CHAPTER V.

HOW THE MATERIALS WERE BROUGHT TOGETHER.

These changes in the heavens, though slow, produce Like change on sea and land.

MILTON

Probably every mountain climber, resting for a brief space on a loose boulder, or seeking the shade of some overhanging piece of rock, has often asked himself, "How were all these rocks made?" The question must occur again and again to any intelligent person on visiting a mountain for the first time, or even on seeing a mountain-range in the distance. He may well ask his companions how these great ramparts of the earth were built up. But unless he possesses some knowledge of the science of geology, which tells of the manifold changes which in former ages have taken place on the earth, or unless, in the absence of such knowledge, he chance to meet with a geologist, his question probably remains unanswered. Such questions, however, can be very satisfactorily answered,--thanks to the labours of zealous seekers after truth, who have given the best part of their lives to studying the rocks which are found everywhere on the surface of the earth, and the changes they undergo. Geology is a truly English science; and Englishmen may well cherish gratefully the memories of its pioneers,--Hutton, Playfair, Lyell, and others, who have made the way so clear for future explorers.

The story of the hills as written on their own rocky tablets and on the very boulders lying loose on their sloping sides, and interpreted by geologists, is a long one; for it takes us far back into the dim ages of the past, and like the fashionable novel, may be divided into three parts, or volumes. To those who follow the stony science it is quite as fascinating as a modern romance, and a great deal more wonderful, thus illustrating the force of the old saying, "Truth is stranger than fiction."

The three parts of our story may be best expressed by the three following inquiries:

I. How were the materials of which mountains are built up brought together and made into hard rock?

II. How were they raised up into the elevated positions in which we now find them?

III. How were they carved out into all their wonderful and beautiful features of crag and precipice, peaks and passes?

A mountain group, with its central peak or spire, its long ridges, steep walls, towers, buttresses, dark hollows, and carved pinnacles standing out against the sky, has well been compared to a great and stately building such as a cathedral or a temple. Mountains are indeed "a great and noble architecture, giving first shelter, comfort, and rest, but covered also with mighty sculpture and painted legend;" and to many they are Nature's shrines, where men may offer their humble praises and prayers to the great Architect who reared them for His children. We have introduced this illustration because it will help us in our inquiry. Suppose we were standing in front of some great cathedral, such as Milan, with all its marble pinnacles, or Notre Dame, with its stately towers, or the minsters of York or Durham in our own country, and trying to picture to ourselves how it was built. No one has lived long enough to watch the completion of one of these great buildings; but for all that, we know pretty well how it was made, even by watching the builder's operations for a short time, or by following, as we often may, the various stages in the construction of a small house. So it is with Nature's work. We cannot, in our little lives, witness the rearing of a great mountain-chain, or even the carving of a single hill; but we can observe for ourselves the slow and continuous operations which in the course of thousands and thousands of years produce such stupendous results. We may learn how the building operations are conducted, though the final results will only be manifested in the far-distant future.

But to return to our cathedral. If we try to picture to ourselves the long years during which it was covered with scaffolding and surrounded by a busy army of workers, we shall soon perceive that the operations may be broadly divided into three heads. _First_, we must inquire how the separate stones of which it is composed were brought together into one place, and we shall at once picture to ourselves groups of men working in stone-quarries,--perhaps a long way off,--busy with their crowbars and hammers, breaking off large blocks of stone, and following the natural divisions of the rock that their rough labour may be lessened; for all rocks will split more easily along certain lines than along others. Sometimes it is easier to follow the "bedding," or natural layers in which the rock was formed; at other times the "joints," or cracks subsequently formed as the rocky materials hardened and contracted in bulk, afford easier lines for the workmen to follow. Others are busily engaged in placing the stony blocks on trollies drawn by horses, that they may be borne along the roads leading from the quarry to the site of the future cathedral. And so, taking a bird's-eye view, we seem to see horses and carts slowly moving on from many a distant quarry, but all converging like the branches of a river to one main channel, and finally depositing their burdens in the stone-yard where the masons are at work. Perhaps bricks are partly employed, in which case we can easily picture to ourselves the brickyards, where some are digging out the soft clay, others moulding it into bricks with wooden moulds, while others again lay them down in rows on the ground to dry, before they are baked in the ovens. And when the bricks are ready for use, the same means of transportation are employed; and cart-loads of them are borne along the country roads until they so reach their destination.

Now, all this may be summed up in the one word "transportation;" and we shall presently inquire how the rocky matter of which the mountains are built was transported.

_Secondly._ We have to inquire how the bricks and stones were raised up. The analogy is not quite perfect in this case; for the mountains were raised up _en bloc_, not bit by bit and stone by stone, as in the case of the cathedral. Still they have been raised somehow. Analogies are seldom complete in every detail; but for all that, our illustration serves well enough, and will help us in following the various processes of mountain building. In these days, the raising of the stones is mostly effected by steam-power applied to big cranes and pulleys. In old days they used cranes and pulleys, but the ropes were pulled by hand-power. In either case the work proceeds slowly; and we can easily picture to ourselves the daily raising of the stones of which the cathedral is composed. "What were the forces at work which slowly raised the mountains?" This question we will endeavour to answer later on (see next chapter). This work may be included in the one word, "elevation."

_And lastly._ We must inquire how the carving of the stately building was effected, how its pinnacles received their shape, and how all those lovely details received their final forms; how the intricate traceries of its windows were made, and the statues carved which adorn its solemn portals. This question is easily answered, for we are all more or less familiar with what goes on in a stone-mason's yard. Under those wooden sheds we see a number of skilled labourers at work, busy with their chisels and mallets, cutting out, according to the patterns made from the architect's detailed drawings, the portions of tracery for windows, or the finials, crockets, and other features of the future building. In another part of the yard may be seen the stone-cutters, working in pairs and slowly pulling backwards and forwards those long saws which, with the help of water and sand, in time cut through the biggest blocks. All this work then may be summed up under the one word, "ornamentation," for it includes the cutting and carving of the stone.

Our three lines of inquiry may now be summed up in these three words, which are easily remembered:--

_Transportation_, _Elevation_, _Ornamentation_.

Taking the first of these subjects for consideration in the present chapter, we have now to inquire into the nature of the materials of which mountains are composed and the means by which they have been brought together and compacted into hard rock.

First, with regard to the nature of the materials which Mother Earth uses to build her rocky ramparts: they are the same as the ordinary rocks of which the earth's crust is composed; and the greater part of them have been formed by the action of water. These are the ordinary "stratified" rocks, which in one form or another meet us almost everywhere, and may be said to be aqueous deposits, or sediments formed in seas and inland lakes. They are always arranged in layers, known to geologists as "strata," because they have been gently laid down, or strewn (Latin, _stratum_), at the bottom of some large body of water. There were pauses in the deposition of the materials, during which each layer had time to harden a little before the next one was formed. This accounts for the stratification. In this way great deposits of sandstone, clay, and limestone, with their numerous varieties, have been in the course of ages gradually piled up, till they have attained to enormous thickness, which at first sight seem almost incredible; but the bed of the seas in which they formed was probably undergoing a slow sinking process that kept pace with the growth of these deposits, otherwise the sea might have been more or less filled up.

And these processes are still going on. In fact, it is entirely by watching what goes on now that geologists are able to explain what took place a very long time ago when there were no human beings on the earth to record the events that took place. And so we argue from the present to the past, from the known to the unknown. In other words, geology is based upon physical geography, which tells us of the changes now in progress on the earth. Thus, sandstone, as frequently met with in different parts of Great Britain, and largely used for building purposes, such as the familiar old red sandstone[20] of South Wales, Hereford, and the north of England and different parts of Scotland, was once soft sand in no way at all different from the sand of the seashore at the present day, or of the sandy bed of the North Sea. In process of time it became hardened, and acquired its characteristic red colour, which is due to oxide of iron. In some places numerous fossil fishes have been discovered in this interesting formation, so intimately associated with the name of Hugh Miller, who first thoroughly explored it; these and other remains entombed therein tell us of the strange forms of life which flourished on the earth during that very old-fashioned period of the world's history; and by putting together all kinds of evidences derived from the rock itself, geologists are able to form a very good idea of the way in which this rock-deposit was accumulated, always, however, basing their conclusions on a thorough knowledge of what goes on at the present day in seas, rivers, and inland lakes.

[20] The reader will find an account of the old red sandstone in the writer's "Autobiography of the Earth" (Edward Stanford, 1890).

In the great series of stratified rocks forming what is commonly called the crust of the earth (an unfortunate term which has survived from the time when the interior of the earth was generally believed to be in a fiery molten condition, and covered by a thin coating of solid rock at the surface), there are besides the sandstones, of which we have just spoken, great deposits of dark-coloured clays, shales, and slates. All these can be accounted for by the geologist. They are simply different states of what was once soft mud. The slates tell us that they have been subjected to very severe pressure, which squeezed their particles till they were elongated and all arranged in one direction, and this is the reason why they split up into thin sheets.

Others, again, represent vast deposits of carbonate of lime, thousands of feet thick and now occupying hundreds of square miles of the earth's surface. Limestone rocks are as abundant in our own country as the sandstones, shales, or slates. The chalk of which the North and South Downs are composed is a familiar example. It is seen again forming Salisbury Plain, in Hampshire and the Isle of Wight, and then it may be traced running up the country in a long band through the counties of Oxford, Cambridge, Lincoln, until it reaches the coast at Flamborough Head in Yorkshire. Then we have the Bath Oölites so much used in building, for they form an admirable "freestone" that can be easily carved and cut in any direction (hence the term "freestone"); and lastly, the great mountain limestone so well developed in South Wales, Yorkshire, and the Lake country. All these were slowly built up at the bottom of the seas which existed in past ages; great beds of gravel formed at the mouths of rivers, and long banks of pebbles and rounded stones collected on the shore of primeval seas, and were ground against each other as now by the action of the waves, until all their corners were rubbed off. Pebble-beds, called by geologists conglomerates, are met with among the stratified rocks; and their story is easily read by studying what takes place at the present day on our seashores.

Now, the sandstones, clays, gravels, and pebble-beds all represent, as will presently be explained, so much material worn away from the surface of the land and swept into the ocean (or in some cases into inland seas and lakes) by streams and rivers, which are the great transporting agents of the world. Hence such deposits of débris, supplied by the constant wear and tear of all rocks exposed to the atmosphere, are truly sedimentary and have a purely mechanical origin. But it is not so with the limestones. The latter were never transported, but grew at the bottom of the sea in very wonderful ways. They have nothing to do with the wear and tear of the land to which the others owe their existence, but represent vast quantities of carbonate of lime extracted from sea water. Sea water contains a certain amount of this substance in a dissolved state, or "in solution," as a chemist would say; and the way in which this is extracted by the agency of various creatures, such as coral polypes and little microscopic creatures that build their shells of carbonate of lime, of great beauty, forms one of the most interesting subjects presented to the student of physical geography. Hence, since limestone can only be accounted for by the agency of living organisms,[21] it is rightly termed an _organic deposit_, and the others are said to be _mechanical deposits_. But both are called "aqueous rocks," because they are formed under water. It is important to distinguish clearly between these two very different methods of rock-formation.

[21] The flints usually found in limestone are also of organic origin.

But although water plays such a very important part in the making of the common rocks around us, yet there are others which have quite a different origin,--rocks which have come up from below the surface of the earth in a heated and molten condition, such as the lavas that flow from volcanoes in active eruptions and the showers of ashes and fine volcanic dust which often attend such eruptions (see chap. viii., pp. 271-272). Some highly heated rocks, though they never rise to the surface to form lava-flows, are forced up with overwhelming pressure from below, and wedge themselves into the sedimentary rocks that overlie them, thus forming what are known as volcanic dykes, and intrusive masses or sheets of once molten rock. In this category we include such rocks as basalt, felstone, pitchstone, and other rocks of fiery origin that have flowed from volcanoes as lava, as well as those like granite, which have cooled and become solid _below_ the surface, and are Plutonic, or deep-seated, igneous rocks. Granite may be exposed to the surface of the earth when the rocks which once overlaid it have been worn away or "denuded." It is frequently seen in the central regions of mountain-chains, where a vast amount of erosion has been effected. Thus we see that heat has played its part in the making of rocks; and for this reason such rocks as we have just mentioned are called _igneous_. Fire and water are therefore very important geological agents; but we should say heat rather than fire, because the latter word might convey a false impression. No rocks can be burned except coal, which may be considered rather as a mineral deposit than as a rock. Some rocks may be heated, and undergo many and various changes in their mineral composition; but they are not capable of combustion.

So far, then, we have learned that the rocks exposed to view on the surface of the earth may be divided into two classes; that is, aqueous and igneous. There is yet a third class, which, though of aqueous origin, has in course of time suffered considerable from the internal heat of the earth and the enormous pressure due to the weight of overlying rocks. Such rocks have been greatly changed from their original condition, both in appearance and in mineral composition, and are said to be "metamorphic," a word which implies change. Thus chalk, or other limestone rock, has been metamorphosed into marble; shales and slates into various kinds of "schists,"[22] such as mica-schist, and even into gneiss, which closely resembles granite. And it is quite possible that even granite may in some cases be the result of the melting and consolidation under great pressure of certain familiar stratified rocks. It is quite conceivable that slate might be converted into granite, for their chemical composition is similar, only the minerals of which it is composed would require to be rearranged and grouped into new compounds. This would seem quite possible; but at present we have no direct proof of such a change having taken place. Even igneous rocks are found in some places to have suffered very considerable change.

[22] Schists are so named from their property of splitting into thin layers. Their structure is crystalline; and the layers, or folia, consist usually of two or more minerals, but sometimes of only one. Thus mica-schist consists of quartz and mica, each arranged in many folia, but it splits along the layers of mica.

In some inland seas, like the Caspian Sea, deposits of rock salt and gypsum may be formed by chemical precipitation, owing to evaporation from the surface.

The various kinds of rock known to geologists may be conveniently arranged as follows:

{ { Clay, shale, slate, etc. { I. Sedimentary. { Sandstones. { { Conglomerates. { Rocks of { { Limestones. aqueous { II. Organic. { Flint. origin. { { Coal. { { III. Chemical. { Rock salt. { { Gypsum, etc.

{ I. Volcanic. { Lavas. Rocks of { { Volcanic ashes, etc. igneous origin. { { II. Plutonic. { Basalt. { { Granite.

Metamorphic rocks { Marbles. of aqueous and { Various kinds of schists. igneous origin. { Gneiss, etc.

So far we have only attempted to state very briefly the different kinds of rocks, and to point out that they were formed in various ways. We must now consider the question of rock-making more closely, and see what we can learn about the wonderful ways in which rocks are made; and it may be instructive to glance at the conflicting opinions on this subject which learned men held not very long ago.

At the end of the last century a great controversy took place on the question of the origin of rocks, and the learned men of the day were divided into two parties. One of these parties, following the teaching of Werner, professor of mining at Freyburg, who inspired great enthusiasm among his disciples, declared that all rocks were formed by the agency of water. This was a very sweeping and of course rash conclusion. But whenever they examined rocks, they found so many clear evidences of the action of water that a powerful impression of the importance of this agency was naturally made on their minds. They found rocks uniformly arranged in great layers which extended for long distances, and containing the remains of animals which must undoubtedly have lived in the seas or estuaries. These layers were further divided into smaller layers, such as clearly were formed by the slow settling down of sand and mud. Others again contained gravels and rounded pebbles, testifying in no uncertain way to the action of water. Even the little grains of sand are obviously water-worn. This teaching was quite sound so long as they confined their attention to clays, sandstones, and limestones; but when they came to basalt and granite, a blind adherence to the views of their master caused them to shut their eyes to the clear evidences of the action of heat, presented by such rocks. The crystalline structure of such rocks; their irregular arrangement, often so different from the uniform disposition of the stratified rocks (although it must be admitted that ancient lava-flows often lie very evenly between aqueous rocks), and the way in which they burst through overlying rocks, thus proving their former molten condition; the signs of alteration exhibited in the aqueous rocks into which they intruded themselves (changes which are obviously due to the action of heat),--these and other evidences were entirely overlooked, and Werner declared that basalt had been found as a sediment under water.

This school of geologists, believing so strongly in the all-powerful influence of Father Neptune, received the not inappropriate title of "Neptunists."

On the other hand, the party who happened to be in districts where granite, basalt, and such igneous rocks abounded were equally impressed with the importance of the powerful agency of heat. To them nearly every rock they met with seemed to show some signs of its action. And since Pluto was the classical deity of the lower regions, and the earth shows evidences in places of greater heat below the surface, this party received the title of "Plutonists;" and so the battle raged hotly for some time between the Neptunists, with their claims for cold water, and the fiery Plutonists of the rival school of Edinburgh, with their subterranean heat. Fire and water are never likely to agree; and they did not do so in this case. But now that the battle is over, and both sides are found to have been partly right and partly wrong,--though the Neptunists have the advantage,--we can afford to smile at the fierceness of the contest, and wonder how it was that each side thought they were so entirely in the right.

Let us now consider the aqueous rocks, and see if we can gain a clear idea of the ways in which they were formed; and first, we will take those of a purely sedimentary origin,--the sandstones, pebble-beds, gravels, and clays. These, as the reader has already probably guessed, have all been transported by means of streams and rivers, and settled down quietly in seas at the mouths of rivers or in inland lakes. There is no trace of the action of heat in the forming of these rocks, though they often show signs of having suffered more or less change from contact with highly heated igneous rocks of later date which forcibly intruded themselves from below; and if the change thus effected were considerable, we should call the rocks so altered metamorphic. But we are now dealing with their original state and how they were made; and of that there is no possible doubt whatever. So for the time being we may call ourselves Neptunists.

Streams and rivers are the great transporting agents whereby the never-failing supply of débris from the waste of the land is unceasingly brought down from the mountains and hills, through the broad valleys and along the great plains, until finally it is flung into the sea. The sea is the workshop where all the sedimentary rocks are slowly manufactured from the raw material brought to it by the rivers. But for the present we must confine our attention to the question of transport. Referring back to our illustration of the cathedral, we may say that streams and rivers play the part of cart and horses. They bring the materials down from the quarry to the scene of action,--the workshop where they are wanted. The quarries, in this case, may be said to be almost everywhere. For wherever rocks and soil are exposed to the action of wind and weather, there is certain to be more or less decay and crumbling away. But it is among the hills and in the higher parts of the mountains that the forces of destruction are most active. How this is brought about will be discussed in the seventh chapter, on the carving of the hills. The frequent slopes covered with loose stones are sufficient evidence of the continual destruction that takes place in these regions.

The transporting powers of rivers are truly prodigious. Looking at a stream or river after heavy rain, we see its waters heavily laden with mud and sand; but it is difficult to realise from a casual glance the vast amount of material that is thus brought down to lower levels. If we could trace the sediment to its source, we must seek it among the rocks of mountains far away. Step by step we may trace it up along the higher courses of the river, then along mountain streams rushing over their rocky beds, tumbling in cascades over broken rocks, or leaping in waterfalls over higher projections of rock, until we come to the deep furrows on the sides of mountains along which loose fragments of rock come tumbling down with the cascades of water that run along these steep channels after heavy rain, leaving at the base of the mountain great fan-shaped heaps of stones.

"Oft both slope and hill are torn Where wintry torrents down have borne, And heaped upon the cumbered land Its wreck of gravel, rocks, and sand."

These accumulations are gradually carried away by the larger mountain streams, which in hurrying them along cause a vast amount of wear and tear; so that their corners are worn off, and they get further and further reduced in size, becoming mere round pebbles lining the bed of the stream, and finally by the time they reach the large slow-moving rivers of the plains are mainly reduced to tiny specks of mud or grains of sand. So then the rivers and streams not only transport sediment, but they manufacture it as they go along. And thus they may be considered as great grinding-mills, where large pieces of stone go in at one end, and only fine sand and mud come out at the other.

The amount of land débris thus transported depends partly on the carrying power of rivers, which varies with the seasons and the annual rainfall; partly on the size of the area drained by a river; and again, partly on the nature of the rocks of which that area is composed.

A stream, moving along at the rate of about half a mile (880 yards) an hour, which is a slow, rate, can carry along ordinary sandy soil suspended in a cloud-like fashion in the water; when moving at the rate of two thirds of a mile (about 1,173 yards) an hour, it can roll fine gravel along its bed; but when the rate increases to a yard in a second, or a little more than two miles an hour, it can sweep along angular stones as large as an egg. But streams often flow much faster than this, and so do rivers when swollen by heavy rain.

A rapid torrent often flows at the rate of eighteen or twenty miles an hour, and then we may hear the stones rattling against each other as they are irresistibly rolled onward; and during very heavy floods, huge masses of rock as large as a house have been known to be moved.

These are the two principal ways in which streams and rivers act as transporting agents: they carry the finer materials in a suspended state (though partly drifting it along their beds); and they push the coarser materials, such as gravel, bodily along. But there is one other way in which they carry on the important work of transportation, which, being unseen, might easily escape our notice. Every spring is busily employed in bringing up to the surface mineral substances which the water has dissolved out of the underground rocks. This invisible material finds its way, as the springs do, to the rivers, and so finally is brought into that great reservoir, the sea. Rain and river water also dissolve a certain amount of mineral matter from rocks lying on the surface of the earth. Now, the material which is most easily dissolved is carbonate of lime. Hence if you take a small quantity of spring or river water and boil it until the whole is evaporated, you will find that it leaves behind a certain amount of deposit. This, when analysed by the chemist, proves to be chiefly carbonate of lime; but it also contains minute quantities of other minerals, such as common salt, potash, soda, oxide of iron, and silica, or flint. All these and other minerals are found to be present in sea water.

The waters of some of the great rivers of the world have been carefully examined at different times, in order to form some idea of the amount of solid matter which they contain, both dissolved and suspended; and the results are extremely important and interesting, for they enable us to form definite conclusions with regard to their capacity for transport. This subject has been investigated with great skill by eminent men of science. The problem is a very complicated one; but it is easy to see that if we know roughly the number of gallons of water annually discharged into the sea by a big river, and the average amount of solid matter contained in such a gallon of water, we have the means of calculating, by a simple process of multiplication, the amount of solid matter annually brought down to the sea by that river. But we must also add the amount of sand, gravel, and stones pushed along its bed. This may be roughly estimated and allowed for. These are some of the results:

The amount of solid matter discharged every year by that great river, the Mississippi, if piled up on a single square mile of the bed of the sea,--say, in the Gulf of Mexico, where that river discharges itself,--would make a great square-shaped pile 268 feet high. But the Gulf Stream, sweeping through this gulf, carries the materials for many and many a mile away; so that in course of time it gradually sinks and spreads itself as a fine film or layer over part of the great Atlantic Ocean. The mud brought down by the great river Amazon spreads so far into the Atlantic Ocean as to discolour the water even at a distance of three hundred miles. The Ganges and the Brahmapootra, flowing into the Bay of Bengal, discharge every year into that part of the Indian Ocean 6,368,000,000 cubic feet of solid matter. This material would in one year raise a space of fifteen square miles one foot in height. The weight of mud, etc., that these rivers bring down is sixty times that of the Great Pyramid of Egypt, or about six million tons.

Or, to put the matter in another way, if a fleet of more than eighty "Indiamen," each with a cargo of fourteen hundred tons of solid matter, sailed down every hour, night and day, for four months, and discharged their burdens into the waters of the Indian Ocean, they would only do what the mighty Ganges does quietly and easily in the four months of the flood season.

It is probable that even the Thames, a small river compared to those just mentioned, manages to bring down, in one way or another, fourteen million cubic feet of solid matter. These few figures may suffice to give the reader some idea of the enormous amount of rock-forming materials brought down to the seas at the present day.

Of course they are spread out far and wide by the numerous ocean currents, some of which flow for hundreds of miles; and so the bed of the sea can only be very slowly raised by their accumulation. Still the geologist can allow plenty of time, for there is no doubt that the world is immensely old; and if we allow thousands of years, we may easily comprehend that deposits of very considerable thickness may in this way accumulate on the floors of the oceans. Also the coasts of continents and islands suffer continual wear and tear at the hands of sea waves; and thus the supply of sediment is increased.

When the geologist comes to study the great rock-masses--hundreds, and even thousands, of feet in thickness--of which mountain-ranges are composed, he finds all those kinds of rock which we have just been considering,--sandstones, shales (or hardened clays), pebble-beds, and limestones,--and endeavours to picture to himself their gradual growth in the ways we have described. In so doing, he is driven to the conclusion that many thousands of years must have been occupied in their construction.

We must now say a few words about those other aqueous rocks which have an organic origin, of which limestone is the chief. It is indeed a startling conclusion that deposits of great thickness, and ranging for very many miles over the earth's surface, have been slowly built up through the agency of marine animals extracting carbonate of lime from the sea. Yet such is undoubtedly the case. Of this important process of rock-building coral reefs are the most familiar example. The great barrier reef along the northeast coast of Australia is about 1,250 miles long, from ten to ninety miles in width, and rises at its seaward edge from depths which in some places certainly exceed eighteen hundred feet. It may be likened to a great submarine wall. Now, all this solid masonry is the work of humble coral polypes (not "insects"), building up their own internal framework or skeleton by extracting carbonate of lime from sea water. Then the breakers dashing against coral reefs produce, by their grinding action, a great deal of fine "coral-sand" and calcareous mud, which covers the surrounding bed of the sea for many miles.

Now, geologists find that some limestone formations met with in the stratified rocks have certainly been formed in this way; for example, certain parts of the great "mountain limestone." This is proved by the fossil corals it contains, and by tracing the old coral reefs; but it is also largely formed by the remains of other graceful calcareous creatures known as encrinites, or "sea-lilies," with long branching arms that waved in the clear water. Such creatures still exist in some deeper parts of the sea, and look more like plants than animals. In former ages they existed in great abundance, and so played an important part as rock-formers,--for their stems, branches, and all are made of little plates of carbonate of lime, beautifully fitting together like the separate bones, or vertebræ, composing the backbone of a fish; and when the creatures died, these little plates no longer held together, but were scattered on the floor of the sea-bed. Shell-fish abounded too, and their shelly remains accumulated into regular shell-beds in some places. But at times mud and sand would come and cover over all these organic deposits.

But of all rocks that have an organic origin, chalk is the most interesting. Geologists were for a long time puzzled to know how this rock could have been formed; but some soundings made in the Atlantic Ocean previous to the laying of the first Atlantic cable led to a very important discovery, which at once threw a flood of light on the question. Samples of the mud lying on the bed of this ocean at considerable distances from the European and American coasts, and at depths varying from one thousand to three thousand fathoms, were brought up by sounding apparatus.

Little was it thought that the dull grey ooze covering a large part of the Atlantic bed would bring a message from the depths of the sea, and furnish the answer to a great geological problem. Yet such was the case; for under the microscope this mud was seen to be chiefly composed of very minute and very beautiful shells, now known as _foraminifera_, and much prized by microscopists. These tiny shells are found at or near the surface of the sea; and after the death of the creatures that inhabit them (which are only lumps of protoplasm with no organs of any kind), the shells slowly sink down to the bed of the ocean. Now, these creatures multiply at so inconceivable a rate that a continuous shower of dead shells seems to be taking place, and the result is the slow accumulation over vast areas of the Atlantic and Pacific oceans of a great deposit of calcareous ooze, which if raised above the sea-level would harden into a rock very similar to chalk.

But this process only takes place in the deeper parts of our seas, far removed from land, where the supply of land-derived materials fails,--for even the finest mud supplied by rivers probably all settles down before travelling two or three hundred miles from its native shores.

Thus we learn that when one agency fails, Nature makes use of another to take up the important work of rock-building. How the other rocks which we mentioned in our list were formed,--such as granite, basalt, and the metamorphic rocks,--we must explain in a future chapter dealing with volcanoes and their work.