CHAPTER X.

THE AGES OF MOUNTAINS, AND OTHER QUESTIONS.

O Earth, what changes hast thou seen!

TENNYSON.

It might naturally be asked at what period in the world's primeval or geological history some particular mountain-range was upheaved; whether it is younger or older than another one perhaps not very far away; and again, whether the mountain-chains of the world have been uplifted all at once, or whether the process of elevation was prolonged and gradual?

Questions such as these are deeply interesting, and present to the geologist some of the most fascinating problems to be met with in the whole range of this science. And though at first sight they might seem hopelessly beyond our reach, yet even here the prospect is by no means unpromising; and it is quite possible to show that they can be answered to some extent. Here we shall find our illustration of the cathedral (see chapter v., pages 143-147) holds good once more.

It is perhaps hardly necessary to explain that by looking at a Gothic cathedral one can say at what period or periods it was built. Perhaps it has a Norman nave, with great pillars and rounded arches. Then the chancel might be Early English, with pointed windows and deep mouldings, and other features that serve to mark the style of the building, and therefore its date,--because different styles prevailed at different periods. Other parts might contain work easily recognised as belonging to the "Perpendicular" period.

Now, as there have been periods in the history of architecture and art, so there have been periods in the history of our earth. What these periods were, and how we have learned to recognise them, we must first very briefly describe.[33]

[33] For a fuller account see the writer's "Autobiography of the Earth."

There are two simple rules by which the age of an ordinary sedimentary rock may be ascertained. This is fixed (1) By its position with regard to others; (2) By the nature of its embedded animal or vegetable remains, known as fossils.

These rules may easily be illustrated by a reference to the methods of the antiquary. For instance, suppose you were going to build a house, and the foundations had just been dug out; you might on examining them find several old layers of soil, showing that the site or neighbourhood had been formerly occupied. You might find in one layer stone implements, in another Roman or early British pottery, and yet again portions of brick or stonework, together with tools or articles of domestic use, belonging, say, to the time of Queen Elizabeth. Now, which of these layers would be the oldest? It is quite clear that the lowest layers must have been there the longest, because the others accumulated on the top of them.

The explorations made of late years under Jerusalem have led to the interesting discovery that the modern city is built up on the remains of thirteen former cities of Jerusalem, all of which have been destroyed in one way or another. Here, again, it is quite clear that the oldest layer of débris must be that which lies at the bottom, and the newest will be the one on the top.

Again, you know that the "Stone Age" in Britain came before the Roman occupation. Those old stone implements were made by a barbarous race, who knew very little of agriculture or the arts of civilisation. Then in succeeding centuries various arts were introduced, many relics of which are found buried in the soil; and hence, since different styles of art and architecture prevailed at different periods, the works of art or industry embedded in any old layers of soil serve to fix the date of those layers.

These layers of soil and débris correspond to the layers or strata of the sedimentary rocks, in which the different chapters of the world's history are recorded. Geology is only another kind of history; and the same principles which guide the archæologist searching buried cities also guide the geologist in reading the stony record. As the illustrious Hutton said, "The ruins of an older world are visible in the present state of our planet." The successive layers of ruin in this case are to be seen in the great series of the stratified rocks; and we may lay it down as an axiom that the lowest strata are the oldest, unless by some subsequent disturbance the order should have been reversed, which, fortunately, is a rare occurrence, though examples are to be found in some mountain-chains with violent foldings.

But it often happens that neither the strata which should come above nor those that lie below can be seen. Then our second rule comes in: We can determine the age of the rock in question by its fossils. The reason of this has perhaps already been guessed by the reader. It is that as different kinds of plants and animals have prevailed at different periods of the world's history, so there have been "styles," or fashions, in creation, as well as in art. At one geological period certain curious types of fishes flourished which are now almost extinct, only a few old-fashioned survivals being found in one or two out-of-the-way places. At another period certain types of reptiles flourished vigorously, and were the leaders in their day; but they have altogether vanished and become extinct. So one type after another has appeared on the scene, played its humble part in the great drama of life; and then--"exit!" another takes its place.

In the oldest and lowest of the series of rocks we find no certain trace of life at all. In the next series we find only lowly creatures, such as shell-fish, corals, and crab-like animals that have no backbone. In a higher group of rocks fishes appear for the first time. Later on, we come across the remains of amphibious creatures for the first time. Then follows (after a long unrecorded interval) an era when reptiles and birds existed in great numbers. After another long interval we come to strata containing many and diverse remains of mammals or quadrupeds. So we have an "Age of Fishes," an "Age of Reptiles," and an "Age of Mammals." Some tribes of these creatures died out, but others lived on to the present day. Thus we see that there has been a continuous progress in life as the world grew older, for higher types kept coming in.

To the geologist fossils are of the greatest possible use, since they help him to determine the age of a particular set of strata, for certain kinds of fossils belong to certain rocks, and to them only.

But the classification of the stratified rocks has been carried farther than this. Practical geologists, working in the field, use fossils as their chief guide in working out the subdivisions of a group of rocks, for certain genera and species of old plants and animals are found to belong to certain small groups of strata. In this way a definite order of succession has been established once for all; and, except in the case of inverted strata already alluded to, this order is invariably found to hold good.

This great discovery of the order of succession of the British stratified rocks, established by their fossil contents, is due to William Smith, the father of English geology. After exploring the whole of England, he published in 1815 a geological map, the result of his extraordinary labours. Before then people had no idea of a definite and regular succession of rocks extending over the country, capable of being recognised to some extent by the nature of the rocks themselves,--whether sandstones, clays, or limestones, etc., but chiefly by their own fossils. They thought the different kinds of rocks were scattered promiscuously up and down the face of the country; but _now_ we know that they do not show themselves in this haphazard way, but have definite relations to each other, like the many volumes of one large book.

By combining the two principles referred to above, geologists have arranged the great series of British stratified rocks into certain groups, each indicating a long period of time. First, they are roughly divided into three large groups, marking the three great eras into which geological time is divided. Secondly, these eras are further divided into certain periods. These periods are again divided into epochs, indicated by local divisions of their rocks. In this way we have something like a historical table. Omitting the small epochs of time, this table is as follows, in descending order:--

_Table of the British Stratified Rocks._

ERA. PERIOD. PREVAILING TYPE.

{ Recent. Cainozoic, { Pleistocene, or { or Tertiary. { Quaternary. Mammals. { Pliocene. { Miocene. { Eocene.

{ Cretaceous. Mesozoic, { Neocomian. or { Jurassic. Reptiles. Secondary. { Triassic. { Permian.

{ Carboniferous. Fishes. { Devonian, and Palæozoic, { Old Red Sandstone. or { Silurian. Creatures without Primary. { Cambrian. a backbone { Archæan,[34] (invertebrates). { or { Pre-Cambrian.

[34] The Archæan rocks are frequently placed in a separate group below the Palæozoic.

The total thickness of all these rocks has been estimated at about one hundred thousand feet, or not far from twenty miles. These names have been given partly from the region in which the rocks occur, partly from the nature of the rocks themselves, and partly for other reasons. Thus the Old Red Sandstone is so called, because it generally, though not always, appears as a dark red sandstone. But the Silurian rocks, which we find in North Wales, receive their name from the Silures, an ancient Welsh tribe; the Cambrian rocks take theirs from Cambria, the old name for North Wales. The Cretaceous rocks are partly composed of chalk, for which the Latin word is _creta_; and so on. The terms "Palæozoic," "Mesozoic," and "Cainozoic" mean "ancient life," "middle life," and "recent or new life," thus indicating that as time went on the various types of life that flourished on the earth became less old-fashioned, and more like those prevailing at the present time. These used to be called "Primary," "Secondary," and "Tertiary;" but the terms were unfortunate, because the primary rocks, as then known, were not the first, or oldest. We have therefore included the Archæan rocks, since discovered, in this primary group. Only one fossil has been found in these rocks, and that is a doubtful one; hence they are sometimes called "Azoic," that is, "without life." The Mesozoic rocks are, as it were, the records of the "middle ages" in the world's history; while the Palæozoic take us back to a truly primeval time.

We have now learned how the geological age of any group of rocks may be determined. Thus, if a series of rocks of unknown age can be shown to rest on undoubtedly Silurian rocks in one place, and in another place to be overlaid or covered by undoubtedly Carboniferous rocks, they will probably belong to the Old Red Sandstone Period. If afterwards we find that they contain some of the well-known fossils of that period, the question of their age is settled at once. But we want more evidence than this. Suppose, now, we find somewhere on the flanks of a mountain-range a series of Permian and Triassic rocks, resting almost horizontally on disturbed and folded Carboniferous strata. Does not that at once prove that the upheaval took place before the Permian Period? Clearly it does, because the Permian rocks have evidently _not_ been disturbed thereby. So now we can fix the date of our range of hills; namely, after the Carboniferous Period and before the Permian Period.

It is by such reasoning that the age of our Pennine range of hills, extending from the north of England into Derbyshire, has been fixed; for the Permian and Triassic strata lie undisturbed on the upheaved arch of Carboniferous rocks of which this chain is composed. Its structure is that of a broken and much denuded anticline, which stands up to form a line of hills only because the Carboniferous limestone is so much harder than the "coal measures," or coal-bearing rocks, on each side of it, that it has not been worn away so fast. In time, this great anticline will be entirely worn away like that of the Weald. It is called the Great Mountain Limestone, because it so often rises up to form high ground. The Mendip Hills in Somersetshire are of about the same date, and they too are largely composed of this great limestone formation.

Of course, a certain amount of up and down movement took place after the hills were upheaved, otherwise the Permian and Triassic rocks could not have been deposited on their sides; but these movements were slight and of a more general kind than those by which strata are thrown into folds.

The main upheaval, by which the rocks now forming the Highlands of Scotland were lifted up and contorted, took place after the Lower Silurian Period, and before that of the Old Red Sandstone; and there is clear evidence that even before the latter period they had not only been greatly altered, or "metamorphosed," by subterranean heat, but that they had suffered enormous denudation. And the work of carving out these mountains has gone on ever since; for even in Old Red Sandstone times they were probably not entirely covered by water. The Highland Mountains are therefore older than the Pennine range.

Geologically Scotland belongs in great part to Scandinavia; and the long line of Scandinavian Mountains is a continuation of the Highlands, and so is of the same age.

Mountain-chains and hill-ranges have been upheaved at various geological periods; and some are very old, while others are much younger.

Turning to the southeast of England, we find the ranges of chalk hills forming the North and South Downs (see page 237). As explained previously, these owe their existence to the upheaval and subsequent denudation of the low arch, or anticline, of the Weald. They are called "escarpments," because they are like lines of cliffs that are being gradually cut back. Now, it is clear that these hills are much newer than either of those we have just considered. Look at the table on page 324, and you will see that the Cretaceous rocks (chalk, etc.) belong to the Mesozoic era. The chalk was the last rock formed during the Cretaceous Period.

So the Wealden arch must have been heaved up after the chalk was formed; that is, ages and ages later than the date of the Pennine range or the Scotch Highlands. From other evidences it has been shown that this anticline was heaved up in the early part of the Cainozoic Era, perhaps during the Miocene Period.

Let us now take the case of the Alps. And here we have an instructive example of a great mountain system formed by repeated movements during a long succession of geological periods. We cannot say that they were entirely raised up at any one time in the world's past history. In the centre of this great range we find a series of igneous and metamorphic rocks, such as granite, gneiss, and crystalline schists. Some of these may belong to the very oldest period,--namely, the Archæan; others are probably Palæozoic and Cainozoic deposits greatly altered by heat and pressure.

The ground from Savoy to Austria began to be an area of disturbance and upheaval towards the close of the Palæozoic Era, if not before; so that crystalline schists and Carboniferous strata were raised up to form elevated land around which Permian conglomerates and shingle-beds were formed,--as on the seashore at the present day.

During the early part of the Mesozoic Era local fractures and certain up and down movements occurred. After this there was a long period of subsidence, during which a series of strata known as Oölites and Cretaceous were deposited on the floor of an old sea.

Towards the close of this long era, a fresh upheaval took place along the present line of the Alps,--an upheaval that was prolonged into the Eocene Period. It was during this latter period that a very extensive formation known as the "Nummulitic limestone" was formed in a sea that covered a large part of Europe and Asia. We have already referred (see chap. v., pp. 169-171) to the way in which limestones have been formed. Nummulites are little shells that were formed by tiny shell-fish.

But after this, the greatest upheaval and disturbance took place,--an upheaval to which the Alps as we now see them are chiefly due. By this means the older Cainozoic strata, once lying horizontally on the floor of the sea, were raised up, together with older rocks, to form dry land, and not only raised up, but crumpled, dislocated, and in some cases turned upside down.

So intense was the compression to which the Eocene rocks were subjected that they were converted into a hard and even crystalline state. It seems almost incredible that these highly altered rocks which look so ancient are of the same date as our London clay and the soft Eocene deposits of the south of England; but in our country the movement that raised up those strata was of the most feeble and gentle kind compared to the violent disturbances that took place in Switzerland.

And here we may point out that the Alps are only a portion of a vast chain of mountains stretching right across Europe and Asia in a general east and west direction, beginning with the Pyrenees and passing through the Alps, the Carpathians, the Caucasus, and the range of Elbruz to the Hindoo-Koosh and the high plateau of Pamir, called "the roof of the world," which stands like a huge fortress, fifteen thousand feet high. Thence it passes to the still higher tracts of Thibet, great plains exceeding in height the highest summits of the Alps, being enclosed between the lofty ramparts of the Himalayas on the south and the Kuen-Lun Mountains on the north; and thence the mountain wall is prolonged in the Yuen-Ling, In-Shan, Khin-Gan, and other ranges till it finally passes to the Pacific Ocean at Behring's Strait.

All these ranges are, as it were, the backbone of the great continental plateau of the Old World, and doubtless are chiefly due to those earth-movements by means of which the Alps were upheaved. The last grand movement, which raised the Mont Blanc range, was probably rather later, and seems to have taken place as late as the Pliocene Period.

At the present day no great movements are taking place in the Alps; but now and then earthquakes visit this region, and serve to remind us that the process of mountain-making is still slowly going on.

Probably there have been times in the history of all these mountain-ranges when movements took place of a more violent and convulsive kind than anything with which we are familiar at the present day; and the age we live in may be one of comparative repose. This is of course somewhat a matter of speculation; and we only allude to it because there has been a tendency on the part of some to carry the theory of uniformity in all geological operations much farther than Hutton or Lyell ever intended. But at the same time there is no need to go back to the old teaching of sudden catastrophes and violent revolutions. We only wish to avoid either of these two extremes and to take a safe middle course.

How rapidly some of these great earth-movements took place it is impossible at present to say; but in several cases it can be shown that they were quite slow, as indicated by the testimony of the rivers. Thus, the rise of the great Uintah Mountains of the Western States was so slow and gradual that the Green River, which flowed across the site of the range, so far from being turned aside as they rose up, has actually been able to deepen its cañon as fast as the mountains were upheaved. So that the two processes, as it were, kept pace with each other, and the river went on cutting out its gorges at the same time that the ground over which it flowed was gently upheaved; and as the land rose the river flowed faster, and therefore acquired more power to cut and deepen its channel. This is a valuable piece of evidence; but in this case we have only a few big broad folds, instead of the violent folding seen in the Alps. However, certain Pliocene strata lying on the southern flanks of the Himalayas show that the rivers still run in the same lines as they occupied before the last great upheaval took place.

We have seen how the substance of the mountains was slowly manufactured by means of such quiet and gentle operations as may be witnessed at the present day; how the rivers of old brought down their burdens as they do now, and flung them into the sea; how the sea spread them out very slowly and compacted them into level layers, to form, in process of time, the hard rocky framework of the plateaux, hills, and mountains of the world; how vast marine accumulations were also slowly manufactured through the agency of countless generations of humble organisms, subtracting carbonate of lime from sea water to form the limestones of future ages; how by slow earth-movements these marine deposits were reared up into dry land; how they have frequently been penetrated by molten rocky matter from below, which occasionally forced its way up to the surface and gave rise to various volcanic eruptions, by means of which the sedimentary rocks were often considerably baked and hardened, and new fissures filled up with valuable metallic ores and precious stones; how lava-flows and great deposits of volcanic ash were mingled with these sedimentary rocks.

Then we endeavoured to follow the history of these rocky layers after their upheaval, and learn how they are affected by the ceaseless operations of rain and rivers and other agents of destruction, so that finally the upheaved ridges of the lands are carved out into all those wonderful features of crag and pinnacle and precipice that give the mountains their present shapes and outlines. All this we were able to account for, without the aid of any imaginary or unnatural causes.

And, lastly, we have seen that even where such causes might seem at first almost indispensable,--when mountains tell us of mighty internal forces crumpling, folding, and fracturing their rocky framework,--yet even there we can account for what we see without supposing them to have been torn and tossed about by any very violent convulsions.

Although the question of the cause, or causes, of earth-movements, whereby continents are upheaved, and the contorting, folding, and crumpling of the rocks of mountains produced, is not at present thoroughly explained, it may perhaps be worth our while to consider briefly some of the views that have been put forward on this difficult subject. The words "upheaval" and "elevation," in reference to movements of the earth's surface, are somewhat misleading, but are used for want of better terms. They would seem to imply that the force which produced mountains was a kind of upward push; whereas, in most cases, and perhaps in all, the force, whatever it was, did not act in an upward direction. So it should be understood that we employ these terms only to indicate that the rocks have somehow been carried up to a higher level, and not as suggesting _how_ the force acted by which they were raised.

It seems pretty clear that in the case of mountain-chains, at least, the force acted in a horizontal direction, as a kind of side-thrust.

This we endeavoured to illustrate in chapter ix. by means of a simple experiment with a sheet of paper; and it was shown how folds similar to those of which Mont Blanc is composed could be imitated by simply pressing the sides of a sheet of paper inwards with one's two hands as it lies on a table. Such lateral pressure, it is thought by many, must be caused by the shrinking of the lower and hotter parts of the earth's crust as they cool, leaving the outer crust unsupported, so that it gradually settles down onto a smaller surface below, and in so doing must inevitably be wrinkled and throw itself into a series of folds (see chapter vi., page 204).

The interior of the earth is hotter than the outside; and since there is good reason to think that the whole earth was once upon a time in a highly heated and perhaps half molten condition, we are compelled to believe that it always has been, and still is, a cooling globe. Now, almost all known substances are found to contract more or less on cooling; and so if the materials of which the earth is mainly composed are at all similar in their nature and properties to those which we find on its surface, it follows that the earth must be contracting at the same time that it is cooling, just as a red-hot poker will contract on being taken out of the fire.

Moreover, we find that hot bodies contract faster than those that are merely warm, so that a red-hot poker contracts more during the first few minutes after it is taken out of the fire than it does after it has passed the red-hot stage. Hence it is easy to see that the interior portions of the earth, which are hotter, must be contracting at a greater rate than its external parts, for they evidently have very little heat to lose. This may seem rather puzzling to the reader at first; for it might be argued that the heat from below _must_ pass through the external layers, or crust, as it is often called. But it should be remembered that this is not the only way in which the earth loses heat. Think of the vast amount of heat given out from the earth every year by volcanic eruptions, and you will see at once that much of the cooling takes place in this way, and not as a direct flow of heat from the interior, as in the case of the poker. A single big lava-stream flowing out from a volcano, and cooling on the surface of the earth, represents so much heat lost forever; and so do the clouds of steam emitted during every eruption; so, again, do even the hot springs that are continually bringing up warm water. If, then, the lower portions of the earth are slowly contracting, they must tend to leave the outer portions of the crust unsupported, so that they would be compelled by their own enormous weight to settle down. Now, we know that something like this happens in coal mines; and as long passages are hollowed out below, the ground begins to "creep," or slowly sink. Think what would be the effect of a slow sinking of any portion of the earth down towards the centre; it would inevitably be curved up and down into numerous folds, as it endeavoured to get itself onto a smaller space, much in the same way that a table-cloth, when thrown onto a table in a kind of arch, settles down in a series of waves, or folds. And this, it is thought, is the way in which it happens that the pressure comes, as we said just now, sideways, instead of from below upwards. It is on this theory that many geologists account for the enormous side-pressure to which rocks have in many cases been subjected.

The evidences of such pressure are many. In some cases fossils have been thereby pulled out of shape and appear considerably distorted; in others, even hard quartz pebbles have been considerably elongated (see chap. ix., pp. 315-316). Then again, we have the little crumplings of all sizes so frequently seen in mica-schists. And lastly, the peculiar property that slates possess of splitting up into thin sheets is found to be due to the same cause; namely, lateral pressure. Slates were originally formed of soft dark mud, and on being subsequently squeezed, by earth-movements, have assumed a structure known as "cleavage," whereby their tiny mud-particles were elongated, and all assumed the same direction, thus giving to the rock this peculiar property of splitting. It can be proved that the pressure came in a direction opposite to that of the planes of cleavage; and it is found that the direction of the cleavage corresponds in a general way with the direction, or trend, of a mountain-chain which is composed partly of slates, as in North Wales. And this discovery helps and harmonises with what we have already said about the cause of the folds in mountain-chains, for the same force, acting sideways, produced the cleavage and the folding, etc.

It has been already stated that in a large number of cases a mountain-range has a central axis, or band, of granite or other crystalline rock. This led some people to suppose that the granite had been driven up from below, and in so doing had thrust up the overlying rocks seen on either flank of the chain; in other words, they believed granite to have been the upheaving agent. And even now we often find unscientific writers speaking of the volcanic forces of upheaval.

Having very little idea of the true structure of mountains, they believed them to consist of a kind of core, or axis, of this igneous rock, with sedimentary rocks sloping away from it on each side. This was a very simple theory of mountain-chains, but unfortunately it will not bear examination. It takes no notice of the folding which is so characteristic of mountain strata, and is quite out of agreement with the facts of the case; so it must be buried among the archives of the past. Mountain-chains are now known to have a much more complicated structure than this,--thanks to the labours of many subsequent observers.

That illustrious astronomer, the late Sir John Herschel, threw out a bold suggestion on this subject, which in the light of recent discoveries with regard to the delicate adjustment between the internal and external forces affecting the earth's surface, is worthy of careful consideration. His idea was that the mere weight of a thick mass of sediment resting on any portion of the earth's crust might cause a certain amount of sinking; and that this would cause portions on either side to swell up. It is certain that as great deposits of sedimentary materials accumulate on the floor of an ocean, that floor slowly sinks, otherwise the sea would become choked up, and dry land would take its place. Now, it is found that every great mountain-chain consists of many thousands of feet of strata thus formed; and more than this: it turns out that a greater thickness of such materials has been formed in regions where we now see mountain-chains than in those continental regions that lie farther away from them. This is an important fact, which was not known in Sir John Herschel's time. One striking example may be mentioned here. In the complicated region of the Appalachian chain the strata are estimated to have a total thickness of eight miles; while in Indiana, where the same strata are nearly horizontal, they are less than one mile thick. Hence it is not impossible that in the mere accumulation, through long periods of time, of vast masses of strata many thousands of feet thick, we may find a potent cause of earth-movements.

The marginal regions of oceans, where most deposition takes place, seem to undergo slow subsidence, while the continents seem in most places to be as slowly rising. Modern geologists are inclined to think that as denudation wears down a continental surface, removing from it a great quantity of solid rocky matter (see chap. v., pp. 161-163), the pressure below is somewhat lessened, or in other words, so much weight is taken off; but that, on the other hand, as this extra amount of material accumulates on the bed of a neighbouring ocean the pressure is increased by a corresponding amount, and so the balance between internal and external forces is upset, and movements consequently take place. We have already seen that the external parts of the earth are much more subject to movements than might have been expected; and for our part, we are willing to believe that in this simple way upheaving forces might be called into play sufficient to account for even the elevation of mountain-chains. For suppose a great mass of strata to continue sinking as they were formed, for long periods of time; what seems to follow? The downward movement would go on until a time would come when the strata, in endeavouring to settle down at a lower level, would (as by the contraction theory above explained) be forced to fold themselves into ridges, and in this way long strips of them might even be elevated into mountain-ranges.

Another ingenious idea was suggested by the late Mr. Scrope, whose work on volcanoes is well known. His idea was that when a large amount of sedimentary material has accumulated on any large area of the bed of the ocean, it somewhat checks the flow of heat from within, and therefore the temperature of the rocks forming part of the earth's crust below will be increased, much in the same manner as a glove checks the escape of heat from the hand and keeps it warm. The consequence of this would be expansion; and as such expansion would be chiefly in a horizontal direction, the area would bulge upwards and cause elevation of the strata resting on it. But there are several difficulties which this theory fails to explain.

And lastly, Professor Le Conte, holding that the contraction theory is unsatisfactory, accounts for earth-movements of all kinds by supposing that some internal parts of the earth cool and contract faster than others. Those parts that cool fastest, according to this theory, are those that underlie the oceanic basins or troughs; while the continental areas, not cooling so rapidly, are left standing up in relief. This theory, which does not seem very satisfactory, is based upon the idea that some parts of the earth's interior may be capable of conducting heat faster than others. We know that some substances, like iron, are good conductors of heat, while others are bad conductors; and it is therefore conceivable that heat may be flowing faster along some parts of the earth than along others; and if so, there would be differences in the rate of contraction.

There are various theories with regard to the nature of the earth's interior. One of these already referred to, but now antiquated, supposes our planet to consist of a thin, solid crust lying on a molten interior, so that the world would be something like an egg with its thin shell and liquid, or semi-liquid, interior. Now, there are grave reasons for refusing to accept this idea. In the first place, a certain slow movement of the earth known as "precession," because it causes the precession of the equinoctial points on the earth's orbit, could not possibly take place as it does if the earth's interior were in this loose and molten condition. That is a matter decided by mathematical calculation, on which we will not dwell further. Secondly, we obtain some very valuable evidence on this abstruse subject from the well-known daily phenomenon of the tides, caused, as the reader is probably aware, by the attractions of the sun and moon; but much more by the moon, because she is nearer, and so exerts a greater pull on the ocean as each part of the world is brought directly under her by the earth's daily rotation on its axis. The waters of our oceans rise up twice each day as they get in a line with the moon, and then begin to fall again. Thus we get that daily ebb and flow seen on our shores. Now, it has been clearly proved by Sir William Thomson, and others, that if any considerable portion of the interior of the earth were in a fluid condition, it too would rise and fall every day as the ocean does. So we should in that case have a tide _below_ the earth as well as on its surface, and the one would tend to neutralise the other, and the ocean tide ought to appear less than it actually is. Even if the earth's crust were made of solid steel, and several hundreds of miles thick, it would yield so much to the enormous pulls exerted by both the sun and moon that it would simply carry the waters of the ocean up and down with it, and we should therefore see no appreciable rise and fall of the water relatively to the land. As a matter of fact, there _is_ a very slight tide in the solid earth below our feet, but so slight that it does not practically affect the tide which we see every day in the ocean. But we wish to show that were the interior of the earth in anything approaching, to a fluid or molten condition, the phenomena of the tides would be very different from what they actually are.

All geologists are therefore agreed that we must consider our earth as a more or less solid body, and not as being something like an india-rubber ball filled with water.

The only question is whether it is entirely solid throughout. Some authorities consider this to be the case. But others venture to think that while the great mass of the globe is solid, there may be a thin liquid layer lying somewhere below the surface. Sir William Thomson calculates that there must be a solid crust at least two thousand or twenty-five hundred miles thick (the diameter of the earth is about eight thousand miles) and that the mass of the earth "is on the whole more rigid certainly than a continuous solid globe of glass of the same diameter."

One other question with regard to the earth's interior may be mentioned in conclusion. Astronomers have calculated the weight of our planet, and the result is curious; for it turns out to be _at least twice as heavy as the heaviest rocks that are found on or near the surface_. It is about five and a half times as heavy as a globe of water of the same size would be, whereas most rocks with which we are acquainted are about two and a half, or at most three times heavier than water. This fact seems to open out curious consequences; for instance, it is quite possible that metals (which are of course much heavier than water) may exist in the earth's interior in considerable quantities. The imagination at once conjures up vast quantities of gold and silver. What is the source of the gold and silver, and other metals found in mineral veins? This question cannot as yet be fully answered. Very small quantities of various metals have been detected in sea-water; and so some geologists look upon the sea as the source from which metals came. But it is possible that they were introduced from below,--perhaps by the action of steam and highly heated water during periods of volcanic activity,--and that their source is far down below in the depths of the earth.

But perhaps we have already wandered too far into the regions of speculation.

Such are some of the interesting problems suggested by the study of mountains, and they add no small charm to the science of geology.

And as we leave the mountains behind us, refreshed by their bracing air, and strengthened for another season of toil and labour by a brief sojourn among their peaks and passes, we come away with a renewed sense of the almost unlimited power of the unhasting operations of Nature, and the wisdom and beneficence of the Great Architect of the Universe, who made and planned those snowcapped temples as symbols of His strength, who was working millions of years ago as He is working to-day, and to whom a thousand years are as one day.

INDEX.

Agents of transportation, 161.

Ages of strata, how determined, 317-333.

Air, composition of, 209.

Alpine animals, 124. plants, 103, 114.

Alps, the history of, 330. (See also Ruskin.)

Ancients, the, their dread of the mountains, 3.

Andes, the, elevation of, 189.

Animals, behaviour of, before an avalanche or earthquake, 95.

"Anticline," 237, 303, 327.

Appalachian Mountains, denudation of the, 239, 305-309.

Aqueous rocks, 154.

Archæan Era, 324.

Arctic flora, 121.

"Arthur's Seat," 277.

Ashes, volcanic, 245, 251, 260.

Atlantic ooze, 172.

Atmosphere, effects produced by the, 209. rarefaction of, 79.

Avalanches, 89.

Badger, the, in Alps, 128.

Baltic Sea, changes in, 182.

Barrier reef, of Australia, 170.

Basalt, of Hebrides, 278. of Snowdon, 272.

Basin, the Great, of United States, 313.

Bear, brown, 125. black, 126.

Beaver, the, in Alps, 128.

Bergfalls, 97.

Bernina, the, fall of rocks from, 98.

Bird, Miss (Mrs. Bishop), on eruption of Kilauea, 262.

Birds, of Alps, 134.

Blueness of the sky, 75.

Bombs, volcanic, 253.

Bonney, Prof., on mountain legends, 23. on effects of the Alps in Europe, 48. on wind on mountain-tops, 84. on Alpine plants, 115. on forms of mountains, 294.

Boulders, erratic, 225.

Bouquetin, the, in Alps, 133.

Britain, Great, rainfall of, 42.

Building up of mountains, 174.

Butterflies, in Alps, 138.

Buzzard, the, in Alps, 136.

Cader Idris, volcano rocks of, 272.

Cainozoic Era, 324.

Callao, 189.

Cambrian rocks, 296, 324.

Canisp Mountain, 297.

Cañons of Colorado, 221.

Carbonic acid in atmosphere, 210.

Carboniferous Period, 324.

Catastrophes, 215.

Caves, human remains, etc., in, 31.

Celsius, on elevation of Gulf of Bothnia, 178.

Chalk, Cretaceous rocks composed of, 325. origin of. See Limestones.

Challenger, H. M. S., expedition of, 251.

Chamois, the, in Alps, 130.

Characteristics of mountain races, 14.

China clay, 292.

Classification of rocks, 157.

Cleavage of slates, 151, 340.

Coniferous trees, region of, 111.

Contortions in strata, 298, 311.

Contraction and expansion of rocks, 208.

Contraction theory of earth-movements, 338.

Coral reefs, 170.

Cotopaxi, 259.

Crystalline schists, 312.

Darwin, Charles, on elevation of the Andes, 189.

Deciduous trees, mountain region of, 110.

Dent de Mayen, 99.

Dent du Midi, fall of rock from, 98.

Denudation, 220, 229, 288, 312.

Devonian rocks, 324.

Diablerets, fall of rock from, 98.

Dislocations of mountain rocks, 313, 315.

Dust, volcanic, 245, 260.

Dykes, 245.

Eagle, the golden, 136.

Earth-pillars in Tyrol, 221.

Earthquakes, 95, 102, 196. effects of, 198, 336. causes of, 198, 200. Lucretius on, 199.

Earth-tremors, 194.

Elevation of mountains, 146, 200, 202, 299, 336. continents, 298-299.

Encrinites, 171.

Eocene Period, 324.

Equador and Peru, earthquake of, 197.

Eras, geological, 324.

Eruptions, volcanic, 247.

Fairies, 5.

Falcon, the, in Alps, 136.

"Fan-structure," 310.

"Faults" and fractures, 200, 313.

Features characteristic of mountains, 177.

Ferns, 118.

Fishes, Age of, 322.

Fissures, 268.

Föhn, the, 84.

Foraminifera, 172.

Fox, the, in Alps, 127.

Frog, the, in Alps, 137.

Frost, effects of, on mountain rocks, 212.

Game-birds, in Alps, 137.

Ganges and Brahmapootra, 167.

Geikie, Sir A., on influence of Scottish scenery, 21. on the Highland plateau, 284. on the mountains of West Sutherland, 296.

Giant's Causeway, basalt of, 279.

Glace, Mer de, 229.

Glacial drifts, 227.

Glacial region of vegetation in Alps, 116.

Glaciers, erosive power of, 228.

Glare from snow in Alps, 76.

Gneiss, 156, 292.

Gold and silver in mountains, 61. in the earth, 350.

Grampians, 276.

Granite, 210. weathering of, 291. in mountain-chains, 312.

Greenland, elevation of, 186.

Green slates and porphyries, 275.

Gulf Stream, 42.

Hare, the, in Alps, 128.

Hawaii, 256.

Heat, effects of, on rocks, 154, 156, 160. underground, of the earth, 338, 345.

Hebrides, former volcanic action in, 278.

Height, influence of, on vegetation, 107.

Herculaneum, 254.

Highest cluster of houses in the world, 79.

Highlands of Scotland, 284.

Himalayas, description of, 6.

Hutton, 142, 320.

Iberian, or pre-Celtic race, 30.

Ice Age, the, 65, 123.

Ice, as a geological agent, 223.

Igneous rocks, 155.

Imbaburu, eruption of mud from, 259.

Implements of stone, 31.

Jackdaw, the, in Alps, 136.

Jura Mountains, 300, 306.

Jurassic rocks, 324.

Kilauea, eruption of. (See Bird, Miss.)

Kite, the, in Alps, 136.

Krakatoa, 252.

Labrador, elevation of, 192.

Lake District, denudation of, 220. volcanic rocks of, 275.

Lakes, origin of, 47.

Lateral pressure, applied to mountains, 310, 315, 337.

Lichens and mosses. (See Ruskin.)

Limestones, origin of, 151, 153, 169.

Lisbon, earthquake at, 197.

Livingstone, on splitting of rocks, 212.

Lizard, the, in Alps, 137.

Lyell, Sir Charles, 333.

Lynx, the, in Alps, 128.

Mal de montagne, 80.

Mammals, age of, 322.

Marmot, the, in Alps, 129.

Mauna Loa, eruption of, 256.

Mendip Hills, 327.

Mer de Glace. (See Glace.)

Metals, precious, 60. in the earth, 349.

Metamorphic rocks, 156, 157, 298, 330.

Mica-schist, 156, 293.

Miller, Hugh, 150.

Milne, Prof., on earth-pulsations, 193.

Minor cones of volcanoes, 246.

Miocene Period, 278, 324.

Mississippi, denudation by the, 232.

Moel Tryfaen, raised beach in, 186.

Mont Blanc, 310.

Monte Conto, downfall of, in 1618, 101.

Monte Nuovo, 248.

Moraines, 225.

Mountain limestone, 152.

Mountains, as barriers between nations, 26. as reservoirs of water, 43. human wants supplied by, 58. influence of, on climate, 62. causing movements in the atmosphere, 65. as backbones of continents, 67. floras of, 103-124. forms of, how determined, 282. general features of, 177, 283. structure of, how determined, 308. elevation of, 174, 313. formed by huge dislocations, 313. Ruskin on uses of, 68. " on a scene on the Jura, 300. " on flowers of, 107.

Mud-flows from volcanoes, 259.

"Needles," the, of Colorado, 221.

Neptunists and Plutonists, 160.

New England, elevation of, 192.

New Zealand, elevation of, 190.

Nummulites, 331.

Old Red Sandstone, 150, 324.

Olive region, the, 107.

Organically formed rocks, 157.

Ornamentation of mountains, 147.

Oxygen, in air, 209.

Palæozoic Era, 324.

Permian rocks, 324.

Pleistocene rocks, 324.

Pliocene, 324.

Plutonists, 160.

Pompeii, buried up, 254.

Precious stones in mountains, 277.

Primary Era, 324.

Pulsations of the earth. (See Milne.)

Quinag, 297.

Rabbit, the, in Alps, 128.

Raised beaches, 185.

Raven, the, in Alps, 136.

Red clay, of Atlantic Ocean, 252.

Reptiles, Age of, 323.

Righi Mountain, fall of rock from, 99.

Rivers, transporting power of, 161-168.

Roches Moutonnées, 227.

"Rocking Stones," 292.

Ross and Sutherland, mountains of, 315.

Rossberg, the, fall of rock from, 99-101.

Ruskin, on effect of tourists in Switzerland, 21. on effects of scenery on mythology, 22. on uses of mountains, 50. on formation of soil, 55. on lichens and mosses, 119. on the Alps, 289. on a scene in the Jura Mountains, 300.

Santorin, island of, 257.

Scandinavia, elevation of, 180.

Scenery, influence of rocks on, 219.

Schists. (See Mica-schist.)

Scotland, former volcanic action in, 275.

Sea-beaches, 183.

Sea-level, constancy of, 179.

Secondary Era, 324.

Serapis, Temple of, 187.

Silurian Period, 324. volcanic rocks of, 272.

Shearing of rocks in mountains, 316.

Skaptar Jökull, lava-flow from, 255, 260.

Smith, William, 323.

Snake River Plain, 258.

Snow, lambent glow of, 77.

Snowdon, volcanic rocks of, 272. denudation of, 239.

Spectre of the Brocken, the, 78.

Stability of the earth, 174, 314.

Stanley, Dean, on capture of Canaan, 32.

Stone Age, 31.

Storms on mountains, 81.

Stratified rocks, table of, 324. how formed, 148, 176.

Striæ, glacial, 227.

Submerged forests, 192.

Suilven Mountain, 297.

Sunsets, 71.

Sutherland, West, mountains of, 296.

Taurentum, destroyed by downfall of rocks, 97.

Thames, solid matter transported by, 168.

Thunder-storms, in Alps, 86.

Tomboro, eruption at, 260.

"Tors," 292.

Tourmente, the, 83.

Transportation by rivers, 161, 166-169. by glaciers, 224.

Triassic Period, 324.

Types of plants and animals at different periods, 106.

Upheaval theory of mountains, 247.

Uses of mountains, 33.

"Valleys, how carved out, 214-230.

Vesuvius, history of, 250.

Vines, the region of, in Alps, 109.

Volcanoes, number of active, 242. old ideas about, 244. structure of, described, 244. volcanic rocks of Great Britain, 271.

Vulture, the bearded, 134.

Wall of Antoninus, 185.

Waterfalls, origin of, 218.

Water-vapour, in air, 34. condensation of, by mountains, 34.

Waves of population, 30.

Weald, the denudation of, 235-239. structure of, 303.

Werner, 158.

Wild-cat, in Alps, 128.

Wolf, the, in Alps, 126.

Zones of climate on the earth, 63.

Transcriber's note: A "List of Illustrations II" has been added to the text, for the convenience of the reader, to display Illustrations that were not included in the original "Illustrations" section. The original spelling of words, especially for place names, has been retained.

End of Project Gutenberg's The Story of the Hills, by H. N. Hutchinson