walls of the orifice in which the rock has solidified.
Some of the most remarkable examples of necks ever figured and described are those to which attention was called by Captain Dutton as displayed in the Zuni plateau of New Mexico, where, amid wide denuded sheets of basalt, numerous prominent crags mark the sites of eruptive vents. The basalt of these eminences is columnar, the columns standing or lying in all sorts of attitudes, and in most cases curved.[28] In the Upper Velay, in Central France, numerous conspicuous domes and cones of phonolite rise amidst the much-worn basalt-plateau of that region (Fig. 345). Many instances will be cited in later chapters from the British Isles.
[Footnote 28: U.S. Geol. Survey, 6th Annual Report, 1884-85, p. 172.]
iii. Distribution of Vents in Relation to Geological Structure-lines
Where the positions of true volcanic necks can be accurately determined, it is interesting to study their distribution and their relation to the main lines of geological structure around them. Sometimes a distinct linear arrangement can be detected in their grouping. Those of the Lower Old Red Sandstone of Central Scotland, for instance, can be followed in lines for distances of many miles (Map No. III). Yet when we try to trace the connection of such an arrangement with any known great lines of dislocation in the terrestrial crust, we can seldom establish it satisfactorily. In the case of the Scottish Old Red Sandstone just cited, it is obvious that the vents were opened along a broad belt of subsidence between the mountains of crystalline schist on the north, and those of convoluted Silurian strata on the south, either margin of that belt being subsequently, if not then, defined by lines of powerful fault. No vents have risen along these faults, nor has any relation been detected between the sites of the volcanic foci and dislocations in the area of ancient depression.
Indeed, it may be asserted of the vents of Britain that they are usually entirely independent of any faults that traverse at least the upper visible part of the earth's crust. They sometimes rise close to such lines of fracture without touching them, but they are equally well developed where no fractures are to be found. Now and then one of them may be observed rising along a line of fault, but such a coincidence could hardly fail occasionally to happen. From the evidence in the British Isles, it is quite certain that if volcanic vents have, as is possible, risen preferably along lines of fissure in the terrestrial crust, these lines are seldom those of the visible superficial faults, but must lie much deeper, and are not generally prolonged upward to the surface. The frequent recurrence of volcanic outbursts at successive geological periods from the same or adjacent vents seems to point to the existence of lines or points of weakness deep down in the crust, within reach of the internal molten magma, but far beneath the horizon of the stratified formations at the surface, with their more superficial displacements.
While sometimes running in lines, old volcanic vents of the Vesuvian and Puy types often occur also in scattered groups. Two or three may be found together within an area of a few hundred yards. Then may come an interval where none, or possibly only a solitary individual, may appear. And beyond that space may rise another sporadic group. These features are well exhibited by the Carboniferous and Permian series of Scotland, to the account of which the reader is referred.
A large neck may have a number of smaller ones placed around it, just as a modern Vesuvian cone has smaller parasitic cones upon its flanks. An instructive example of this arrangement is to be seen at the great vent of the Braid Hills belonging to the Lower Old Red Sandstone and described in Chapter xx. Other instances may be cited from the Carboniferous and Permian volcanic series (see Figs. 90, 148, 213).
Not infrequently the irregularities in the ground-plan of a neck, as already remarked, may be accounted for on the supposition that they mark the site of more than one vent. Sometimes, indeed, it is possible to demonstrate the existence of two or even more vents which have been successively opened nearly on the same spot. The first orifice having become choked up, another has broken out a little to one side, which in turn ceasing to be effective from the same or some other cause, has been succeeded by a third (Fig. 29). The three cones and craters of the little island of Volcanello supply a singularly perfect recent instance of this structure (Fig. 214). Here the funnel has twice shifted its position, each cone becoming successively smaller and partially effacing that which preceded it. In Auvergne, the Puy de Pariou has long been celebrated as an example of a fresh cinder-cone partially effacing an earlier one. In the much denuded Palæozoic volcanic tracts of Britain, where the cones have long since disappeared and only the stumps of the volcanic cylinders are left, many illustrations occur of a similar displacement of the funnel, especially among the volcanoes of the Carboniferous system.
Among the irregularities of necks that may indicate a connection with lines of fissure, reference may be made here to dykes or dyke-like masses of agglomerate which are sometimes to be seen among the volcanic districts of Britain. In these cases the fragmentary materials, instead of lying in a more or less cylindrical pipe, appear to fill up a long fissure. We may suppose that the explosions which produced them did actually occur in fissures instead of in ordinary vents. The remarkable Icelandic fissures with their long rows of cinder cones are doubtless, at least in their upper parts, largely filled up with slag and scoriæ. Some illustrations of this structure will be given in the account of the Carboniferous volcanic rocks of Scotland (see No. 1 in Fig. 22).
There is yet another consideration in regard to the form and size of necks which deserves attention. Where the actual margin of a neck and its line of vertical junction with the rocks through which it has been drilled can be seen, there is no room for dispute as to the diameter of the original funnel, which must have been that of the actual neck. But in many cases it is impossible to observe the boundary; not merely because of superficial soil or drift, but occasionally because the volcanic detritus extends beyond the actual limits of the funnel. In such cases the necks have retained some portion of the original volcanic cone which accumulated on the surface around the eruptive vent. It may even chance that what appears to be a large neck would be considerably reduced in diameter, and might be shown to include more than one pipe if all this outer casing could be removed from it. In Fig. 30, for example, a section is given of a neck (n) from which on the right-hand side all the cone and surrounding tuffs (t) have been removed by denudation, the original form of the volcano being suggested by the dotted lines. On the left side, however, the tuffs which were interstratified with the contemporaneous sediments are still connected with the neck, denudation not having yet severed them from it. The overlying strata (l, l) which originally overspread the extinct volcano have been bent into an anticline, and the neck of the vent has thus been laid bare by the removal of the crest of the arch.
The instances where a structure of this kind is concealed are probably fewer in number in proportion to their antiquity. But among Tertiary cones they may perhaps not be so rare. The possibility of their occurrence should be kept in view during the investigation of extinct volcanoes. The term Neck ought not properly to be applied to such degraded volcanic cones. The true neck still remains preserved in the inside of them. As illustrative of the structure here referred to, I may cite the example of the Saline Hill (Fig. 148) and of Largo Law (Fig. 226), both in Fife.
iv. Metamorphism in and around Volcanic Vents--Solfataric Action
The prolonged ascent of hot vapours, stones, dust and lava, in the funnel of a volcano must necessarily affect the rocks through which the funnel has been driven. We may therefore expect some signs of alteration in the material forming the walls of a volcanic neck. The nature of the metamorphism will no doubt depend, in the first place, on the character and duration of the agents producing it, and in the second, on the susceptibility of the rocks to undergo change. Mere heat will indurate rocks, baking sandstone, for instance, into quartzite, and shales into porcellanite. But there will almost invariably be causes of alteration other than mere high temperature. Water-vapour, for instance, has probably always been one of the most abundant and most powerful of them. The copious evolution of steam from volcanoes is one of their most characteristic features at the present day, and that it was equally so in past time seems to be put beyond question by the constantly recurring vesicular structure in ancient lavas and in the lapilli and ejected blocks of old agglomerates and tuffs. Direct experiment has demonstrated, in the hands of various skilful observers, from the time of Sir James Hall to that of Professor Daubrée, how powerfully rocks are acted upon when exposed to superheated vapour of water under great pressure. But the steam of volcanoes often contains other vapours or mineralizing agents dissolved in it, which increase its metamorphic influence. The mineral acids, for instance, must exert a powerful effect in corroding most minerals and rocks. At the Solfatara of Naples and at other volcanic orifices in different parts of Italy, considerable alteration is seen to be due to this cause.
Bearing these well-known facts in mind, we may be prepared to find various proofs of metamorphism around and within old volcanic vents. The surrounding rocks are generally much hardened immediately contiguous to a neck, whether its materials be fragmental or massive. Sandstones, for example, are often markedly bleached, acquire the vitreous lustre and texture of quartzite, lose their usual fissility, break irregularly into angular blocks, and on an exposed surface project above the level of the unaltered parts beyond. Shales are baked into a kind of porcelain-like substance. Coal-seams are entirely destroyed for economic purposes, having been burnt into a kind of cinder or fused into a blistered slag-like mass. Limestones likewise lose their usual bluish-grey tint, become white and hard, and assume the saccaroid texture of marble.
The distance to which this metamorphism extends from the wall is, among the exposed necks in Britain, smaller than might be anticipated. Thus I have seldom been able to trace it among those of Carboniferous or Permian age for more than 15 or 20 yards in ordinary arenaceous and argillaceous strata, even where every detail of a neck and its surroundings has been laid bare in plan upon a beach. The alteration seems to reach furthest in carbonaceous seams, such as coals.
It is evident that the element of time must enter into the question of the amount of metamorphism produced in the terrestrial crust immediately surrounding a volcanic pipe. A volcano, of which the eruptions begin and end within an interval of a few days or hours, cannot be expected to have had much metamorphic influence on the rocks through which its vent was opened. On the other hand, around a funnel which served for many centuries as a channel for the escape of hot vapours, ashes or lava to the surface, there could hardly fail to be a considerable amount of alteration. The absence or comparatively slight development of metamorphism at the Carboniferous and Permian necks of Scotland may perhaps be regarded as some indication that these volcanoes were generally short-lived. On the other hand, more extensive alteration may be taken as pointing to a longer continuance of eruptive vigour.
The same causes which have induced metamorphism in the rocks surrounding a volcanic vent might obviously effect it also among the fragmentary materials by which the vent may have been filled up. When the eruptions ceased and the funnel was left choked with volcanic debris, hot vapours and gases would no doubt still continue for a time to find their way upward through the loose or partially compacted mass. In their ascent they would permeate this material, and in the end produce in it a series of changes similar to, and possibly even more pronounced than, those traceable in the walls of the vent. Instances of this kind of metamorphism will be cited in the following chapters (see in particular p. 404).
v. Inward Dip of Docks towards Necks
One concluding observation requires to be made regarding the relation of old volcanic necks to the rocks which immediately surround them. Where a vent has been opened through massive rocks, such as granite, felsite, andesite or basalt, it is generally difficult or impossible to determine whether there has been any displacement of these rocks, beyond the disruption of them caused by the explosions that blew out the orifice. But where the pipe has been drilled through stratified rocks, especially when these still lie nearly flat, the planes of stratification usually supply a ready test and measure of any such movement. Investigation of the volcanic rocks of Britain has shown me that where any displacement can be detected at a neck, it is almost invariably in a downward direction. The strata immediately around the vent tend to dip towards it, whatever may be their prevalent inclination in the ground beyond (Fig. 24). This is the reverse of the position which might have been expected. It is so frequent, however, that it appears to indicate a general tendency to subsidence at the sites of volcanic vents. After copious eruptions, large cavernous spaces may conceivably be left at the roots of volcanoes, and the materials that have filled the vents, losing support underneath, will tend to gravitate downwards, and if firmly welded to their surrounding walls may drag these irregularly down with them. Examples of such sagging structures are abundantly to be seen among the dissected vents of the Carboniferous and Permian volcanic series of Scotland.
vi. Influence of Contemporaneous Denudation upon Volcanic Cones
It must be remembered that former vents, except those of the later geological periods, are revealed at the surface now only after extensive denudation. As a rule, the volcanoes that formed them appeared and continued in eruption during periods of general subsidence, and were one by one submerged and buried beneath subaqueous deposits. We can conceive that, while a volcanic cone was sinking under water, it might be seriously altered in form and height by waves and currents. If it consisted of loose ashes and stones, it might be entirely levelled, and its material might be strewn over the floor of the sea or lake in which it stood. But, as has been already pointed out, the destruction of the cone would still leave the choked-up pipe or funnel from which the materials of that cone had been ejected. Though, during the subsidence, every outward vestige of the actual volcano might disappear, yet the agglomerate or lava that solidified in the funnel underneath would remain. And if these materials had risen some way within the cone or crater, or if they reached at least a higher level in the funnel than the surrounding water-bottom or land-surface, the destruction of the cone might leave a projecting knob or neck to be surrounded and covered by the accumulating sediments of the time. It is thus evident that the levelling of a cone of loose ashes during gradual subsidence, and the deposition of a contemporary series of sedimentary deposits, might give rise to a true neck, which would be coeval with the geological period of the volcano itself.
In practice it is extremely difficult to decide how far any now visible neck may have been reduced to the condition of a mere stump or core of a volcano before being buried under the stratified accumulations of its time. In every case the existence of the neck is a proof of denudation, and perhaps, in most cases, the chief amount of that denudation is to be ascribed not to the era of the original volcano, but to the comparatively recent interval that has elapsed since, in the progress of degradation, the volcanic rocks, after being long buried within the crust, were once more laid bare by the continuous waste and lowering of the level of the land.
vii. Stages in the History of old Volcanic Vents
Let us now try to follow the successive stages in the history of a volcano after its fires had quite burnt out, and when, slowly sinking in the waters of the sea or lake wherein it had burst forth, it was buried under an ever-growing accumulation of sedimentary material. The sand, mud, calcareous ooze, shell-banks, or whatever may have been the sediment that was gathering there, gradually crept over the submerged cone or neck, and would no doubt be more or less mixed with any volcanic detritus which waves or currents could stir up. If the cone escaped being levelled, or if it left a projecting neck, this subaqueous feature would be entombed and preserved beneath these detrital deposits. Hundreds or thousands of feet of strata might be laid down over the site of the volcano, which would then remain hidden and preserved for an indefinite period, until in the course of geological revolutions it might once again be brought to the surface.
These successive changes involve no theory or supposition. They must obviously have taken place again and again in past time. That they actually did occur is demonstrated by many examples in the British Isles. I need only refer here to the interesting cases brought to light by mining operations in the Dairy coal-fields of Ayrshire, which are more fully described in Chapter xxvii. (p. 433). In that district a number of cones of tuff, one of which is 700 feet in height, have been met with in the course of boring and mining for ironstone and coal. The well-known mineral seams of the coal-field can be followed up to and over these hidden hills of volcanic tuff which in the progress of denudation have not yet been laid bare (Fig. 146).
The subsidence which carried down the water-bottom and allowed the volcanic vents to be entombed in sedimentary deposits may have been in most cases tolerably equable, so that at any given point these deposits would be sensibly horizontal. But subsequent terrestrial disturbances might seriously affect this regularity. The sedimentary formations, piled above each other to a great depth, and acquiring solidity by compression, might be thrown into folds, dislocated, upheaved or depressed. The buried volcanic funnels would, of course, share in the effects of these disturbances, and eventually might be so squeezed and broken as to be with difficulty recognizable. It is possible that some of the extreme stages of such subterranean commotions are revealed among the "Dalradian" rocks of Scotland. Certain green schists which were evidently originally sediments, and probably tuffs, are associated with numerous sills and bosses of eruptive material. The way in which these various rocks are grouped together strikingly suggests a series of volcanic products, some of the crushed bosses recalling the forms of true necks in younger formations. But they have been so enormously compressed and sheared that the very lavas which originally were massive amorphous crystalline rocks have passed into fissile hornblende-schists.
Among the Palæozoic systems of Britain, however, where considerable fracture and displacement have taken place, examples of successive stages in the reappearance of buried volcanic cones and necks may be gathered in abundance. As an illustrative diagram of the process of revelation by the gradual denudation of an upheaved tract of country, Fig. 31 may be referred to (compare also Fig. 147).
Here three volcanic vents are represented in different stages of re-emergence. In the first (A) we see a cone and funnel which, after having been buried under sedimentary deposits (s, s,) have been tilted up by subterranean movements. The overlying strata have been brought within the influence of denudation, and their exposed basset edges along the present surface of the land (g, g) bear witness to the loss which they have suffered. Already, in the progress of degradation, a portion of the volcanic materials which, ejected from that vent, were interstratified with the contemporaneous sediments of the surrounding sea-floor, has been exposed at t. A geologist coming to that volcanic intercalation would be sure that it pointed to the existence of some volcanic vent in the neighbourhood, but without further evidence he would be unable to tell whether it lay to right or left, whether it was now at the surface or lay still buried under cover of the stratified deposits which were laid down upon it.
In the second or central example (B) we have a pipe and cone which have been similarly disturbed. But in this case denudation has proceeded so far as to reveal the cone and even to cut away a portion of it, as shown by the dotted lines to the right hand. Owing, however, to the general inclination of the rocks towards the left, that side of the cone, together with the tuffs or lavas connected with it, still lies buried and protected under cover of the sedimentary formations (s, s).
The third example (C) shows a much more advanced stage of destruction. Here the whole of the cone has been worn away. All the lavas and tuffs which were ejected from it towards the right have likewise disappeared, and strata older than the eruptions of this vent now come to the surface there. To the left, however, a little portion of its lavas still remains at l, though all the intervening volcanic material has been removed. That solitary fragment of the outpourings of this volcano once extended further to the left hand, but the occurrence of the large dislocation (f) has carried this extension for down below the surface. The vent in this instance, owing to its position, has suffered more from denudation than the other two. Yet, judged by the size of its neck, it was probably larger than either of them, and threw out a more extensive pile of volcanic material. Its funnel has been filled with agglomerate (a), through which a central plug of lava (p) has ascended, and into which dykes or veins (d, d), the last efforts of eruption, have been injected.
This diagram will serve to illustrate the fact already so often insisted on, that although denudation may entirely remove a volcanic cone, and also all the lavas and tuffs which issued from it, the actual filled-up pipe cannot be so effaced, but is practically permanent.
Chapter VI
Underground Phases of Volcanic Action--continued. II. Subterranean Movements of the Magma: i. Dykes and Veins; ii. Sills and Laccolites; iii. Bosses (Stocks, Culots)--Conditions that govern the Intrusion of Molten Rock within the Terrestrial Crust.
II. Subterranean Movements of the Magma
In the foregoing pages attention has been more specially directed to those aspects of volcanic energy which reveal themselves above ground and in eruptive vents. We have now to consider the various ways in which the molten magma is injected into the crust of the earth.
Such injection must obviously take place during the expulsion of volcanic materials to the surface. If the explosive violence of an eruption, or the concomitant movements of the earth's crust, should lead to ruptures among the subterranean rocks, the molten magma will be forced into these rents. It is evident that this may happen either with or without any discharge of lava at the surface. It may be either entirely a plutonic, that is, a deep-seated phenomenon, or it may be part of a truly volcanic series of events.
It is clear that, by the study of old volcanoes that have had their structure laid bare by denudation, we may hope to obtain fresh light in regard to some of the more deeply-seated features of volcanic energy, which in a modern volcano are entirely concealed from view. A little reflection will convince us that the conditions for consolidation within the crust are so different from those at the surface that we may expect them to make themselves visible in the internal characters of the rocks.
An essential distinction between underground propulsions of molten rock and superficial outflows of the same material lies in the fact that while the latter are free to take any shape which the form and slope of the ground may permit, the subterranean injections, like metal poured into a mould, are always bounded by the walls of the aperture into which they are thrust. According, therefore, to the shape of this aperture a convenient classification of such intrusions may be made. Where the molten material has risen up vertical fissures or irregular cracks, it has solidified as Dykes and Veins. Where it has been thrust between the divisional planes either of stratified or unstratified rocks, so as to form beds, these are conveniently known as Sills, Laccolites or Intrusive Sheets. Where it has taken the form of large cylindrical masses, which, ascending through the crust, appear at the surface in rounded, elliptical or irregularly-shaped eminences, these are called Bosses (Stocks, Culots).
Further contrasts between the superficial and subterranean consolidation of molten material are to be found in the respective textures and minute structures of the rocks. The deep-seated intrusions are commonly characterized by a general and markedly greater coarseness of crystallization than is possessed by lavas poured out at the surface. This difference of texture, obviously in great measure the result of slower cooling, shows itself in acid, intermediate, and basic magmas. A lava which at the surface has cooled as a fine-grained, compact black basalt, in which neither with the naked eye nor with the lens can the constituent minerals be distinctly determined, may conceivably be represented at the roots of its parent volcano by a coarse-textured gabbro, in which the felspars and pyroxenes may have grown into crystals or crystalline aggregates an inch or more in length. Mr. Iddings has pointed out that the various porphyrites which form the dykes and sills of Electric Peak are connected with a central boss of coarsely crystalline diorite.[29] Examples of the same relation from different volcanic centres in Britain will be cited in later chapters.
[Footnote 29: 12th Ann. Rep. U.S. Geol. Survey (1890-91), p. 595.]
This greater coarseness of texture is shown by microscopic examination to be accompanied by other notable differences. In particular, the glassy residuum, or its devitrified representatives, which may be so frequently detected among the crystals of outflowing lavas, is less often traceable in the body of subterranean intrusive rocks, though it may sometimes be noticed at their outer margins where they have been rapidly chilled by contact with the cool upper part of the crust into which they have been impelled. Various minerals, the constituents of which exist in the original magma, but which may be hardly or not all recognisable in the superficial lavas, have had leisure to crystallize out in the deep-seated intrusions and appear sometimes among the components of the general body of the rock, or as well-terminated crystals in its drusy cavities.
Considerable though the variations may be between the petrographical characters of the intrusive and extrusive rocks of a given district and of the same eruptive period, they appear generally to lie within such limits as to suggest a genetic relation between the whole series. Conditions of temperature and pressure, and the retention or escape of the absorbed vapours which play so large a part in volcanic activity, must exercise great influence on the crystallization of constituent minerals, and on the consolidation and ultimate texture of the rocks. Slow cooling under great pressure and with the mineralizing vapours still largely retained seems to be pre-eminently favourable for the production of a holocrystalline texture in deep-seated portions of the magma, while rapid cooling under merely atmospheric pressure and with a continuous disengagement of vapours, appears to be required for the finer grain, more glassy structure, and more vesicular character of lavas poured out at the surface.
Besides these differences, however, there is evidence of a migration of the constituent minerals in the body of large intrusive masses before consolidation. In particular, the heavier and more basic constituents travel towards the cooling margin, leaving the central portions more acid. This subject will be more fully considered in connection with the internal constitution of Bosses, and some British examples will then be cited.
Reference, however, may here be made to one of the most exhaustive and instructive studies of the relations of the subterranean and superficial erupted rocks of an old volcano, which will be found in the monograph by Mr. Iddings on Electric Peak and Sepulchre Mountain in the Yellowstone Park of Western America. From the data there obtainable he draws the deduction that one parent magma, retaining the same chemical composition, may result in the ultimate production of rocks strikingly different from each other in structure and mineralogical constitution, yet chemically identical. Electric Peak includes the central funnel filled up with coarsely crystalline diorite, and having a connected series of sills and dykes of various porphyrites. Sepulchre Mountain, separated from its neighbouring eminence by a fault of 4000 feet, displays some of the superficial discharges from the vent--coarse breccias with andesite-lavas. These rocks are not chemically distinguishable from the intrusive series, but the lavas are, on the whole, more glassy, while the materials of the bosses, sills and dykes are more crystalline. The latter display much more visible quartz and biotite.[30]
[Footnote 30: 12th Ann. Rep. U.S. Geol. Survey, 1890-91. As already stated, the eruptions of this volcanic centre became progressively more acid, and this change appears to be exhibited by the extrusive lavas as well as by the intrusive rocks.]
By practice in the field, supplemented by investigation with the aid of the microscope, a geologist acquires a power of discriminating with fair accuracy, even in hand specimens, the superficial from the subterranean igneous rocks of an old volcanic district.
Denudation, while laying bare the underground mechanism of an ancient volcano, has not always revealed the evidence of the actual structural relations of the rocks, or has first exposed and then destroyed it. Sometimes a mass of eruptive rock has been worn down and left in such an isolated condition that its connection with the rest of the volcanic network cannot be determined. So far as its position goes, it might perhaps be either a remnant of a lava-stream or the projecting part of some deeper-seated protrusion. But its texture and internal structure will often enable a confident opinion to be expressed regarding the true relations of such a solitary mass.
i. Dykes and Veins
For the study of these manifestations of volcanic energy, the British Isles may be regarded as a typical region. It was thence that the word "dyke" passed into geological literature. Thousands of examples of both dykes and veins may be seen from the Outer Hebrides southwards across the length and breadth of the southern half of Scotland, far into the north of England and towards the centre of Ireland. They may be found cutting the crests of the mountains and extending as reefs below the level of the sea. They are thus exposed in every conceivable divergence of position and in endless varieties of enclosing rock. Moreover, they can be shown to represent a vast range of geological time. One system of them belongs to some remote part of the Archæan periods, another is as young as the older Tertiary ages.
Full details regarding these interesting relics of volcanic activity will be given in later chapters, especially in Chapters xxxiv. and xxxv. It may suffice here to note that each of the three types of old volcanoes above described has, in Britain, its accompaniment of dykes and veins. The plateaux, however, present by far the most abundant and varied development of them. The dykes of this series are characterized not only by their prodigious numbers in and around some of the plateaux, but by the long distances to which they may be traced beyond these limits. They are chiefly found in connection with the Tertiary basalt-plateaux, though the Carboniferous andesite-plateaux present a feebler display of them. The Tertiary dykes are pre-eminently distinguished by their persistent rectilinear lines, sometimes for distances of many miles, and their general north-westerly direction. They form a vast system extending over an area of some 40,000 square miles. Throughout that wide region their persistence of direction and of petrographical characters point to the former existence of one or more reservoirs of an andesitic and basaltic magma underneath the northern half of Britain, and to the rupture of the crust overlying this subterranean reservoir by thousands of parallel fissures. They thus constitute perhaps the most astonishing feature in the volcanic history of Tertiary time.
The dykes and veins connected with the puys are mainly to be found at or close to the vents. Not infrequently they traverse the agglomerates of the necks, and are sometimes to be traced to a central pipe or core of basalt.
The larger cones are likewise intersected with similar vertical, inclined or tortuously irregular walls of intruded lava. Occasionally a radiate arrangement may be observed in such cases, like that noticeable at some modern volcanoes, the dykes diverging from the eruptive centre.
Many dykes exist regarding which there is no evidence to connect them with any actual volcanic rocks. They have been injected into fissures, but whether this took place during volcanic paroxysms, or owing to some subterranean movements which never culminated in any eruption, cannot be decided.
The question of the age of dykes, like that of intrusive masses of all kinds, is often difficult or impossible to decide. A dyke must of course be younger than the rocks which it traverses, and a limit to its antiquity is thus easily fixed. But we cannot always affirm that because a dyke stops short of a particular rock, or series of rocks, it is older than these. The Hett Dyke, in the north of England, rises through the Coal-measures, but stops at the Magnesian Limestone; yet this cessation does not necessarily imply that the dyke was in place before the deposition of that limestone. The structure may have arisen from the dyke-fissure having ended at the bottom of the limestone. Where dykes rise up to the base of an unconformable formation without in any single case entering it, and where fragments of them are enclosed in that formation, they must be of higher antiquity, and must have been laid bare by extensive denudation before the unconformable strata were deposited upon them. The great system of dykes in the Lewisian Gneiss of the north-west of Scotland is in this way proved to be much more ancient than the Torridon Sandstones under which it passes (Figs. 35, 36).
Where two dykes cross each other, it is sometimes not difficult to decide upon their relative antiquity. In intrusive rocks, the finest-grained parts are those which lie nearest the outer margin, where the molten material was rapidly chilled by coming in contact with cool surfaces of rock. Such "chilled margins" of closer grain are common characteristics of dykes. Wherever a dyke carries its chilled margin across another dyke, it must be the younger of the two, and wherever such a margin is interrupted by another dyke, it must belong to the older.
As a rule, the uprise of molten material in a fissure has so effectually sealed it up that in the subsequent disturbances of the terrestrial crust the fissure has not been reopened, though others may have been produced near it, or across it. Sometimes, however, the enormous tension to which the crust was exposed opened the fissure once more, sometimes even splitting a dyke along its centre, and a new ascent of molten rock took place within the rent. Hence double or treble or compound dykes have been produced. The second or later infillings are generally somewhat different from the original dyke. Occasionally, indeed, they present a strong contrast to it. Thus, among the dykes of Skye examples occur where the centre is occupied by an acid granophyre, while the sides are occupied by dykes of basalt. Instances of this compound type of dyke will be given in the account of the Tertiary volcanic rocks of Britain.
It is obvious that in a wide fissure the central portion may remain molten for some time after the sides have consolidated. If the fissure served as a channel for the ascent of lava to the surface, it is conceivable that the central still fluid part might be driven out and be replaced by other material from below, and that this later material might differ considerably in composition from that which first filled the opening. Such, according to Mr. Iddings, has been the probable history of some of the dykes at the old volcano of Electric Peak.[31] But we can hardly suppose that this explanation of compound dykes can have any wide application. It could only hold good of broad fissures having an outlet, and is probably inadmissible in the case of the numerous compound dykes not more than 10 or 15 feet in diameter, where the several bands of rock are sharply marked off from each other. The abrupt demarcation of the materials in these dykes, their closer texture along their mutual boundaries, the indications of solution of the older parts of the group by the younger, and of injection of the latter into the former, show that they belong to separate and unconnected intrusions. These questions will be again referred to in the account of the British Tertiary dykes (Chapter xxxv. vol. ii. p. 159).
[Footnote 31: 12th Ann. Rep. U.S. Geol. Survey (1890-91), p. 587.]
Another kind of compound dyke has arisen from the manner in which the original fissure has been produced. While, in general, the dislocation has taken the form of a single rectilinear rent, which on opening has left two clean-cut walls, cases occur where the rupture has followed several parallel lines, and the magma on rising into the rents appears as two or more vertical sheets or dykes, separated by intervening partitions of the surrounding rock. Examples of this structure are not infrequent among the Tertiary dykes of Scotland. One of these may be noticed rising through the cliffs of Lewisian gneiss on the east coast of the island of Lewis, south of Stornoway. One of the most extraordinary instances of the same structure yet observed is that described by Professor A. C. Lawson from the Laurentian rocks at the mouth of White Gravel River, on the N.E. coast of Lake Superior. In a breadth of only about 14 feet no less than 28 vertically intrusive sheets or dykes of diabase, from 1 inch to 6½ inches broad, rise through the granite, which is thus split into 27 thin sheets. The diabase undoubtedly cuts the granite, some of the sheets actually anastomosing and sending veins into the older rock.[32]
[Footnote 32: American Geologist (1894), p. 293.]
From the evidence supplied by the modern eruptions of Iceland, it is evident that gaping fissures, which are filled by ascending lava and thereby converted into dykes, in many instances serve as channels by which molten rock escapes to the surface. It would be interesting if any test could be discovered whereby those dykes could be distinguished which had ever established a connection with the outer air. If the lava continued to ascend in the fissures, and to pour out in superficial streams for a long time, the rocks on either side would be likely to undergo considerably more metamorphism than where there was only one rapid injection of the magma, which would soon cool. Possibly in the much greater alteration of the same rocks by some dykes than by others, a sign of such a connection with the surface may survive. This subject will be again referred to in the account of the Tertiary dykes of Britain in Book VIII., where the whole of the phenomena of this phase of volcanic action will be fully discussed (see vol. ii. p. 163).
ii. Sills and Laccolites
The word "sill," derived from a remarkable sheet of eruptive rock in the north of England, known as the Great Whin Sill (Chapter xxix.), is now applied as a convenient general term to masses of intrusive material, which have been injected between such divisional planes as those of stratification, and which now appear as sheets or beds (Fig. 33). These masses are likewise called Intrusive Sheets, and where the injected material has accumulated in large blister-like expansions, these are known as Laccolites (Fig. 34).
Sills vary from only an inch or two up to 500 feet or more in thickness. Lying, as they frequently do, parallel with strata above and below them, they resemble in some respects true lava-sheets erupted contemporaneously with the series of sediments among which they are intercalated. And, indeed, cases occur in which it is hardly possible to decide whether to regard a given mass as a sill or as a superficial lava. In general, however, sills exhibit the coarser texture above referred to as specially characteristic of subterranean eruptive masses. Moreover they are usually, though not always, free from the vesicular and amygdaloidal structures of true surface-lavas. Their under and upper surfaces, unlike the more scoriaceous parts of lavas, are commonly much closer in grain than the general body of the mass; in other words, they possess chilled borders, the result of more rapid consolidation by contact with cooler rock. Again, instead of conforming to the stratification of the formations among which they lie, as truly interstratified lavas do, they may be seen to break across the bedding and pursue their course on a higher or lower platform. The strata that overlie them, instead of enclosing pieces of them and wrapping round irregularities on their surface, as in the case of contemporaneously erupted lava-sheets, are usually indurated, sometimes even considerably altered, while in many cases they are invaded by veins from the eruptive sheet, or portions of them are involved in it, and are then much hardened or metamorphosed.
The petrographical character of the sills in a volcanic district depends primarily on the constitution of the parent magma, whence both they and the outflowing lavas have issued. Where the lavas are rhyolites or felsites the sills are acid, where basalts have been erupted the sills are basic, though there has often been a tendency towards the appearance of more acid material, such as trachyte. As we have seen, considerable differences in petrographical characters may arise between the intrusive and extrusive offshoots from the same parent magma during the course of a volcanic cycle. This question will be more appropriately discussed together with the leading characters of Bosses.
Between the upper and under surface of a thick sill considerable petrographical variation may sometimes be observed, especially where the rock is of basic constitution. Differences both of texture and even to some extent of composition can be detected. Sometimes what have been called "segregation veins" traverse the mass, consisting of the same minerals as the general body of the rock, but in larger crystals and in somewhat different proportions. That these veins belong to the period of original consolidation appears to be shown by the absence of fine-grained, chilled margins, and by the way in which the component crystals of the veins are interlocked with those of the body of the rock. Other veins of finer grain and more acid composition probably belong to a later phase of consolidation, when, after the separation and crystallization of the more basic minerals, the more acid mother liquor that remained was, in consequence of terrestrial movements, injected into cracks in the now solidified, though still highly heated, rock. Examples of these features will be cited from various geological formations in the following chapters.
Reference has already been made to the difference occasionally perceptible between the constitution of the upper and that of the under portions of superficial lavas. A similar variation is sometimes strongly marked among sills, especially those of a basic character, the felspars remaining most abundant above, while the olivines and augites preponderate below. Mr. Iddings has observed some excellent illustrations of this character in the great series of sills connected with the volcanic pipe of Electric Peak in the Yellowstone country.[33] Some examples of the same structure will subsequently be cited from the Carboniferous volcanic series of Central Scotland.
[Footnote 33: "Electric Peak and Sepulchre Mountain," _12th Ann. Rep. U.S. Geol. Survey_ (1890-91), p. 584.]
The greatest extreme of difference which I have observed in the petrographical characters of any group of sills is that displayed by the Tertiary gabbros of Skye. These rocks occur as sheets interposed among the bedded basalts, and injected between each other in such a manner as to form thick piles of rudely stratified sills. They possess a remarkable banded structure, due to the aggregation of their component minerals in distinct layers, some of which are dark in colour, from the abundance of their iron-ore, pyroxene and olivine; while others are light-coloured, from the predominance of their felspar. From the manner in which the component minerals of one band interlace with those of the contiguous bands, it is quite certain that the structure is not due to successive injections of material among already consolidated rocks, but belongs to the original conditions of expulsion of the gabbro as a whole. It seems to indicate that the magma which supplied the sills was at the time of its extrusion heterogeneous in composition, and that the banding arises from the simultaneous or rapidly successive protrusion of different portions of this variously-constituted magma. The details of the structure will be described in the general account to be given of the Tertiary volcanic rocks (Chapters xliii. and xliv.).
Besides such visible differences in the composition of sills, others much less obtrusive may occasionally be detected with the aid of microscopic or chemical research. The outer parts of some sills are thus discovered to be more basic or more acid than the inner portions. Or evidence may be obtained pointing to the probable melting down of surrounding rocks by the erupted magma, with a consequent local change in the chemical and mineralogical constitution of the mass.
In regard to their position in the geological structure of an old volcanic district I may here remark that sills, seldom entirely absent, are more especially developed either among the rocks through which the volcano has driven its vent, or about the base of the erupted lavas and tuffs. Many illustrations of this distribution will be described from the various volcanic areas of Britain belonging to Palæozoic and Tertiary time. At the base of the great Cambrian and Lower Silurian volcanic series of Merionethshire, sills are admirably developed, while among the basaltic eruptions which closed the long volcanic record in the north of Ireland and the Inner Hebrides, they play a notable part.
From the frequent place which sills take at the base of a volcanic series, it may be inferred that they generally belong to a late phase in the history of an eruptive episode or cycle, when the orifices of discharge had become choked up, and when the volcanic energy found an easier passage laterally between the strata underneath the volcanic pile or between the sheets of that pile itself, than upward through the ever-increasing thickness of ejected material.
While there is an obvious relation between most sills and some eruptive centre in their neighbourhood, cases occur in which no trace of any contemporaneous volcano can be found, but where the intrusive sheet remains as the sole evidence of the movements of the subterranean magma. The Great Whin Sill, one of the most extensive intrusive sheets in the British Isles, is an instance of this kind. Though this large mass of injected material can be traced for a distance of about 80 miles, and though the strata beneath and above it are well exposed in innumerable sections, no evidence has yet been detected to show that it was connected with any vent that formed a volcano at the surface (see vol. ii. p. 2). The absence of this evidence may, of course, arise from the failure of denudation to uncover the site of the vent, which may possibly still remain buried under the Carboniferous strata that overlie the sill towards the south-east. But it may be due to the non-existence of any such vent. We can quite conceive that volcanic energy should sometimes have failed to complete the formation of an actual volcano. Aided by subterranean movements, it might have been potent enough to disrupt the lower parts of the terrestrial crust, to propel the molten magma into fissures, even to inject it for many miles between the planes of stratification, which would be lines of least resistance, and yet in default of available rents, might have been unable to force its way through the upper layers and so reach the surface. Examples of such incompleted volcanoes are perhaps to be recognized among solitary sills, which not infrequently present themselves in the geological structure of Britain. But the positive decision of this question is almost always frustrated by the imperfection of the evidence, and the consequent possibility that a connected vent may still lie concealed under overlying strata.
Besides the more usual intrusions of molten material in the form of sheets of which the vertical thickness bears but a small proportion to the horizontal extent, there occur also large and thick cakes of intruded material in which the vertical thickness may approach, or perhaps even surpass, the horizontal diameter. These dome-shaped or irregular expansions form a connecting link between ordinary sills and the bosses to be subsequently described. They have received the name of Laccolites from Mr. G. K. Gilbert, who worked out this peculiar type of structure in the case of the Henry Mountains in southern Utah[34] (Fig. 34). The same type has since been found distributed over Arizona and Colorado, and it has been recognized as essentially that of many eruptive masses or bosses in all parts of the world.
[Footnote 34: "Geology of the Henry Mountains," _U.S. Geog. and Geol. Survey of the Rocky Mountain Region_, 1877. For a review of the whole subject of laccolites in Western America see a paper by Mr. Whitman Cross, in the _14th Annual Report of the Director of the U.S. Geological Survey_, 1892-93 (pub. 1895), p. 157.]
In Western America, owing in large measure to the previously undisturbed condition of the sedimentary formations, the relations of the injected igneous material to these formations can be satisfactorily ascertained. The geological structure of the various isolated laccolites thus clearly presented, helps to explain the structure of other intrusive bodies which, having been injected among plicated and dislocated rocks, do not so readily admit of interpretation.
In Colorado, Utah and Arizona the eruptive magma, usually a porphyrite, diorite or quartz-porphyry, has risen in one or more pipes, and has then intruded itself laterally between the planes of the sedimentary formations which, over the centre of intrusion, have been pushed upward into a vast dome-shaped or blister-like elevation. The horizon on which this lateral and vertical expansion of the intruded material took place would seem to have lain several thousand feet below the surface. It ranges from the Cambrian to the Tertiary formations. Subsequent denudation has cut down the upraised mantle of sedimentary layers, and has revealed more or less of the igneous rock underneath, which is thus allowed to protrude and to be affected by atmospheric erosion. In this way, wide plains of horizontal or gently undulating Secondary and Tertiary strata have been diversified by the appearance of cones, detached or in groups, which have become more peaked and varied in outline in proportion as their original sedimentary covering has been removed from them. The largest of the laccolitic masses in the Henry Mountains is about 7000 feet deep and about 4 miles in diameter. Less than one-half of the cover of overarching strata has been removed, and denudation has cut deeply into the remaining part.
That the type of structure, so well exhibited among the Henry Mountains, has not been more abundantly recognized elsewhere probably arises from the fact not that it is rare, but that the conditions for its development are seldom so favourable as in Western America. Obviously where stratified rocks have been much disturbed, they cease to furnish definite or regular platforms for the reception of eruptive material, and to afford convenient datum-lines for estimating what was probably the shape of the intruded magma. We may believe that the effect of the propulsion of eruptive material is usually to upheave the overlying crust, and thus to give rise to a laccolitic form of intrusion. The upheaval relatively to the surrounding country will be apt to be practically permanent, the intruded body of rock being welded to the surrounding formations, and forming in this way a solid and resisting core directly united by pipes or funnels with the great magma-reservoir underneath. On the other hand, where the molten rock, instead of consolidating underground, has been copiously discharged at the surface, its emission must tend towards the production of cavernous spaces within the crust. The falling in of the roofs of such caverns will give rise to shocks of earthquakes. Subsequent uprisings of the magma may fill these spaces up, and when the rock has solidified in the form of laccolites or bosses, it may effectually put an end there to further eruptions.
Some contact metamorphism may be observed along the upper and under surfaces of large sills. The rocks over the American laccolites have sometimes been highly altered. But as the change is the same in kind as that attendant upon Bosses, though generally less in degree, it will be considered with these intrusive masses. The problems in terrestrial physics suggested by the intrusion of such thick and persistent masses of eruptive material as those which form sills and laccolites will likewise be discussed in connection with the mechanism of the remaining intrusive masses which have now to be described.
iii. Bosses (Stocks, Culots)
The term Boss has been applied to masses of intrusive rock which form at the surface rounded, craggy or variously-shaped eminences, having a circular, elliptical or irregular ground-plan, and descending into the terrestrial crust with vertical or steeply-inclined sides (Fig. 28). Sometimes they can be seen to have pushed the surrounding rocks aside. In other places they seem to occupy the place of these rocks through which, as it were, an opening has been punched for the reception of the intrusive material.
Occasionally, more especially in the case of large bosses, like those in which granite so frequently appears, the eruptive mass may be observed to rise here and there in detached knobs through the surrounding rocks, or to enclose patches of these, in such a manner as to indicate that the large body of eruptive material terminates upward in a very irregular surface, of which only the more prominent parts project through the cake of overlying rocks. In true bosses, unlike sills or laccolites, we do not get to any bottom on which the eruptive material rests. Laccolites, indeed, may be regarded as intermediate between the typical sill and the typical boss. The difference between a laccolite and a boss lies in the fact that the body of the laccolite does not descend into an unknown depth in the crust, but lies upon a platform on which it has accumulated, the magma having ascended by one or more ducts, which generally bear but a small proportion in area to the mass of the laccolite. The boss, on the other hand, is not known to lie on any horizon, nor to proceed from smaller ducts underneath, but plunges as a great pillar or irregular mass, which may frequently be noticed to widen downwards into the crust. There can be no doubt, however, that many masses of eruptive rock, which, according to the definition here given, should be called bosses, would be found to be truly laccolites if their structure below ground could be ascertained. It is obvious that our failure to find any platform on which the body of a boss lies, may arise merely from denudation having been as yet insufficient to lay such a platform bare. It is hardly probable that a boss several miles in diameter should descend as a column of that magnitude to the magma-reservoir from which its material came. More probably it has been supplied through one or more smaller ducts. The large boss now visible at the surface may thus be really a laccolitic expansion on one or more horizons. M. Michel Lévy lays stress on the general widening of granitic bosses as they descend into the crust.[35] While his observations are supported by many illustrations from all parts of the globe, and are probably true of the deeper-seated masses of granite, it is no less true that numerous examples have been met with where a granite boss is sharply marked off from the rocks which it has invaded and on which it may be seen to lie. Apart from the cases where granite seems to form part of a vast internal, once molten mass, into which its encircling gneisses seem to graduate, there are others in which this rock, as now visible, has been injected into the crust as a boss or as a laccolite. Instances will be described in later chapters where such bosses have risen through Cambrian, Silurian, Devonian and Carboniferous formations. It may be said that between such granitic intrusions and volcanic operations no connection can be traced. But reasons will be brought forward in later chapters to regard some of the granitic bosses as parts of the mechanism of Palæozoic volcanoes. It will also be shown that among the intrusive rocks of the Tertiary volcanic series of Britain there occur bosses of truly granophyric and granitic material. Hence, though mainly what is called a "plutonic" rock, granite has made its appearance among the subterranean protrusions of volcanoes.
[Footnote 35: M. Michel Lévy, Bull. Carte Géol. France, No. 35, tome v. (1893), p. 32. The view stated in the text is also that adopted by Prof. Brögger with reference to the granite of the Christiania district. "Die Eruptivgesteine des Kristianiagebietes."]
It is no doubt true that many intrusive masses, which must be included under the general name of bosses, have probably had no connection whatever with volcanic action properly so called. They are plutonic injections, that is, portions of the subterranean magma which have been intruded into the terrestrial crust during its periods of disturbance, and have not been accompanied with any superficial discharges, which are essential in truly volcanic energy. It has been proposed to draw a distinction between such deep-seated intrusions and those which represent volcanic funnels.[36] If this were always practicable it would certainly be desirable. But the distinction is not one that can in every case be satisfactorily drawn. Even in regard to granitic bosses, which may generally be assumed to be plutonic in origin, the British examples just referred to have in all likelihood been connected with undoubted volcanic outbursts. Without, therefore, attempting here to separate the obviously volcanic necks of eruptive material from the probably plutonic bosses, I propose to describe briefly the general characters of bosses considered as a group of intrusive rocks, together with the phenomena which accompany them, and the conditions under which they may have been injected.
[Footnote 36: M. Michel Lévy, Bull. Carte Géol. France, No. 35, tome v. (1893).]
Bosses, whether of plutonic or volcanic origin, are frequently not merely single masses of eruptive rock, but are accompanied with a system of dykes and veins, some of which can be traced directly into the parent-mass, while others traverse it as well as the surrounding rocks. Hence the history of a boss may be considerably more complex than the external form of the mass might suggest.
The petrographical characters of bosses link them with the other underground injections of igneous material, more especially with sills and laccolites. Indeed, on mere lithological grounds no satisfactory line could be drawn between these various forms of intrusive rocks. The larger the mass the more coarsely crystalline it may be expected to be. But the whole range of structure, texture and composition, from those of the narrowest vein to those of the widest boss, constitutes one connected series of gradations.
Acid, intermediate and basic rocks are abundantly displayed among the bosses. Huge masses of granite, granophyre, quartz-porphyry, felsite or rhyolite, represent the acid series. Intermediate varieties consist of trachyte, phonolite, diorite, andesite or other rock. The basic bosses include varieties of gabbro, dolerite, basalt, picrite, and other compounds.
In a boss of large size, a considerable range of texture, composition and structure may often be observed. The rock is generally much coarser in grain than that of thin sills or dykes. Sometimes it exhibits a finer texture along the margin than in the centre, though this variation is not usually so marked as in sills and dykes. The rapidly-chilled and therefore more close-textured selvage seems to have been developed much more fully in small than in large masses of eruptive material. The latter, cooling more slowly, allowed even their marginal parts to retain their heat, and sometimes perhaps even their molten condition, longer than small injections. Some influence must also have been exercised by the temperature of the rocks into which the eruptive material was intruded. Where this temperature was high, as in deep-seated parts of the crust, it would allow the intrusive magma to cool more slowly, and thus to assume a more coarsely crystalline condition. The absence of a close grain round the margins of granitic bosses may be due to this cause.
But a much more important distinction may be traced between the central and marginal parts of some large bosses and thick sills. I have already alluded to the fact that while the middle of a large intrusive mass may be decidedly acid, taking even the form of granite, the outer borders are sometimes found to be much more basic, passing into such a rock as gabbro, or even into some ultra-basic compound. Between these extremes of composition no sharp division is sometimes discoverable, such as might have been expected had the one rock been intruded into the other. The differences graduate so insensibly into each other as to suggest that originally the whole mass of the rock formed one continuous body of eruptive material. It is possible that in some cases the magma itself was heterogeneous at the time of intrusion.[37] But the frequency of the distribution of the basic ingredients towards the outer margin, and the acid towards the centre, points rather to a process of differentiation among the constituents of the boss before consolidation. In some instances the differentiation would appear to have taken place before crystallization to any great extent had set in, because the minerals ultimately developed in the central parts differ from those at the sides. In other cases, the transference of material would seem to have been in progress after the component minerals had crystallized out of the magma, for they are the same throughout the whole intrusive mass, but differ in relative proportions from centre to circumference.[38]
[Footnote 37: The Tertiary gabbros of the Inner Hebrides have already been cited, and will be more fully described in a later chapter as exhibiting the heterogeneousness of an eruptive magma.]
[Footnote 38: See Messrs. Dakyns and Teall, Quart. Journ. Geol. Soc. xlviii. (1892), p. 104; Prof. Brögger, op. cit. 1. (1894), p. 15; Mr. A. Harker, op. cit. p. 320; Prof. Iddings, Journ. Geol. Chicago, i. (1893), p. 833; Bull. Phil. Soc. Washington, ii. (1890), p. 191; 1892, p. 89.]
As illustrations of these features I may cite two good examples, one from Scotland and one from England. The mass of Garabol Hill, in the Loch Lomond district, consists mainly of granite, occupying an area of about 12½ square miles. Messrs. Dakyns and Teall have shown that while the central portions consist of granite, the south-eastern margin affords a remarkable series of intermediate rocks, such as hornblende-biotite-granite, tonalite (quartz-mica-diorite), diorite and augite-diorite, which lead us outwards into highly basic compounds, including wehrlites (olivine-diallage rocks), picrites (olivine-augite rocks), serpentine (possibly representing dunites, saxonites, and lherzolites), and a peculiar rock consisting essentially of enstatite, diallage, brown hornblende and biotite. The authors regard the whole of these widely different rocks as the products of one original magma, the more basic marginal area having consolidated first as peridotites, followed by diorites, tonalites and granites in the order of increasing acidity. The most acid rock in the whole series consists of felspar and quartz, is almost devoid of ferro-magnesian minerals, and occurs in narrow veins in the granite and tonalite. It indicates that after the segregation and consolidation of the whole boss, ruptures occurred which were filled in by the ascent of the very latest and most acid remaining portion of still fluid magma.[39]
[Footnote 39: Messrs. Dakyns and Teall, Quart. Journ. Geol. Soc. xlviii. (1892), p. 104.]
The case of Carrock Fell in Cumberland has been described by Mr. A. Harker, who has ascertained that the gabbro of this boss has in its central portions a specific gravity of less than 2·85 and a silica-percentage sometimes as high as 59·46, whilst its marginal zone gives a specific gravity above 2·95 and a silica-percentage as low as 32·50. The migration of the heavy iron ores towards the margin is readily apparent to the naked eye, and is well established by chemical analysis, the oxides of iron amounting in the centre to 6·24 (Fe_{2}O_{3} 3·60, FeO 2·64), and at the margin to 25·54 (Fe_{2}O_{3} 8·44, FeO 17·10).[40] Neither in this instance nor in that of Garabol Hill has any evidence been noticed which would suggest that the basic and acid rocks belong to different periods of intrusion. They pass so insensibly into each other as to form in each case one graduated mass.
[Footnote 40: Mr. A. Harker, op. cit. p. 320.]
From these and other examples which have been observed, it is difficult to escape the conclusion that the differences between the basic margin and the acid centre are due to some process of segregation or differentiation while the mass was still in a liquid condition, and its constituents could pass from one part of the boss to another. According to Professor Brögger, it may be stated as a general law that differentiation sets in during consolidation, and is determined by, and dependent on, the laws of crystallization in a magma, in so far as the compounds which, on given conditions, would first crystallize out, diffuse themselves towards the cooling margin so as to produce in the contact-stratum a peculiar chemical composition in the still liquid material before crystallization takes place.[41]
[Footnote 41: This general conclusion is stated by Professor Brögger from his investigation of the rocks of Gran, Quart. Journ. Geol. Soc. l. (1894), p. 36.]
If during the process of differentiation, and before consolidation, injections of the magma occur, they may be expected to differ in character according to the portion of the magma from which they are derived. Professor Brögger believes that among the basic eruptive rocks of Gran in the Christiania district, one and the same magma has in the bosses solidified as olivine-gabbro-diabases, and in the dykes as camptonites, bostonites, pyroxenites, hornblendites, and more acid augite-diorites.[42]
[Footnote 42: Quart. Journ. Geol. Soc. l. (1894), p. 35.]
Various opinions have been propounded as to the cause or causes of this so-called differentiation, but none of them are entirely satisfactory. We must await the results of further exploration in the field and of continued research in the laboratory.
What appears to have taken place within a subterranean molten magma which has been propelled into the earth's crust as a boss or laccolite, with or without a connected system of dykes, may possibly be made to throw some light on the remarkable changes in the characters of lavas successively erupted from the same vent during the continuance of a volcanic cycle. Whether or not any such process of differentiation can be proved to take place within a subterranean volcanic reservoir, the sequence of erupted lavas bears a curious resemblance to the order in which the constituents of some large bosses succeed each other from margin to centre. The earliest lavas may be of an intermediate or even basic character, but they generally tend to become more acid. Nevertheless alternations of basic and acid lavas which have been noted in various districts would seem to show that if there be a process of differentiation in the magma-basins, it is not regular and continuous, but liable to interruption and renewal. The return to basic eruptions, which so often marks the close of a volcanic cycle, is likewise not easily explicable on the supposition of continuous differentiation.
Where no sensible evidence of differentiation is traceable in the general body of a large intrusive mass, indications that some such process has there been in progress are perhaps supplied by the more acid dykes or veins, and the so-called "segregation veins," which have been already alluded to as traversing large intrusive masses. Though these portions differ to a greater or less extent in texture and composition from the main substance of the boss, the differences are not such as to prevent us from regarding them as really parts of the same parent magma. The veins, which are more acid than the rock that they traverse, may be regarded as having emanated from some central or deeper-seated part of a boss, which still remained fluid after the marginal or upper portion had consolidated sufficiently far to be capable of being rent open during subterranean disturbance. But that the mass, though coherent enough to be fissured, still remained at a high temperature, may be inferred from the general absence of chilled edges to these veins. The evidence of differentiation supplied by "segregation veins" has been referred to in the case of Sills.
The study of the petrographical variations in the constitution of large eruptive bosses has a twofold interest for the geologist. In the first place, it affords him material for an investigation of the changes which a volcanic magma undergoes during its eruption and consolidation, and thereby provides him with some data for an elucidation of the cause of the sequence of erupted products during a volcanic cycle. In the second place, it yields to him some interesting analogies with the structures of ancient gneisses, and thus helps towards the comprehension of the origin and history of these profoundly difficult but deeply fascinating rocks.
Bosses, like sills, occur in the midst of volcanic sheets, and also as solitary protrusions. Where they rise amidst interstratified lavas and tuffs they may often be recognized as occupying the position of volcanic vents. They are then necks, and their characters in this connection have already been given. Where, however, as so frequently happens, they appear among rocks in which no trace of any contemporaneous volcanic material is to be detected, their relation to former volcanic activity remains uncertain.
Of this doubtful nature some of the most notable examples are supplied by the great granitic bosses which occur so frequently among the older Palæozoic rocks of Britain. The age of these can sometimes be approximately fixed, and is then found to correspond more or less closely with some volcanic episode. Thus the granite-bosses of Galloway, in the south of Scotland, disrupt Upper Silurian strata, but are older than the Upper Old Sandstone. Hence they probably belong to the period of the Lower Old Red Sandstone, which was eminently characterized by the vigour and long continuance of its volcanoes. The granite of Arran and of the Mourne Mountains can be shown by one line of reasoning to be younger than surrounding Carboniferous formations, by other arguments to be probably later than the Permian period, and by a review of the whole evidence to form almost certainly part of the volcanic history of Tertiary time.
But even where it can be shown that the uprise of a huge boss of eruptive material was geologically contemporaneous with energetic volcanic action, this coincidence may not warrant the conclusion that the boss therefore marks one of the volcanic centres of activity. Each example must be judged by itself. There have, doubtless, been many cases of the intrusion of molten material in bosses, as well as in sills, without the establishment of any connection with the surface. Such incompleted volcanoes have been revealed by denudation after the removal of a great thickness of superincumbent rock. The evidence which would have decided the question to what extent any of them became true volcanic vents has thus been destroyed. We can only reason tentatively from a careful collation of all the facts that are now recoverable. Illustrations of this kind of reasoning will be fully given in subsequent chapters.
It has been supposed that a test for the discrimination of a subterranean protrusion from a true volcanic chimney may be found in the condition of the surrounding rocks, which in the case of the prolonged flow of molten matter up a vent would be likely to undergo far more metamorphism than would be the case in the injection of a single eruptive mass.[43] But, as has been already pointed out, no special or excessive metamorphism of the encircling rocks is noticeable around many vents. There is certainly no more alteration contiguous to numerous true necks than around bosses, which there is no reason to suppose ever communicated directly with the surface, and which were probably the result of a single intrusion. We must always remember that the denudation which has revealed these bosses has generally removed the evidence of their upward termination and of their possible connection with any volcanic ejections. Many of them may mark the sites of true vents from which only single eruptions took place. The opening of a volcanic vent does not necessarily imply a prolonged ascent of volcanic material. In a vast number of cases the original eruption was the first and last effort of the volcano, so that in such circumstances there seems no more reason for much alteration of the walls of the chimney than for the metamorphism of the rocks round a boss, laccolite, sill or dyke.
[Footnote 43: See, for example, Mr. Harker, Quart. Journ. Geol. Soc. l. (1894), p. 329.]
The metamorphism produced by intrusions of molten material upon the rocks with which they have come in contact has long been studied. Its amount varies so greatly in different cases that the conditions on which it has specially depended are not easily determined. Three factors have obviously been of great importance--first, the bulk of the intruded material; secondly, the chemical composition and lithological texture and structure of the rocks affected; and thirdly, the constitution and temperature of the invading magma.
1. It is clear that a huge boss of eruptive material will be likely to effect much more alteration of the surrounding rocks than a small boss, sill or dyke. Its initial temperature will probably be higher at the time of its assuming its final place than that of the same material after it has found its way into the narrower space of a thin sill or dyke. It will likewise take much longer to cool. Hence the influence of its heat and its vapours will continue to act long after those of the dyke or sill have ceased to manifest themselves.
2. It is equally evident that much of the resultant metamorphism will depend on the susceptibility of the rocks to change. An obdurate material such as pure quartz-sand, for example, will resist further alteration than mere hardening into quartzite. Shales and mudstones may be indurated into cherty substances of various textures. Limestones and dolomites, on the other hand, may become entirely crystalline, and may even have new minerals, such as garnet, tremolite, pyroxene, etc., developed in them. Hence in comparing the amount of metamorphism attendant on two separate bosses we must always take into account the nature of the rocks in which it has been induced.
3. But perhaps the most effective cause of variation in the nature and amount of contact metamorphism has been the constitution of the eruptive magma. A broad distinction may be drawn between the alteration produced by basic and by acid rocks. The intrusion of basic material has often produced singularly little change, even when the eruptive mass has been of considerable size. The greatest amount of alteration is to be found where the basic boss has caught up and enveloped portions of the surrounding rocks. Thus where the gabbro of Carrock Fell has invaded the basic Lower Silurian lavas of the Lake District, the enveloped portions of the latter show considerable modification. Their groundmass becomes darker and more lustrous, the felspars assume a clearer appearance and lose some of their conspicuous inclusions, the pyroxenic constituents are converted into pale amphibole, and the glassy base disappears. At the actual line of contact the felspars of the lavas have become disengaged from their original matrix, which seems to have been dissolved and absorbed in the gabbro-magma. Brown mica has been exceptionally developed in the altered lava. At the same time, a change is noticeable in the character of the gabbro itself near the contact. Brown mica is there to be seen, though not a constituent of the rock elsewhere. The eruptive material has incorporated the basic groundmass of the lavas, leaving the felspars undissolved.[44]
[Footnote 44: Mr. Harker, Quart. Journ. Geol. Soc. vol. l. (1894), p. 331.]
Much more serious are the changes produced by intrusions of acid material, though here again the metamorphism varies within wide limits, being sometimes hardly perceptible, and in other cases advancing so far as to convert mere sedimentary material into thoroughly crystalline rocks. Small sills and dykes of felsite and granophyre may produce very slight change even upon shales and limestones, as may be seen among the eruptive rocks of Skye and Raasay. Large bosses of granophyre, and still more of granite, have been accompanied with the most extensive metamorphism. Round these eruptive masses every gradation may be traced among sandy and argillaceous sediments, until they pass into crystalline mica-schists, which do not appear to be distinguishable from rocks of Archæan age. Admirable examples of this extreme alteration may be observed around the great granite bosses of Galloway.[45] Again, among calcareous rocks a transition may be traced from dull grey ordinary fossiliferous limestones and dolomites into pure white crystalline marbles, full of crystals of tremolite, zoisite, garnet and other minerals. The alteration of the fossiliferous Cambrian limestones of Strath in Skye by the intrusive bosses of Tertiary granite well illustrates this change.[46]
[Footnote 45: See Explanation to Sheet 9 of the Geological Survey of Scotland, p. 22; Prof. Bonney and Mr. Allport, Proc. Roy. Soc. xvi. (1889); Miss Gardiner, Quart. Journ. Geol. Soc. vol. xlvi. (1890), p. 569.]
[Footnote 46: Macculloch, Trans. Geol. Soc. vol. iii. (1816), p. 1; Description of the Western Isles, vol. i. p. 322. See also _Quart. Journ. Geol. Soc._ vol. xiv. (1857), p. 1; and vol. xliv. (1888), p. 62.]
Without entering further here into the wide subject of contact metamorphism, to which a large literature has now been devoted, we may note the effects which have been produced in the eruptive material itself by its contact with the surrounding rocks. Not only have these rocks been altered, but very considerable modifications have likewise taken place in the active agent of the change.
Sometimes the alteration of the invading material has been effected without any sensible absorption of the mineral constituents of the rocks invaded. This appears to be the case in those instances where sheets of basalt, intruded among coals or highly carbonaceous shales, have lost their compact crystalline character and have become mere clays. In the coal-fields of Britain, where many examples of this change have been noted, the igneous material is known as "white trap." The iron oxides have been in great part removed, or, together with the lime of the component minerals, have been converted into carbonates. Traces of the original felspar crystals may still be detected, but the groundmass has been changed into a dull, earthy, friable and decomposed substance.
Nearly always, however, the alteration of the intrusive magma has resulted from the incorporation of portions of the surrounding rocks. Reference has been made above to the alteration of the Carrock Fell gabbro by the absorption of some of the basic lavas around it. But still more remarkable is the change produced in some acid rocks by the incorporation of basic material into their substance. Professor Sollas has described in great detail a remarkable instance of this effect in the probably Tertiary eruptive rocks of the Carlingford district in the north-east of Ireland. He has ascertained that the eruptive gabbro of that district is older than the granite, for it is traversed by granophyre dykes which enclose pieces of it. The granophyre dykes, on the other hand, often show a lithoidal or chilled margin, which is not visible in the gabbro. He believes that the gabbro is not only older than the acid protrusions, but was already completely solid, traversed by contraction-joints, and probably fractured by earth-movements, before the injection of the granophyric material, which at the time of its intrusion was in a state of extreme fluidity, for it has found its way into the minutest cracks and crevices. He has especially studied the alteration produced by the granophyre upon the enclosed pieces of basic rock. The diallage, isolated from the other constituents of the gabbro, may commonly be seen to have broken up into numerous granules, like the augite grains of basalt, while in some cases biotite and hornblende have been developed with the concomitant excretion of magnetite. The acid rock itself has undergone considerable modification owing to the incorporation of basic material into its substance. Professor Sollas distinguishes the following varieties of the rock:--Biotite-granophyre, biotite-amphibole-granophyre, augite-granophyre, diallage-amphibole-augite-granophyre.[47]
[Footnote 47: Trans. Roy. Irish Acad. xxx. (1894), part xii. p. 477.]
Similar phenomena have been described by Mr. Harker as occurring where granophyre has invaded the gabbro of Carrock Fell.[48] The same observer has more recently detected some interesting examples furnished by injections of Tertiary granophyre in the agglomerates of Skye. The acid rock is roughly estimated by him to have taken up about one-fourth of its bulk of gabbro fragments. He has investigated the minute structure of the rock thus constituted, and has been able to recognize the augite of the original gabbro, in various stages of alteration and completely isolated, the other minerals having been dissolved in the acid magma.[49]
[Footnote 48: Quart. Journ. Geol. Soc. li. (1895), p. 183.]
[Footnote 49: Op. cit. lii. The metamorphism produced upon fragments of different kinds of foreign material enclosed within various igneous rocks has in recent years been studied in great detail by Professor Lacroix--Les Enclaves des Roches Volcaniques, Macon, 1893.]
It is not easy to comprehend the conditions under which large masses of molten material have been injected into the crust of the earth. The two main factors in volcanic action--terrestrial contraction and the energy of the vapours in the magma--have no doubt played the chief part in the process. But the relative share of each and the way in which the enormous load of overlying rock has been overcome are not readily intelligible.
Let us first consider for a moment the pressure of the superincumbent crust under which the injection in many cases took place. The Whin Sill of England may serve as a good illustration of the difficulties of the problem. This notable mass of intrusive rock has been forced between the stratification planes of the Carboniferous Limestone series in one, or sometimes more than one, sheet. It stretches for a horizontal distance of not less than 80 miles with an average thickness of between 80 and 100 feet. From the area over which it can be traced its total extent underground must be at least 400 square miles (see Chapter xxix.).
In any single section the Whin Sill might be supposed to be a truly interstratified sheet, so evenly does it seem to be intercalated between the sedimentary strata. But here and there it diverges upward or downward in such a way as to prove it to be really a vast injected sheet. The age of the injection cannot be precisely fixed. It must be later than the Carboniferous Limestone. There is no trace of any stratigraphical break in the Carboniferous system of the region traversed by the sill. If the injection took place during the Carboniferous period, it does not appear to have been attended with any local disturbance, such as we might suppose would have been likely to accompany the extravasation of so enormous a mass of igneous material. If the date of injection be assigned to the next volcanic episode in the geological history of Britain--that of the Permian period--it will follow that the Whin Sill was intruded into its present position under the superincumbent weight of the whole of the Carboniferous system higher than the platform followed by the injected rock. The overlying body of strata would thus exceed 5000 feet in thickness, or in round numbers would amount at least to an English mile. The pressure of this mass of superincumbent material, at the depth at which the injected magma was forced between the strata, must have been so gigantic that it is difficult to believe that the energy of the magma would have been able to achieve of itself so stupendous a task as the formation of the Great Whin Sill.
The volume of injected material is likewise deserving of special attention. Many sills exceed 300 or 400 feet in thickness; and some laccolites must enormously surpass these limits. The intrusion of so vast a body of new material into the terrestrial crust will necessitate either a corresponding elevation of that part of the crust overlying the injected magma or a subsidence of that part underlying it, or some combination of both movements. It is conceivable that, where the body of protruded magma was large and the thickness of overlying crust was small, the expansive force of the vapours under high tension in the molten rock may have sufficed for the uplift. This result will be most likely to be effected around a volcanic chimney where the magma has the least amount of overlying load, and encounters that relief from pressure which enables it to become a powerful agent in terrestrial physics.
But in the case of the larger bodies of injected rock, especially where they do not seem to have been accompanied by the opening of any volcanic vents, the propulsion of the igneous material into the crust has probably been effected as a consequence of disturbance of the terrestrial crust. When the strain of contraction leads to the pushing upward of the terrestrial areas intervening between wide regions of subsidence, even though the differential movement may be slight, the isogeotherms undergo deformation. The intensely hot nucleus is squeezed upward, and if in the process of compression ruptures take place in the crust, and cavities in it are consequently opened, the magma will at once be forced into them. Such ruptures may be expected to take place along lines of weakness. Rocks will split along their stratification-planes, and the tendency to separation along these lines may be aided by the readiness of the energetic magma to find its way into and to enlarge every available opening. Hence we may expect that, besides vertical fractures, leading to the production of dykes and bosses, there will often be horizontal thrusts and ruptures, which will give rise to the formation of sills.
There is still another feature of terrestrial contraction which may help us to follow the behaviour of the magma within the crust. Plication of the crust is one of the most characteristic results of the contracting strain. Where a great series of sedimentary formations has been violently compressed so that its component strata have been thrown into rapid folds and squeezed into a vertical position, the portion of the crust thus treated may possibly be on the whole strengthened against the uprise of molten material through it. But the folding is often accompanied with dislocation. Not only are the rocks thrown into endless plications, but portions of them are ruptured and even driven horizontally over other parts. Such greatly disturbed areas of the crust are not infrequently found to have been plentifully injected with igneous rocks in the form of dykes, veins, sills, laccolites and bosses.
The elevation of a mountain-chain is known to be accompanied with a diminution of density in the crust underneath. Mr. O. Fisher has suggested that along such lines of terrestrial uplift there may be a double bulge in the crust, one portion rising to form the upheaved land and the other sinking down into the hot nucleus. If the lighter descending crust were there melted it might form a magma ready to be poured out as lava on the opening of any vent. The lava thus ejected would be of the lighter kinds. It has been remarked as certainly a curious fact that the lavas which issue from high mountain ranges are generally much more acid than the heavy basic lavas which are so characteristic of volcanoes close to the level of the sea.
But even where no actual mountain-chain is formed, there are gentle undulations of the crust which no doubt also affect the isogeotherms. If any series of disturbances should give rise to a double system of such undulations, one crossing the other, there would be limited dome-shaped elevations at the intersections of these waves, and if at the same time actual rupture of the crust should take place, the magma might find its way upward under such domes and give rise to the formation of laccolitic intrusions. Cessation of the earth-movements might allow the intruded material slowly to solidify without ever making an opening to the surface and forming a volcano. Doubtless many sills, laccolites and bosses represent such early or arrested stages in volcanic history.
Propelled into the crust at a high temperature, and endowed with great energy from the tension of its absorbed vapours and gases, the magma will avail itself of every rent which may be opened in the surrounding crust, and where it has succeeded in reaching the surface, its own explosive violence may enable it to rupture the crust still further, and open for itself many new passages. Thus an eruptive laccolite or boss is often fringed with veins, dykes and sills which proceed from its mass into the rocks around.
The question how far an ascending mass of magma can melt down its walls is one to which no definite answer can yet be given. Recent observations show that where the difference in the silica percentage between the magma and the rock attacked is great, there may be considerable dissolution of material from this cause. Allusion has already been made to Mr. Harker's computation that some of the acid granophyres of Skye have melted down about a fourth of their bulk of the basic gabbros. If such a reaction should take place between the magma of a boss, sill or laccolite and the rocks among which it has been intruded, great changes might result in the composition of the intruded rock. We are not yet, however, in possession of evidence to indicate that absorption of this kind really takes place on an extensive scale within the earth's crust. If it did occur to a large extent, we should expect much greater varieties in the composition of eruptive rocks than usually occur, and also some observable relation between the composition of the igneous material and that of the rocks into which it has been injected. But enough is not yet known of this subject to warrant any decided opinion regarding it.
Chapter VII
Influence of Volcanic Rocks on the Scenery of the Land--Effects of Denudation.
As considerable popular misapprehension exists respecting the part which volcanism has played in the evolution of the existing topography of the earth's surface, and as the British Isles, from their varied geological structure, offer special facilities for the discussion of this subject, it may not be out of place to devote a final section of the present Introduction to a consideration of the real topographical influence of volcanic action.
With modern, and especially with active, volcanoes we need not here concern ourselves. Their topographical forms are well known, and give rise to no difficulty. The lofty cones of the Vesuvian type, with their widespread lavas and ashes, their vast craters and their abundant parasitic volcanoes; the crowded, but generally diminutive, cones and domes of the puy type, so well displayed in Auvergne, the Eifel and the Bay of Naples; and the vast lava deserts of the plateaux, so characteristically developed in Iceland and Western America, illustrate the various ways in which volcanic energy directly changes the contours of a terrestrial surface.
But the circumstances are altered when we deal with the topographical influence of long extinct volcanoes. Other agencies then come into play, and some caution may be needed in the effort to disentangle the elements of the complicated problem, and to assign to each contributing cause its own proper effect.
Reference has already been made to the continuous denudation of volcanic hills from the time that they are first erupted. But the comparative rapidity of the waste and the remarkable topographical changes which it involves can hardly be adequately realized without the inspection of an actual example. A visit to the back of Monte Somma, already alluded to, will teach the observer, far more vividly than books can do, how a volcanic cone is affected by daily meteoric changes. The sides of such a cone may remain tolerably uniform slopes so long as they are always being renewed by deposits from fresh eruptions. But when the volcanic activity ceases, and the declivities undergo no such reparation, they are rapidly channelled by the descent of rain-water, until the furrows grow by degrees wide and deep ravines, with only narrow and continually-diminishing crests between them. If unchecked by any fresh discharge of volcanic material, the degradation will at last have removed the whole cone.
It is thus obvious that purely volcanic topography, that is, the terrestrial scenery due directly to the eruption of materials from within the earth, can never become in a geological sense very old. It can only endure so long as it is continually renewed by fresh eruptions, or where it is carried down by subsidence under water and is there buried under a cover of protecting sediments. When, therefore, we meet with volcanic rocks of ancient date exposed at the surface, we may be quite certain that their present contours are not those of the original volcano, but have been brought about by the processes of denudation.
It is true that, in the general erosion of the surface of the land, volcanic rocks of ancient date sometimes rise into wonderfully craggy heights, including, perhaps, cones and deep crater-like hollows, which to popular imagination betoken contours left by now extinguished volcanic fires. Examples of such scenery are familiar in various parts of Britain; but the resemblance to recent volcanic topography is deceptive. There are, indeed, a few hills wherein the progress of denudation seems not as yet to have entirely removed the lavas and tuffs that gathered round the original vents. Some of the tuff-cones of eastern Fife, for example, present cases of this kind. Again, the great granophyre domes and cones of the Tertiary volcanic series of the Inner Hebrides, though they have undoubtedly been extensively denuded, may possibly retain contours that do not greatly differ from those which these protruded bosses originally assumed under the mass of rock which has been removed from them. Nevertheless, putting such doubtful exceptions aside, we may confidently affirm that hills composed of ancient volcanic material give no clue to the forms of the original volcanoes.
It can hardly be too often repeated that the fundamental law in the universal decay and sculpture of the land is that the waste is proportioned to the resistance offered to it: the softer rocks are worn down with comparative rapidity, while the harder varieties are left projecting above them. As a general rule, volcanic rocks are more durable than those among which they are interstratified, and hence project above them, but this is not always the case. No universal rule can, indeed, be laid down with regard to the relative durability of any rocks. While, therefore, topographic contours afford a valuable indication of the nature and disposition of the rocks below the surface, they cannot be relied upon as in all circumstances an infallible guide in this respect. No better proof can be offered of the caution that is needed in tracing such contours back to their origin than is furnished by the old volcanic rocks of Britain. These eruptive masses, consisting usually of durable materials and ranging through a vast cycle of geological time, usually rise into prominent features and thus support the general law. But they include also many easily eroded members, which, instead of forming eminences, are worn into hollows. They include, in short, every type of scenery, from featureless plains and rolling lowlands to craggy and spiry mountains.
The first point, then, which is established in an investigation of the topographical influence of old volcanic rocks is that their prevailing prominence arises from relative durability amidst universal degradation. When we proceed further to inquire why they vary so much from each other in different places, and how their complicated details of feature have been elaborated, we soon learn that such local peculiarities have arisen mainly from variations in the internal structure and grouping of the rocks themselves.
Here again the general law of sculpture comes into play. The local features have depended upon the comparative resistance offered to the sculpturing agents by the different portions of a volcanic series. Each distinct variety of rock possesses its own characteristic internal structure. The lines along which atmospheric disintegration will most effectually carry on its carving work are thus already traced in the very substance and architecture of the rock itself. Each rock consequently yields in its own way to the processes of disintegration, and thus contributes its own distinctive share to topographical feature.
Among the massive rocks abundant examples of such special types of weathering may be cited, from the acid and basic series, and from superficial lavas as well as from intrusive bosses and sills. Acid bosses, such as those of granite, granophyre and quartz-porphyry, tend to weather into blocks and finally into sand, and as this tendency is somewhat uniformly distributed through the rocks, they are apt to assume rounded, dome-shaped or conical forms which, at a distance, may seem to have smooth declivities, but on examination are generally found to be covered with a slowly-descending sheet of disintegrated blocks and debris (Fig. 346). When less prone to decay, and especially where traversed by a strongly-defined system of vertical joints, they may shoot up into tower-like heights, with prominent spires and obelisks. Basic bosses, when their materials decay somewhat rapidly, give rise to analogous topographical forms, though the more fertile soils which they produce generally lead to their being clothed with vegetation. Where they consist of an obdurate rock, much jointed and fissured, like the gabbro of the Inner Hebrides, they form exceedingly rugged mountains, terminating upward in serrated crests and groups of aiguilles (Figs. 331, 333).
Acid lavas that have been superficially erupted weather into irregularly craggy hills, like the flanks of Snowdon. Those of intermediate composition, where they have accumulated in thick masses, are apt to weather into conical forms, as may be seen among the Cheviot, Pentland and Garleton Hills (Figs. 109, 110, 133); but where they have been poured out in successive thin sheets they have built up undulating plateaux with terraced sides, as among the Ayrshire and Campsie Fells and the hills of Lorne (Figs. 99, 107). Basic lavas have issued in comparatively thin sheets, frequently columnar or slaggy, forming flat-topped hills and terraced escarpments, such as are typically developed among the Tertiary basalt-plateaux of the Inner Hebrides and the Faroe Islands (Figs. 11, 265, 283, 284, 286).
One of the most frequent causes of local peculiarities of topography among old volcanic rocks is the intercalation of very distinct varieties of material in the same volcanic series. Where, for instance, lavas and tuffs alternate, great inequalities of surface may be produced. The tuffs, being generally more friable, decay faster and give rise to hollows, while the lavas, being more durable, project in bold ridges or rise into mural escarpments (Fig. 265). Again, where dykes weather more readily than the rocks which they traverse, they originate deep narrow clefts, while where they weather more slowly than the rocks around them, they project as dark ribs. Thus in Skye some dykes which rise through the obdurate gabbro are marked by chasms which reach up even to the highest crests of the mountains (Fig. 333), while of those which run in the pale crumbling granophyre, some stand up as black walls that can be followed with the eye across the ridges even from a long distance.
Many further illustrations of these principles might be cited here from the old volcanic districts of Britain. But they will present themselves successively in later chapters. For my present purpose it is enough to show that the scenery of these districts is not directly due to volcanic action, but is the immediate result of denudation acting upon volcanic rocks, modified and directed by their geological structure.
It may, however, be useful, in concluding the discussion of this subject, to cite some typical volcanic regions in the British Isles as illustrations of the relations between geology and topography, which, besides impressing the main lesson here enforced, may serve also to show some of the striking contrasts which geology reveals between the present and former conditions of the surface of the globe. Among these contrasts none are more singular than those offered by tracts where volcanic action has once been rife, and where the picture of ancient geography presented in the rocks differs so widely from the scenery of the same places to-day as to appeal vividly to the imagination.
The first district to which I may refer where ancient volcanic rocks are well developed is that of Devonshire. The story of the Devonian volcanoes will be told in some detail in later chapters, when it will be shown that the eruptions were again and again renewed during a long course of ages. Yet, abundant as the intercalated lavas and tuffs are, they can hardly be said to have had any marked effect on the scenery, though here and there a harder or larger mass of diabase rises into a prominent knoll or isolated hill. When the amount of volcanic material in this region is considered, we may feel some surprise at the trifling influence which it has exerted in the general denudation of the surface.
To one who wanders over the rich champaign of southern Devonshire, and surveys from some higher prominence the undulating tree-crowned ridges that slope down into orchard-filled hollows, and the green uplands that sweep in successive waves of verdure to the distant blue tors of Dartmoor, the scene appears as a type of all that is most peaceful, varied and fertile in English landscape. In the trim luxuriance that meets the eye on every side, the hand of man is apparent, though from many a point of vantage no sound may be heard for a time to show that he himself is anywhere near us. Yet ever and anon from the deep lanes, hidden out of sight under their canopy of foliage, there will come the creak of the groaning waggon and the crack of the waggoner's whip, as evidence that there are roads and human traffic through this bosky silent country.
Amid so much quiet beauty, where every feature seems to be eloquent of long generations of undisturbed repose, it must surely stir the imagination to be told that underneath these orchards, meadows and woodlands lie the mouldering remnants of once active and long-lived volcanoes. Yet we have only to descend into one of the deep lanes to find the crumbling lavas and ashes of the old eruptions. The landscape has, in truth, been carved out of these volcanic rocks, and their decomposition has furnished the rich loam that nourishes so luxuriant a vegetation.
Not less impressive is the contrast presented between the present and former condition of the broad pastoral uplands of the south of Scotland. Nowhere in the British Islands can the feeling of mere loneliness be more perfectly experienced than among these elevated tracts of bare moorland. They have nothing of the grandeur of outline peculiar to mountain tracts. Sometimes, for miles around one of their conspicuous summits, we may see no projecting knob or pinnacle. The rocks have been gently rounded off into broad featureless hills, which sink into winding valleys, each with its thread of streamlet and its farms along the bottom, and its scattered remnants of birch-wood or alder-copse along its slopes and dingles. Across miles of heathy pasture and moorland, on the summits of this great tableland, we may perchance see no sign of man or his handiwork, though the bleating of the sheep and the far-off barking of the collie tell that we are here within the quiet domain of the south-country shepherd.
In this pastoral territory, also, though they hardly affect the scenery, volcanic rocks come to the surface where the foldings of the earth's crust have brought up the oldest formations. Their appearance extends over so wide an area as to show that a large part of these uplands lies on a deeply-buried volcanic floor. A whole series of submarine volcanoes, extending over an area of many hundreds of square miles, and still in great part overlain with the accumulated sands and silts of the sea-bottom, now hardened into stone, underlies these quiet hills and lonely valleys.
A contrast of another type meets us in the broad midland valley of Scotland. Around the city of Edinburgh, for instance, the landscape is diversified by many hills and crags which show where harder rocks project from amidst the sediments of the Carboniferous system. On some of these crags the forts of the early races, the towers of Celt and Saxon, and the feudal castles of the middle ages were successively planted, and round their base clustered for protection the cots of the peasants and the earliest homesteads of the future city. Beneath these crags many of the most notable events in the stormy annals of the country were transacted. Under their shadow, and not without inspiration from their local form and colour, literature, art and science have arisen and flourished. Nowhere, in short, within the compass of the British Isles has the political and intellectual progress of the people been more plainly affected by the environment than in this central district of Scotland.
When now we inquire into the origin and history of the topography which has so influenced the population around it, we find that its prominences are relics of ancient volcanoes. The feudal towers are based on sills and dykes and necks. The fields and gardens, monuments and roadways, overlie sheets of lava or beds of volcanic ashes. Not only is every conspicuous eminence immediately around of volcanic origin, but even the ranges of blue hills that close in the distant view to south and north and east and west are mainly built up of lavas and tuffs. The eruptions of which these heights are memorials belong to a vast range of geological ages, the latest of them having passed away long before the advent of man. But they have left their traces deeply engraven in the rocky framework of the landscape. While human history, stormy or peaceful, has been slowly evolving itself during the progress of the centuries in these fertile lowlands, the crags and heights have remained as memorials of an earlier history when Central Scotland continued for many ages to be the theatre of vigorous volcanic activity.
As a final illustration of the influence of volcanic rocks in scenery, and of the contrast between their origin and their present condition, I may cite the more prominent groups of hills in the Inner Hebrides. In the singularly varied landscapes of that region three distinct types of topography attract the eye of the traveller. These are best combined and most fully developed in the island of Skye. Throughout the northern half of that picturesque island, the ground rises into a rolling tableland, deeply penetrated by arms of the sea, into which it slopes in green declivities, while along its outer borders it plunges in ranges of precipice into the Atlantic. Everywhere, alike on the cliffs and the inland slopes, long parallel lines of rock-terrace meet the eye. These mount one above another from the shores up to the flat tops of the highest hills, presenting level or gently-inclined bars of dark crag that rise above slopes of debris, green sward and bracken. It is these parallel, sharply-defined bars of rock, with their intervening strips of verdure, that give its distinctive character to the scenery of northern Skye. On hillside after hillside and in valley after valley, they reappear with the same almost artificial monotony. And far beyond the limits of Skye they are repeated in one island after another, all down the chain of the Inner Hebrides.
In striking contrast to this scenery, and abruptly bounding it on the south, rise the Red Hills of Skye--a singular group of connected cones. Alike in form and in colour, these hills stand apart from everything around them. The verdure of the northern terraced tableland here entirely disappears. The slopes are sheets of angular debris,--huge blocks of naked stone and trails of sand, amidst which hardly any vegetation finds a footing. The decay of the rock gives it a pale yellowish-grey hue, which after rain deepens into russet, so that in favourable lights these strange cones gleam with a warm glow as if they, in some special way, could catch and reflect the radiance of the sky.
Immediately to the west of these pale smooth-sloped cones, the dark mass of the Cuillin Hills completes the interruption of the northern tableland. In almost every topographical feature these hills present a contrast to the other two kinds of scenery. Their forms are more rugged than those of any other hill-group in Britain (Fig. 331). Every declivity among them is an irregular pile of crags, every crest is notched like a saw, every peak is sharpened into a pinnacle. Instead of being buried under vast sheets of their own debris, these hills show everywhere their naked rock, which seems to brave the elements as few other rocks can do. Unlike the pale Red Hills, they are dark, almost black in tone, though when canopied with cloud they assume a hue of deepest violet.
Each of these three distinct types of topography owes its existence to the way in which a special kind of volcanic rock yields to the influences of denudation. The terraced tableland of the north is built up of hundreds of sheets of basaltic lava, each of the long level ledges of brown rock marking the outcrop of one or more of these once molten streams. The black rugged mass of the Cuillin Hills consists of a vast protruded body of eruptive material, which, in the form of endless sills and bosses of gabbro and dolerite, has invaded the basalt-plateau, and has now been revealed by the gradual removal of the portion of that plateau which it upraised. The pale cones and domes of the Red Hills mark the place of one of the last protrusions in the volcanic history of Britain--that of large masses of an acid magma, which broke through the basalt-plateau and also disrupted the earlier gabbro.
In no part of North-Western Europe has volcanic activity left more varied and abundant records of its operations than in these three contiguous tracts of Skye. It is interesting therefore to note the striking contrast between the former and the present landscapes of the region. The lavas of the basaltic tableland crumble into a rich loam, that in the mild moist climate of the Hebrides supports a greener verdure than any of the other rocks around will yield. The uplands have accordingly become pasture-grounds for herds of sheep and cattle. The strips of lowland along the valleys and in the recesses of the coast-line furnish the chief tracts of arable land in the island, and are thus the main centres of the crofter population. The bays and creeks of the much-indented shores form natural harbours, which in former days attracted the Norse sea-rovers, and supplied them with sites for their settlements. Norse names still linger on headland and inlet, but the spirit of adventure has passed away, and a few poor fishing-boats, here and there drawn up on the beach, are usually the only token that the islanders make any attempt to gather the harvest of the sea.
The mountain groups which so abruptly bound the basalt-plateau on the south, and present in their topographical features such distinctive scenery, comprise a region too lofty, too rugged and too barren for human occupation. The black Cuillins and the pale Red Hills are solitudes left to the few wild creatures that have not yet been exterminated. The corries are the home of the red deer. The gabbro cliffs are haunts of the eagle and the raven. Where patches of soil have gathered in the crannies of the gabbro, alpine plants find their home. In the chasms left by the decay of the dykes between the vertical walls of their fissures, the winter snows linger into summer, and conceal with their thick drifts the mouldering surface of the once molten rock beneath them. On every side and at every turn a mute appeal is made to the imagination by the strange contrasts between the quiet restfulness of to-day, when the sculpture-tools of nature are each busily carving the features of the landscape, and the tumult of the time when the rocks, now so silent, were erupted.
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The general discussion of the subject of Volcanism in this Introduction will, I hope, have prepared the reader who has no special geological training for entering upon the more detailed descriptions in the rest of this treatise. As already stated, the chronological order of arrangement will be followed. Beginning with the records of the earliest ages, we shall follow the story of volcanic action down to the end of the latest eruptions.
Each great geological system will be taken as a whole, representing a long period of time, and its volcanic evolution will be traced from the beginning of the period to the close. Some variety of treatment is necessarily entailed by the wide range in the nature and amount of the evidence for the volcanic history of different ages. But where practicable, an outline will first be given of what can be gathered respecting the physical geography of each geological period in Britain. In the description which will then follow of the volcanic phenomena, an account of the general characters of the erupted rocks will precede the more detailed narrative of the history of the volcanic eruptions in the several regions where they took place. References to the published literature of each formation will be given in the first part of each section, or will be introduced in subsequent pages, as may be found most convenient.