Читать книгу Fragments of Earth Lore: Sketches & Addresses Geological and Geographical - Geikie James - Страница 4

Mountains have long had a fascination for lovers of nature. Time was, however, when most civilised folk looked upon them with feelings akin to horror; and good people, indeed, have written books to show that they are the cursed places of the earth – the ruin and desolation of their gorges and defiles affording indubitable proof of the evils which befell the world when man lapsed from his primitive state of innocence and purity. All this has changed. It is the fashion now to offer a kind of worship to mountains; and every year their solitudes are invaded by devotees – some, according to worthy Meg Dods, “rinning up hill and down dale, knapping the chuckie-stanes to pieces wi’ hammers, like sae mony roadmakers run daft – to see, as they say, how the warld was made” – others trying to transfer some of the beauty around them to paper or canvas – yet others, and these perhaps not the least wise, content, as old Sir Thomas Browne has it, “to stare about with a gross rusticity,” and humbly thankful that they are beyond the reach of telegrams, and see nothing to remind them of the fumun et opes strepitumque Romæ. But if the sentiment with which mountains are regarded has greatly changed, so likewise have the views of scientific men as to their origin and history. Years ago no one doubted that all mountains were simply the result of titanic convulsions. The crust of the earth had been pushed up from below, tossed into great billows, shivered and shattered – the mountains corresponding to the crests of huge earth-waves, the valleys to the intervening depressions, or to gaping fractures and dislocations. This view of the origin of mountains has always appeared reasonable to those who do not know what is meant by geological structure, and in some cases it is pretty near the truth. A true mountain-chain, like that of the Alps, does indeed owe its origin to gigantic disturbances of the earth’s crust, and in such a region the larger features of the surface often correspond more or less closely with the inclination of the underlying rocks. But in many elevated tracts, composed of highly disturbed and convoluted strata, no such coincidence of surface-features and underground structure can be traced. The mountains do not correspond to great swellings of the crust – the valleys neither lie in trough-shaped strata, nor do they coincide with gaping fractures. Again, many considerable mountains are built up of rocks which have not been convoluted at all, but occur in approximately horizontal beds. Evidently, therefore, some force other than subterranean action must be called upon to explain the origin of many of the most striking surface-features of the land.

Every geologist admits – it is one of the truisms of his science – that corrugations and plications are the result of subterranean action. Nor does any one deny that when a true mountain-chain was first upheaved the greater undulations of the folded strata probably gave rise to similar undulations at the surface. Some of the larger fractures and dislocations might also have appeared at the surface and produced mural precipices. So long a time, however, has elapsed since the elevation of even the youngest mountain-chains of the globe that the sub-aërial agents of erosion – rain, frost, rivers, glaciers, etc. – have been enabled greatly to modify their primeval features. For these mountains, therefore, it is only partially true that their present slopes coincide with those of the underlying strata. Such being the case with so young a chain as the Alps, we need not be surprised to meet with modifications on a still grander scale in mountain-regions of much greater antiquity. In many such tracts the primeval configuration due to subterranean action has been entirely remodelled, so that hills now stand where deep hollows formerly existed, while valleys frequently have replaced mountains. And this newer configuration is the direct result of erosion, guided by the mineralogical composition and structural peculiarities of the rocks.

It is difficult, or even impossible, for one who is ignorant of geological structure to realise that the apparently insignificant agents of erosion have played so important a rôle in the evolution of notable earth-features. It may be well, therefore, to illustrate the matter by reference to one or two regions where the geological structure is too simple to be misunderstood. The first examples I shall give are from tracts of horizontal strata. Many readers are doubtless aware of the fact that our rock-masses consist for the most part of the more or less indurated and compacted sediments of former rivers, lakes, and seas. Frequently those ancient water-formed rocks have been very much altered, so as even sometimes to acquire a crystalline character. But it is enough for us now to remember that the crust of the globe, so far as that is accessible to observation, is built up mostly of rocks which were originally accumulated as aqueous sediments. Such being the case, it is obvious that our strata of sandstone, conglomerate, shale, limestone, etc., must at first have been spread out in approximately horizontal or gently inclined sheets or layers. We judge so from what we know of sediments which are accumulating at present. The wide flats of our river valleys, the broad plains that occupy the sites of silted-up lakes, the extensive deltas of such rivers as the Nile and the Po, the narrow and wide belts of low-lying land which within a recent period have been gained from the sea, are all made up of various kinds of sediment arranged in approximately horizontal layers. Now, over wide regions of the earth’s surface the sedimentary strata still lie horizontally, and we can often tell at what geological period they became converted into dry land. Thus, for example, we know that the elevated plateau through which the river Colorado flows is built up of a great series of nearly horizontal beds of various sedimentary deposits, which reach a thickness of many thousand feet. It is self-evident that the youngest strata must be those which occur at the surface of the plateau, and they, as we know, are of lacustrine origin and belong to the Tertiary period. Now, American geologists have shown that since that period several thousands of feet of rock-materials have been removed from the surface of that plateau – the thickness of rock so carried away amounting in some places to nearly 10,000 feet. Yet all that prodigious erosion has been effected since early Tertiary times. Indeed, it can be proved that the excavation of the Grand Ca¤on of the Colorado, probably the most remarkable river-trench in the world, has been accomplished since the close of the Tertiary period, and is therefore a work of more recent date than the last great upheaval of the Swiss Alps. The origin of the ca¤on is self-evident – it is a magnificent example of river-erosion, and the mere statement of its dimensions gives one a forcible impression of the potency of sub-aërial denudation. The river-cutting is about 300 miles long, 11 or 12 miles broad, and varies from 3000 to 6000 feet in depth.

Take another example of what denuding agents have done within a recent geological period. The Faröe Islands, some twenty in number, extend over an area measuring about 70 miles from south to north, and nearly 50 miles from west to east. These islands are composed of volcanic rocks – beds of basalt with intervening layers of fine fragmental materials, and are obviously the relics of what formerly was one continuous plateau, deeply trenched by valleys running in various directions. Subsequent depression of the land introduced the sea to these valleys, and the plateau was then converted into a group of islands, separated from each other by narrow sounds and fiords. Were the great plateau through which the Colorado flows to be partially submerged, it would reproduce on a larger scale the general phenomena presented by this lonely island-group of the North Atlantic. The flat-topped “buttes” and “mesas,” and the pyramidal mountains of the Colorado district would form islands comparable to those of the Faröes. Most of the latter attain a considerable elevation above the sea – heights of 1700, 2000, 2500, and 2850 feet being met with in several of the islands. Indeed, the average elevation of the land in this northern archipelago can hardly be less than 900 feet. The deep trench-like valleys are evidently only the upper reaches of valleys which began to be excavated when the islands formed part and parcel of one and the same plateau – the lower reaches being now occupied by fiords and sounds. It is quite certain that all these valleys are the work of erosion. One can trace the beds of basalt continuously across the bottoms, and be quite sure that the valleys are not gaping cracks or fractures. Now, as the strata are approximately horizontal, it is obvious that the hollows of the surface have nothing whatever to do with undulations produced by earth-movements. The sub-aërial erosion of the islands has resulted in the development of massive flat-topped and pyramidal mountains. These stand up as eminences simply because the rock-material which once surrounded them has been gradually broken up and carried away. Nothing can well be more impressive to the student of physical geology than the aspect presented by these relics of an ancient plateau. Standing on some commanding elevation, such as Nakkin in Suderöe, one sees rising before him great truncated pyramids – built up of horizontal beds of basalt rising tier above tier – the mountains being separated from each other by wide and profound hollows, across which the basalt-beds were once continuous. Owing to the parallel and undisturbed position of the strata, it is not hard to form an estimate of the amount of material which has been removed during the gradual excavation of the valleys. In order to do so we have simply to measure the width, depth, and length of the valleys. Thus in Suderöe, which is 19 miles long and 6 miles broad, the bottoms of the valleys are 1000 feet at least below the tops of the mountains, and some of the hollows in question are a mile in width. Now, the amount of rock worn away from this one little island by sub-aërial erosion cannot be less than that of a mass measuring 10 miles in length by 6 miles in breadth, and 800 feet in thickness. And yet the Faröe Islands are composed of rocks which had no existence when the soft clays, etc., of the London Basin were being accumulated. All the erosion referred to has taken place since the great upheaval of the Eocene strata of the Swiss Alps.

But if the evidence of erosion be so conspicuous in regions composed of horizontal strata, it is not less so in countries where the rocks are inclined at various angles to the horizon. Indeed, the very fact that inclined strata crop out at the surface is sufficient evidence of erosion. For it is obvious that these outcrops are merely the truncated ends of beds which must formerly have had a wider extension. But while the effects produced by the erosion of horizontal strata are readily perceived by the least-informed observer, it requires some knowledge of geological structure to appreciate the denudation of curved or undulating strata. And yet there is really no mystery in the matter. All we have to do is by careful observation to ascertain the mode of arrangement of the rocks – this accomplished, we have no difficulty in estimating the minimum erosion which any set of strata may have experienced. An illustration may serve to make this plain. Here, for example, is a section across a region of undulating strata. Let the line A B represent the surface of the ground, and C D be any datum line – say, the sea-level. An observer at A, who should walk in the direction of B, would cross successively eight outcrops of coal; and, were he incapable of reading the geological structure of the ground, he might imagine that he had come upon eight separate coal-seams. A glance at the section, however, shows that in reality he had met with only two coals, and that the deceptive appearances, which might be misread by an incautious observer, are simply the result of denudation. In this case the tops of a series of curved or arched beds have been removed (as at E), and, by protracting the lines of the truncated beds until they meet, we can estimate the minimum amount of erosion they have sustained. Thus, if the strata between o and p be 300 feet thick, it is self-evident that a somewhat greater thickness of rock must have been removed from the top of the anticlinal arch or “saddleback” at E.

Again, let us draw a section across strata which have been fractured and dislocated, and we shall see how such fractures likewise enable us to estimate the minimum amount of erosion which certain regions have experienced. In we have a series of strata containing a bed of limestone L, and a coal-seam a. The present surface of the ground is represented by the line A B. At F the strata are traversed by a fault or dislocation – the beds being thrown down for say 500 feet on the low side of the fault – so that the coal at a² occurs now at a depth of 500 feet below its continuation at a¹. At the surface of the ground there is no inequality of level – the beds overlying the coal (a²) having been removed by denudation. Were the missing rocks to be replaced, they would occupy the space contained within the dotted lines above the present surface A B. Such dislocations are of common occurrence in our coal-fields, and it is not often that they give rise to any features at the surface. We may thus traverse many level or gently-undulating tracts, and be quite unconscious of the fact that geologically we have frequently leaped up or dropped down for hundreds of feet in a single step. Nay, some Scottish streams and rivers flow across dislocations by which the strata have been shifted up or down for thousands of feet, and in some places one can have the satisfaction of sitting upon rocks which are geologically 3000 yards below or above those on which he rests his feet. In other words, thousands of feet of strata have been removed by denudation from the high sides of faults. These, as I have said, often give rise to no feature at the surface; but, occasionally, when “soft” rocks have been shifted by dislocations, and brought against “hard” rocks, the latter, by better resisting denudation than the former, cause a more or less well-marked feature at the surface, and thus betray the presence of a fault to the geologist. The phenomena presented by faults, therefore, are just as eloquent of denudation as is the truncated appearance of our strata; and only after we have carefully examined the present extension and mutual relations of our rock-masses, their varied inclination, and the size of the dislocations by which they are traversed, can we properly appreciate the degree of erosion which they have sustained. Before we are entitled to express any opinion as to the origin of the surface-features of a country, we must first know its geological structure. Until we have attained such knowledge, all our views as to the origin of mountains are of less value than the paper they are written upon.

I have spoken of the evidence of denudation which we find in our truncated and dislocated rock-masses; there is yet another line of evidence which I may very shortly point out. As every one knows, there exist in this and many other countries enormous masses of igneous rocks, which have certainly been extruded from below. Now, some of these rocks, such as granite, belong to what is called the plutonic class of rocks; they are of deep-seated origin – that is to say, they never were erupted at the surface, but cooled and consolidated at great depths in the earth’s crust. I need not go into any detail to show that this is the case – it is a conclusion based upon incontrovertible facts, and accepted by every practical geologist. When, therefore, we encounter at the actual surface of the earth great mountain-masses of granite, we know that in such regions enormous denudation has taken place. The granite appears at the surface simply because the thick rock-masses under which it solidified have been gradually removed by erosion.

The facts which I have now briefly passed in review must convince us that erosion is one of the most potent factors with which the geologist has to deal. We have seen what it has been able to effect in certain tracts composed of strata which date back to a recent geological period, such as the plateau of the Colorado and the pyramidal mountains of the Faröe Islands. If in regions built up of strata so young as the rocks of those tracts the amount of erosion be so great, we may well expect to meet with evidence of much more extensive denudation in regions which have been subjected for enormously longer periods to the action of the eroding agents.

The study of geological structure, or the architecture of the earth’s crust, has enabled us to group all mountains under these three principal heads: —

1. Mountains of Accumulation

2. Mountains of Elevation

3. Mountains of Circumdenudation

1. Mountains of Accumulation. – Volcanoes may be taken as the type of this class of mountains. These are, of course, formed by the accumulation of igneous materials around the focus or foci of eruption, and their mode of origin is so generally understood, and, indeed, so obvious, that I need do no more than mention them. Of course, they are all subject to erosion, and many long-extinct volcanoes are highly denuded. Some very ancient ones, as those of our own country, have been so demolished that frequently all that remains are the now plugged-up pipes or flues through which the heated materials found a passage to the surface – all those materials, consisting of lavas and ashes, having in many cases entirely disappeared. In former times volcanic eruptions often took place along the line of an extensive fissure – the lava, instead of being extruded at one or more points, welled-up and overflowed along the whole length of the fissure, so as to flood the surrounding regions. And this happening again and again, vast plateaux of igneous rock came to be built up, such as those of the Rocky Mountains, Iceland, the Faröes, Antrim and Mull, Abyssinia and the Deccan. These are called plateaux of accumulation, and all of them are more or less highly denuded, so that in many cases the plateaux have quite a mountainous appearance. Of course, plateaux of accumulation are not always formed of igneous rocks. Any area of approximately horizontal strata of aqueous origin, rising to a height of a thousand feet or more above the sea, would come under this class of plateau – the plateau of the Colorado being a good example. Although that plateau is of recent origin, yet its surface, as we have seen, has been profoundly modified by superficial erosion; and this is true to a greater extent of plateaux which have been much longer exposed to denudation. It is obvious that even mountains and plateaux of accumulation often owe many of their present features to the action of the surface-agents of change.

2. Mountains of Elevation. – We have seen that the strata which enter most largely into the composition of the earth’s crust, so far as that is open to observation, consist of rocks which must originally have been disposed in horizontal or approximately horizontal layers. But, as every one knows, the stratified rocks are not always horizontally arranged. In Scotland they rarely are so. On the contrary, they are inclined at all angles from the horizon, and not infrequently they even stand on end. Moreover, they are often traversed by dislocations, large and small. No one doubts that these tilted and disturbed rocks are evidence of wide-spread earth-movements. And it has been long known to geologists that such movements have happened again and again in this and many other countries where similar disturbed strata occur. Some of these movements, resulting in the upheaval of enormous mountain-masses, have taken place within comparatively recent geological times. Others again date back to periods inconceivably remote. The Pyrenees, the Alps, the Caucasus, the Himalaya, which form the back-bone of Eurasia, are among the youngest mountains of the globe. The Highlands of Scotland and Scandinavia are immeasurably more ancient; they are, in point of fact, the oldest high grounds in Europe, nor are there any mountain-masses elsewhere which can be shown to be older. But while the Alps and other recent mountains of elevation still retain much of their original configuration, not a vestige of the primeval configuration of our own Highlands has been preserved; their present surface-features have no direct connection with those which must have distinguished them in late Silurian times. Our existing mountains are not, like those of the Alps, mountains of elevation.

The structure of a true mountain-chain is frequently very complicated, but the general phenomena can be readily expressed in a simple diagram. Let be a section taken across a mountain-chain, i. e. at right angles to its trend or direction. The dominant point of the chain is shown at B, while A and C represent the low grounds. Now, an observer at A, advancing towards B, would note that the strata, at first horizontal, would gradually become undulating as he proceeded on his way – the undulations getting always more and more pronounced. He would observe, moreover, that the undulations, at first symmetrical, as at a, would become less so as he advanced – one limb of an arch or anticline, as it is termed, being inclined at a greater angle than the other, as at b. Approaching still nearer to B, the arches or anticlines would be seen eventually to bend over upon each other, so as to produce a general dip or inclination of the strata towards the central axis of the chain. Crossing that axis (B), and walking in the direction of the low grounds (C), the observer would again encounter the same structural arrangement, but of course in reverse order. Thus, in its simplest expression, a true mountain-chain consists of strata arranged in a series of parallel undulations – the greater mountain ridges and intervening hollows corresponding more or less closely to the larger undulations and folds of the strata. Now, could these plicated strata be pulled out, could the folds and reduplications be smoothed away, so as to cause the strata to assume their original horizontal position, it is obvious that the rocks would occupy a greater superficial area. We see, then, that such a mountain-chain must owe its origin to a process of tangential or lateral thrusting and crushing. The originally horizontal strata have been squeezed laterally, and have yielded to the force acting upon them by folding and doubling up. It seems most probable that the larger contortions and foldings which are visible in all true mountain-chains, owe their origin to the sinking down of the earth’s crust upon the cooling and contracting nucleus. During such depressions of the crust the strata are necessarily subjected to enormous lateral compression; they are forced to occupy less space at the surface, and this they can only do by folding and doubling-back upon themselves. If the strata are equally unyielding throughout a wide area, then general undulation may ensue; but should they yield unequally, then folding and contortion will take place along one or more lines of weakness. In other words, the pressure will be relieved by the formation of true mountain-chains. Thus, paradoxical as it may seem, the loftiest mountains of the globe bear witness to profound depression or subsidence of the crust. The Andes, for example, appear to owe their origin to the sinking down of the earth’s crust under the Pacific; and so in like manner the Alps would seem to have been ridged up by depression of the crust in the area of the Mediterranean. Mountain-chains, therefore, are true wrinkles in the crust of the earth; they are lines of weakness along which the strata have yielded to enormous lateral pressure.

A glance at the geological structure of the Alps and the Jura shows us that these mountains are a typical example of such a chain; they are mountains of elevation. In the Jura the mountains form a series of long parallel ridges separated by intervening hollows; and the form or shape of the ground coincides in a striking manner with the foldings of the strata. In these mountains we see a succession of symmetrical flexures, the beds dipping in opposite directions at the same angle from the axis of each individual anticline. There each mountain-ridge corresponds to an anticline, and each valley to a syncline, or trough-shaped arrangement of strata. But as we approach the Alps the flexures become less and less symmetrical, until in the Alps themselves the most extraordinary convolutions and intricate plications appear, the strata being often reversed or turned completely upside down.

Though it is true that the slopes of this great mountain-chain not infrequently correspond more or less closely to the slope or inclination of the underlying rocks, it must not be supposed that this correspondence is often complete. Sometimes, indeed, we find that the mountains, so far from coinciding with anticlines, are in reality built up of synclinal or basin-shaped strata; while in other cases deep and broad valleys run along the lines of anticlinal axes. All this speaks to enormous erosion. A study of the geological structure of the Alps demonstrates that thousands of feet of rock have been removed from those mountains since the time of their elevation. A section drawn across any part of the chain would show that the strata have been eroded to such an extent, and the whole configuration so profoundly modified, that it is often difficult, or even impossible, to tell what may have been the original form of the surface when the chain was upheaved. And yet the Alps, it must be remembered, are of comparatively recent age, some of their highly-confused and contorted rocks consisting of marine strata which are of no greater antiquity than the incoherent clays and sands of the London Tertiary basin. Now, when we reflect upon the fact that, in the case of so young a mountain-chain, the configuration due to undulations of the strata has been so greatly modified, and even in many places obliterated, it is not hard to believe that after sufficient time has elapsed – after the Alps have existed for as long a period, say, as the mountains of middle Germany – every mountain formed of anticlinal strata shall have disappeared, and those synclines which now coincide with valleys shall have developed into hills. The reader who may have paid little or no attention to geological structure and its influence upon the form of the ground, will probably think this a strange and extravagant statement; yet I hope to show presently that it is supported by all that we know of regions of folded strata which have been for long periods of time subjected to denudation.

3. Mountains of Circumdenudation. – In countries composed of undulating and folded strata which have been for long ages exposed to the action of eroding agents, the ultimate form assumed by the ground is directly dependent on the character of the rocks, and the mode of their arrangement. The various rock-masses which occur in such a neighbourhood as Edinburgh, for example, differ considerably in their power of resisting denudation. Hence the less readily eroded rocks have come in time to form hills of less or greater prominence. Such is the case with the Castle Rock, Corstorphine Hill, the Braids, the Pentlands, etc. These hills owe their existence, as such, to the fact that they are composed of more enduring kinds of rock than the softer sandstones and shales by which they are surrounded, and underneath which they were formerly buried to great depths. Some hills, again, which are for the most part built up of rocks having the same character as the strata that occur in the adjacent low grounds, stand up as prominences simply because they have been preserved by overlying caps or coverings of harder rocks – rocks which have offered a stronger resistance to the action of the denuding agents. The Lomond Hills are good examples. Those hills consist chiefly of sandstones which have been preserved from demolition by an overlying sheet of basalt-rock.

But the mode in which rocks are arranged is a not less important factor in determining the shape which the ground assumes under the action of the agents of erosion. Thus, as we have already seen, flat-topped, pyramidal mountains, and more or less steep-sided or trench-like valleys, are characteristic features in regions of horizontal strata. When strata dip or incline in one general direction, then we have a succession of escarpments or dip-slopes, corresponding to the outcrops of harder or less readily eroded beds, and separated from each other by long valleys, hollows, or undulating plains, which have the same trend as the escarpments. This kind of configuration is well exemplified over a large part of England. The general dip or inclination of the Mesozoic or Secondary strata throughout that country, between the shores of the North Sea and the English Channel, is easterly and south-easterly – so that the outcrops of the more durable strata form well-defined escarpments that face the west and north-west, and can be followed almost continuously from north to south. Passing from the Malvern Hills in a south-easterly direction, we traverse two great escarpments – the first coinciding with the outcrop of the Oolite, and forming the Cotswold Hills; and the second corresponding to the outcrop of the Chalk, and forming the Chiltern Hills. The plains and low undulating tracts that separate these escarpments mark the outcrops of more yielding strata – the low grounds that intervene between the Cotswolds and the Malvern Hills being composed of Liassic and Triassic clays and sandstones. In Scotland similar escarpments occur, but owing to sudden changes of the dip, and various interruptions of the strata, the Scottish escarpments are not so continuous as those of the sister-country. Many of the belts of hilly ground in the Scottish Lowlands, however, exemplify the phenomena of escarpment and dip-slope. Thus, the Sidlaws in Forfarshire consist of a series of hard igneous rocks and interbedded sandstones and flags – the outcrops of which form a succession of escarpments with intervening hollows. The same appearances recur again and again all over the Lowlands. Wherever, indeed, any considerable bed of hard rock occurs in a series of less enduring strata – the outcrop of the harder rock invariably forms a well-marked feature or escarpment. As examples, I may refer to Salisbury Crags, Craiglockhart Hill, Dalmahoy Crags, the Bathgate Hills, King Alexander’s Crag, etc. All these are conspicuous examples of the work of denudation – for it can be demonstrated that each of these rock-masses was at one time deeply buried under sandstones and shales, and they now crop out at the surface, and form prominent features simply because the beds which formerly covered and surrounded them have been gradually removed.

From what has now been said it will be readily understood that in regions composed of strata the inclination or dip of which is not constant but continually changing in direction, the surface-features must be more or less irregular. If the strata dip east the outcrops of the harder beds will form escarpments facing the west, and the direction of the escarpments will obviously change with the direction of the dip. Undulating strata of variable composition will, in short, give rise to an undulating surface, but the superficial undulations will not coincide with those of the strata. On the contrary, in regions consisting of undulating strata of diverse consistency the hills generally correspond with synclinal troughs – or, in other words, trough-shaped strata tend to form hills; while, on the other hand, arch-shaped or anticlinal strata most usually give rise to hollows. This remarkable fact is one of the first to arrest the attention of every student of physical geology, and its explanation is simple enough. An anticlinal arrangement of strata is a weak structure – it readily succumbs to the attacks of the denuding agents; a synclinal arrangement on the contrary, is a strong structure, which is much less readily broken up. Hence it is that in all regions which have been exposed for prolonged periods to sub-aërial denudation synclinal strata naturally come to form hills, and anticlinal strata valleys or low grounds. In the case of a mountain-chain so recently elevated as that of the Alps, the mountain-ridges, as we have seen, often coincide roughly with the greater folds of the strata. Such anticlinal mountains are weakly built, and consequently rock-falls and landslips are of common occurrence among them – far more common, and on a much larger scale, than among the immeasurably older mountains of Scandinavia and Scotland. The valleys of the Pyrenees, the Alps, and the Apennines, are cumbered with enormous chaotic heaps of fallen rock-masses. From time to time peaks and whole mountain-sides give way, and slide into the valleys, burying hamlets and villages, and covering wide tracts of cultivated land. Hundreds of such disastrous rock-falls have occurred in the Alps within historical ages, and must continue to take place until every weakly-formed mountain has been demolished. The hills and mountains of Scotland have long since passed through this phase of unstable equilibrium. After countless ages of erosion our higher grounds have acquired a configuration essentially different from that of a true mountain-chain. Enormous landslips like that of the Rossberg are here impossible, for all such weakly-constructed mountains have disappeared.

A little consideration will serve to show how such modifications and changes have come about. When strata are crumpled up they naturally crack across, for they are not elastic. During the great movements which have originated all mountains of elevation, it is evident that the strata forming the actual surface of the ground would often be greatly fissured and shattered along the crests of the sharper anticlinal ridges. In the synclinal troughs, however, although much fissuring would take place, yet the strata would be compelled by the pressure to keep together. Now, when we study the structure of such a region as the Alps, we find that the tops of the anticlines have almost invariably been removed, so as to expose the truncated ends of the strata – the ruptured and shattered rock-masses having in the course of time been carried away by the agents of erosion. Such mountains are pre-eminently weak structures. Let us suppose that the mountains represented in the diagram consist of a succession of strata, some of which are more or less permeable by water, while others are practically impermeable. It is obvious that water soaking down from the surface will find its way through the porous strata (p), and come out on the slopes of the mountains along the joints and cracks (c) by which all strata are traversed. Under the influence of such springs and the action of frost, the rock at the surface will eventually be broken up, and ever and anon larger and smaller portions will slide downwards over the surface of the underlying impermeable stratum. The undermining action of rivers will greatly intensify this disintegrating and disrupting process. As the river deepens and widens its valley (v), it is apparent that in doing so it must truncate the strata that are inclined towards it. The beds will then crop out upon the slopes of the valley (as at b, b), and so the conditions most favourable for a landslip will arise. Underground water, percolating through the porous beds (p), and over the surface of the underlying impermeable beds (i, i, i), must eventually bring about a collapse. The rocks forming the surface-slopes of the mountain will from time to time give and slide into the valley, or the whole thickness of the truncated strata may break away and rush downwards; and this process must continue so long as any portion of the anticlinal arch remains above the level of the adjacent synclinal troughs.

Thus it will be seen that an anticlinal arch is a weak structure – a mountain so constructed falls a ready prey to the denuding agents; and hence in regions which have been exposed to denudation for as long a period as the Scottish or Scandinavian uplands, a mountain formed of anticlinally arranged strata is of very exceptional occurrence. When it does appear, it is only because the rocks of which it is composed happen to be of a more enduring character than those of the adjacent tracts. The Ochil Hills exemplify this point. These hills consist of a great series of hard igneous rocks, which are arranged in the form of a depressed anticlinal arch – the low grounds lying to the north and south being composed chiefly of sandstones and shales. Here it is owing to the more enduring character of the igneous rocks that the anticlinal arch has not been entirely removed. We know, however, that these igneous rocks were formerly buried under a great thickness of strata, and that their present appearance at the surface is simply the result of denudation.

If an anticlinal arch be a weak structure, a synclinal arrangement of strata is quite the opposite. In the case of the former each bed has a tendency to slip or slide away from the axis, while in a syncline it is just the reverse – the strata being inclined towards and not away from the axis. Underground water, springs, and frost are enabled to play havoc with anticlinal strata, for the structure is entirely in their favour. But in synclinal beds the action of these powerful agents is opposed by the structure of the rocks – and great rock-falls and landslips cannot take place. Synclinal strata therefore endure, while anticlinal strata are worn more readily away. Even in a true mountain-range so young as the Alps, denudation has already demolished many weakly-built anticlinal mountains, and opened up valleys along their axes; while, on the other hand, synclinal troughs have been converted into mountains. And if this be true of the Alps, it is still more so of much older mountain-regions, in which the original contours due to convolutions of the strata have entirely disappeared.

The mountains of such regions, having been carved out and modelled by denuding agents, are rightly termed mountains of circumdenudation, for they are just as much the work of erosion as the flat-topped and pyramidal mountains which have been carved out of horizontal strata. The Scottish Highlands afford us an admirable example of a mountainous region of undulating and often highly-flexed strata, in which the present surface-features are the result of long-continued erosion. As already remarked, this region is one of the oldest land-surfaces in the world. In comparison with it, the Pyrenees, the Alps, and the Himalayas are creations of yesterday. The original surface or configuration assumed by the rocks composing our Highland area at the time when these were first crushed and folded into anticlines and synclines had already been demolished at a period inconceivably more remote than the latest grand upheaval of the Alps. Even before the commencement of Old Red Sandstone times, our Archæan, Cambrian, and Silurian rocks had been planed down for thousands of feet, so that the bottom beds of the Old Red Sandstone were deposited upon a gently undulating surface, which cuts across anticlines and synclines alike. In late Silurian and early post-Silurian times the North-west Highlands probably existed as a true mountain-chain, consisting of a series of parallel ranges formed by the folding and reduplication of the strata. The recent observations of my friends, Professor Lapworth and Messrs. Peach and Horne, in Sutherland, have brought to light the evidence of gigantic earth-movements, by which enormous masses of strata have been convoluted and pushed for miles out of place. We see in that region part of a dissected mountain-chain. The mountain-masses which are there exposed to view are the basal or lower portions of enormous sheets of disrupted rock, the upper parts of which have been removed by denudation. In a word, the mountains of Sutherland are mountains of circumdenudation – they have been carved out of elevated masses by the long-continued action of erosion. To prove this, one has only to draw an accurate section across the North-west Highlands, when it becomes apparent that the form or shape of the ground does not correspond or coincide with the convolutions of the strata, and that a thickness of thousands of feet of rock has been denuded away since those strata were folded and fractured. All over the Highlands we meet with similar evidence of enormous denudation. The great masses of granite which appear at the surface in many places are eloquent of the result produced by erosion continued for immeasurable periods of time. Every geologist knows that granite is a rock which could only have been formed and consolidated at great depths. When, therefore, such a rock occurs at the surface, it is evidence beyond all doubt of prodigious erosion. The granite has been laid bare by the removal of the thick rock-masses underneath which it cooled and consolidated.

A glance at any map of Scotland will show that many river-valleys, and not a few lakes, of the Highlands have a north-east and south-west trend. This trend corresponds to what geologists call the strike of the strata. The rocks of the Highlands have been compressed into a series of folds or anticlines and synclines, which have the direction just stated – namely, north-east and south-west. A careless observer might therefore rashly conclude that these surface-features resembled those of the Jura – in other words, that the long parallel hollows were synclinal troughs, and that the intervening ridges and high grounds were anticlinal arches or saddle-backs. Nothing could be further from the truth. A geological examination of the ground would show that the features in question were everywhere the result of denudation, guided by the petrological character and geological structure of the rocks. Several of the most marked hollows run along the backs of anticlinal axes, while some of the most conspicuous mountains are built up of synclinal or trough-shaped strata. Ben Lawers, and the depression occupied by Loch Tay, are excellent examples; and since that district has recently been mapped in detail by Mr. J. Grant Wilson, of the Geological Survey, I shall give a section to show the relation between the form of the ground and the geological structure of the rocks. This section speaks for itself. Here evidently is a case where “valleys have been exalted and mountains made low.” A well-marked syncline, it will be observed, passes through Ben Lawers, while Loch Tay occupies a depression scooped out of an equally well-defined anticline – a structure which is just the opposite of that which we should expect to find in a true mountain-chain. It will be also noted that Glen-Lyon coincides neither with a syncline nor a fault; it has been eroded along the outcrops of the strata. Many of the north-east and south-west hollows of the Highlands indeed run along the base of what are really great escarpments – a feature which, as we have seen, is constantly met with in every region where the strata “strike” more or less steadily in one direction. In the Highlands the strata are most frequently inclined at considerable angles, so that the escarpments succeed each other more rapidly than would be the case if the strata were less steeply inclined. In no case does any north-east and south-west hollow coincide with a structural cavity. Loch Awe has been cited as an example of a superficial depression formed by the inward dip of the strata on either side. But, as was shown many years ago by my brother, A. Geikie,5 this lake winds across the strike of the strata. Moreover, if it owed its existence to a great synclinal fold, why, he asks, does it not run along the same line as far as the same structure continues? It does not do so: it is not continuous with the synclinal fold, while vertical strata appear in the middle of the lake, where, as my brother remarks, they have clearly no business to be if the sides of the lake are formed by the inward dip of the schists.

The Great Glen, as I mentioned in the preceding article, coincides with a fracture or dislocation – a line of weakness along which the denuding agents had worked for many ages before the beginning of Old Red Sandstone times; and it is possible that smaller dislocations may yet be detected in other valleys. But in each and every case the valleys as we now see them are valleys of erosion; in each and every case the mountains are mountains of circumdenudation; they project as eminences because the rock-masses which formerly surrounded them have been gradually removed. We have only to protract the outcrops of the denuded strata – to restore their continuations – to form some faint idea of the enormous masses of rock which have been carried away from the surface of the Highland area since the strata were folded and fractured. All this erosion speaks to the lapse of long ages. The mountains of elevation which doubtless at one time existed within the Highland area had already, as we have seen, suffered extreme erosion before the beginning of Old Red Sandstone times, much of the area having been converted into an undulating plateau or plain, which, becoming submerged in part, was gradually overspread by the sedimentary deposits of the succeeding Old Red Sandstone period. Those sediments were doubtless derived in large measure from the denudation of the older rocks of the Highlands, and since they attain in places a thickness of 20,000 feet, and cover many square miles, they help us to realise in some measure the vast erosion the Highland area had sustained before the commencement of the Carboniferous period. Nor must we forget that the Old Red Sandstone formation which borders the Highlands has itself experienced excessive denudation: it formerly had a much greater extension, and doubtless at one time overspread large tracts of the Highlands. Again, we have to remember that during the Carboniferous and Permian periods, and the later Mesozoic and Cainozoic eras, the Highlands probably remained more or less continuously in the condition of land. Bearing this in mind, we need not be surprised that not a vestige of the primeval configuration brought about by the great earth-movements of late Silurian times has been preserved. Indeed, had the Highland area, after the disappearance of the Old Red Sandstone inland seas, remained undisturbed by any movement of elevation or depression, it must long ago have been reduced by sub-aërial erosion to the condition of a low-lying undulating plain. But elevation en masse from time to time took place, and so running water and its numerous allies have been enabled to carry on the work of denudation.

Thus in the geological history of the Scottish Highlands we may trace the successive phases through which many other elevated tracts have passed. The Scandinavian plateau, and many of the mountains of middle Germany – such, for example, as the Harz, the Erzgebirge, the Thüringer-Wald, etc. – show by their structure that they have undergone similar changes. First we have an epoch of mountain-elevation, when the strata are squeezed and crushed laterally, fractured and shattered – the result being the production of a series of more or less parallel anticlines and synclines, or, in other words, a true mountain-chain. Next we have a prolonged period of erosion, during which running water flows through synclinal troughs, works along the backs of broken and shattered anticlines, and makes its way by joints, gaping cracks, and dislocations, to the low grounds. As time goes on, the varying character of the rocks and the mode of their arrangement begin to tell: the weaker structures are broken up; rock-falls and landslips ever and anon take place; anticlinal ridges are gradually demolished, while synclines tend to endure, and thus grow, as it were, into hills, by the gradual removal of the more weakly-constructed rock-masses that surround them. Valleys continue to be deepened and widened, while the intervening mountains, eaten into by the rivers and their countless feeders, and shattered and pulverised by springs and frosts, are gradually narrowed, interrupted, and reduced, until eventually what was formerly a great mountain-chain becomes converted into a low-lying undulating plain. Should the region now experience a movement of depression, and sink under the sea, new sedimentary deposits will gather over its surface to a depth, it may be, of many hundreds or even thousands of feet. Should this sunken area be once more elevated en masse – pushed up bodily until it attains a height of several thousand feet – it will form a plateau, composed of a series of horizontal strata resting on the contorted and convoluted rocks of the ancient denuded mountain-chain. The surface of the plateau will now be traversed by streams and rivers, and in course of time it must become deeply cleft and furrowed, the ground between the various valleys rising into mountain-masses. Should the land remain stationary, its former fate shall again overtake it; it will inevitably be degraded and worn down by the sub-aërial agents of erosion, until once more it assumes the character of a low-lying undulating plain.

Through such phases our Highlands have certainly passed. At a very early epoch the Archæan rocks of the north-west were ridged up into great mountain-masses, but before the beginning of the pre-Cambrian period wide areas of those highly-contorted rocks had already been planed across, so that when subsidence ensued the pre-Cambrian sandstones were deposited upon a gently undulating surface of highly convoluted strata. Another great epoch of mountain-making took place after Lower Silurian times, and true mountain-ranges once more appeared in the Highland area. We cannot tell how high those mountains may have been, but they might well have rivalled the Alps. After their elevation a prolonged period of erosion ensued, and the lofty mountain-land was reduced in large measure to the condition of a plain, wide areas of which were subsequently overflowed by the inland seas of Old Red Sandstone times – so that the sediments of those seas or lakes now rest with a violent unconformity on the upturned and denuded edges of the folded and contorted Silurian strata. At a later geological period the whole Highland area was elevated en masse, forming an undulating plateau, traversed by countless streams and rivers, some of which flowed in hollows that had existed before the beginning of Old Red Sandstone times. Since that epoch of elevation the Highland area, although subject to occasional oscillations of level, would appear to have remained more or less continuously in the condition of dry land. The result is, that the ancient plateau of erosion has been deeply incised – the denuding agents have carved it into mountain and glen – the forms and directions of which have been determined partly by the original surface-slopes of the plateau, and partly by the petrological character of the rocks and the geological structure of the ground.

Thus, in the evolution of the surface-features of the earth, the working of two great classes of geological agents is conspicuous – the subterranean and the sub-aërial. The sinking down of the crust upon the cooling nucleus would appear to have given rise to the great oceanic depressions and continental ridges, just as the minor depressions within our continental areas have originated many mountain-chains. In the area undergoing depression the strata are subjected to intense lateral pressure, to which they yield along certain lines by folding up. The strata forming the Alps, which are 130 miles broad, originally occupied a width of 200 miles; and similar evidence of enormous compression is conspicuous in the structure of all mountains of elevation. Great elevation, however, may take place with little or no disturbance of stratification: wide continental areas have been slowly upheaved en masse, and sea-bottoms and low-lying plains have in this way been converted into lofty plateaux.6 Many of the most conspicuous features of the earth’s surface, therefore, are due directly to subterranean action. All those features, however, become modified by denudation, and eventually the primeval configuration may be entirely destroyed, and replaced by contours which bear no direct relation to the form of the original surface. In the newer mountain-chains of the globe the surface-features are still largely those due directly to upheaval; so in some recently elevated plateaux the ground has not yet been cut up and converted into irregular mountain-masses. Many of the more ancient mountain-chains and ranges, however, have been exposed so long to the abrading action of the denuding agents that all trace of their original contour has vanished. And in like manner plateaux of great age have been so highly denuded, so cut and carved by the tools of erosion, that their plateau character has become obscured. They have been converted into undulating mountainous and hilly regions. Everywhere throughout the world we read the same tale of subsidence and accumulation, of upheaval and denudation. The ancient sedimentary deposits which form the major portion of our land-surfaces, are the waste materials derived from the demolition of plains, plateaux, and mountains of elevation. In some mountain-regions we read the evidence of successive epochs of uplift, separated by long intervening periods of erosion, followed by depression and accumulation of newer sediments over the denuded surface. Thus the Alps began to be elevated towards the close of Palæozoic times. Erosion followed, and subsequently the land became depressed, and a vast succession of deposits accumulated over its surface during the long-continued Mesozoic era into early Cainozoic times. Again, a great upheaval ensued, and the Mesozoic and Eocene strata were violently contorted and folded along the flanks of the chain. Then succeeded another period of erosion and depression, which was again interrupted by one or more extensive upheavals. Away from those lines of weakness which we call mountain-chains, we constantly encounter evidence of widespread movements of elevation, during which broad areas of sea-bottom have been upheaved to the light of day, and, after suffering extensive denudation have subsided, to be again overspread with the spoils of adjacent lands, and then upheaved once more. And such oscillations of level have occurred again and again. Looking back through the long vista of the past, we see each continental area in a state of flux – land alternating with sea, and sea with land – mountains and plateaux appearing and disappearing – a constant succession of modifications, brought about by the antagonistic subterranean and sub-aërial agents.