How the Crust of the Earth was Formed

The ups and downs of the planet—Fossil shells discovered before the time of Christ—Fossil fish—A time when the British Isles were not—A great continent of long ago—The surface of the planet—Where the soil came from—How the chalk cliffs were formed—Microscopic creatures—How the rocks were formed—Microscopic plants—How they multiply—Different kinds of rocks—The great coalfields—When the mountains appeared—The Ice Ages

Man discovered, long ago, that different parts of the Earth's crust have had their ups and downs. It became evident to him that the land we know to-day was once at the bottom of the ocean, and that what forms the bed of an ocean to-day was at one time dry land. Fossil shells were found away up on mountains far from the sea, so long ago as 500 B.C. But although some of the ancient philosophers declared that this Earth's surface was different entirely from what it had been at one time, it has taken man a long time to accept that fact. This doctrine, begun two thousand five hundred years ago, was opposed in quite recent times. But those in opposition had to invent strange suggestions to account for sea-shells being buried in the rocks on mountain-tops. One suggestion was that the shells had been carried there by the Flood in the days of Noah. Another was that pilgrim had carried the shells there, but neither of these suggestions appeals to ordinary reason.

Fossil shells are found to-day on mountain ranges three miles above the present level of the sea. Of course, this does not signify that the ocean level was three miles higher when these shells were deposited. The mountain has been forced up, and doubtless the sea-level has fallen also.

By careful study the geologist and the zoologist have been able to map out some of the ups and downs in the past history of our planet. It is quite certain that our British Isles were not always in evidence; more than once they have been at the bottom of the ocean. Just as surely do we know that during another period of long-ago Britain was part of a great continent, and primitive man could walk across valleys now covered by the English Channel and the German Ocean. But there is evidence that some parts of the Earth have remained dry land right away back so far as geological records go. For instance, Norway and Sweden have never been entirely under the sea, nor has the whole of Africa, India, or Australia been hidden completely as our island home has been.

At one time there existed an enormous continent upon which one could have travelled from South America right overland to Africa and thence to India, and again right overland to Australia. This great one-time continent, which we call "Gondwanaland," was a very ancient one. Although it was not the abode of man, it was peopled with plants and animals. Indeed it is the distribution of the fossil remains of these prehistoric plants which tells us that this continent must have existed in the long-ago.

Skeletons of Microscopic Creatures
In the upper illustration we see the actual size of the microscope slide and the group of Radiolaria or Polycystina. The second photograph shows a few of these magnified, while the third photograph shows one of the microscopic obisuts greatly magnified.

It is the geologist who has discovered the past history to the Earth. He has found that its crust has been built up of different layers, laid one upon another. He has been able to make some calculation as to the time which would be occupied in the formation of the different coatings. It was the geologist who declared first that the Earth was many millions of years in the making.

We know that to-day the surface of the planet is composed of soft soil, sand, gravel, and clays, with hard rock coming through at places. Even the amateur geologist is convinced easily that the soft soil has been made from the material which forms the rocks. The storms of past ages have helped to attack the rocks, the changes of temperature even now are helping to crack them, the rain has played its part, and chemical action has taken a leading hand in bringing about what we might call the rotting or decaying of the solid rock. When we open up the ground to cut up the solid rock for building purposes, the life history of the soft soil may be read; the decayed and broken rock is seen to form a subsoil between the soft soil and the hard rock.

We require this layer of soft soil wherever plants are to grow; the better soil the better growth. This soft soil must have existed wherever we find traces of vegetable life buried in the Earth, and there must have been vegetable life wherever there are traces of animal life.

At certain well-known places we find great chalk or limestone cliffs projecting through the Earth's surface. These are the deposits of past oceans, and the discovery of their formation is most interesting. A German professor, early last century, was examining pieces of these limestone rocks by means of his microscope, when he discovered that the rock was composed of very minute shells, not broken fragments of shells, but myriads of complete microscopic shells (see the illustration facing page 26). A million of these shells might be sent by letter-post for a penny.

These are tiny fossil shells, each of which was the home of a living creature in long-past ages. It goes without saying that the bodies of these minute creatures had no definite structures; they might be described as individual specks of jelly-like living substance. These microscopic creatures were named "Foraminifera." They are not all alike; one variety of these tiny creatures formed the rocks from which our Parisian friends have used great quantities of stones to build their houses.

While the Foraminifera were alive they floated about in the great oceans, and at their death their tiny shells sank to the bottom of the ocean. We are picturing what took place long before the creation of man, but similar Foraminifera live and die to-day. If we think of any great chalk cliffs which we know, it is difficult to realise that those vast deposits have been built up by the gradual and imperceptible deposit of almost invisible shells. The thickness of some of these deposits can be measured in hundreds of feet.

The shells of some of those apparently insignificant creatures go to form part of many important mountain ranges; among these are the Alps and the Himalayas. We marvel at the ingenuity of ancient man in building the great Pyramids of Egypt. Is it not much more marvellous that, by means of these small shell-creatures, Nature has built the great stones of which the Pyramids are made?

The ordinary chalk, with which the schoolmaster writes upon his blackboard, is made of minute microscopic shells, which are allied to those found in the Pyramids. This may be seen by examining a very thin slice of the chalk with the aid of a microscope.

When the German scientist made this discovery concerning the constitution of chalk, it was thought that only the softer kinds of rock, such as chalk and limestone, had been formed in this fashion. However, with other methods of preparing microscopic specimens of some harder rocks, it was seen that many hard rocks had been formed in a similar fashion.

These harder rocks had not been formed by the chalky shells which belong to the Foraminifera family, but by even more beautiful microscopic objects. In the illustration facing page 46 we can see a photograph of a few of these objects very greatly magnified. To the unaided vision each of these objects looks like a tiny speck of white dust or sand. The specimens are obtained by crushing a tiny fragment of the hard rock, and then washing the powdered rock in a strong solution of acid. By this means all the chalky matter or lime is dissolved and only these tiny microscopic objects are left. From the treatment given to this specimen it is apparent that these tiny objects are made of some very hard substance which is not easily dissolved by acid. We are not surprised to learn that they are made of that hard, flinty, glass-like substance known as "Silica."

The name of the class of these microscopic objects is "Polycystina," or "Radiolaria," and although they are referred to as shells, they are in reality the skeletons of tiny gelatinous creatures. The hard flinty objects, which are shown in the photographs, do not form a protecting shell; the living substance of the radiolaria is on the outside as well as on the inside of the hard skeleton. The living creatures float near the surface of warm oceans, and at their death they sink, leaving a cemetery of dead skeletons on the bed of the ocean. This deposit, becoming buried, produces in long ages a great variety of hard rocks. As in the ease of the Foraminif era, so we have living representatives of these radiolaria, whose fossil skeletons are sometimes called polycystinae.

Yet another class of microscopic objects take an important part in the formation of some rocks. In the photograph facing page 50 we see specimens of this third class, which are known as "Diatoms." This time we have not to deal with living animals, but with tiny aquatic plants. The microscopic plants possess a hard flinty covering, and it is these coatings which remain after the plant is decomposed. The quantities of these diatoms are enormous. Each diatom grows rapidly and divides into two, each of these two diatoms divides again, and so on, at a very remarkable rate.

Flinty Cases of Microscopic Plants
The upper illustration shows the microscopic slide con attune a very large number of diatoms. The centre illustration shows a few of these tiny objects enlarged, while the third photograph is of a single diatom greatly magnified.

In the process of division the succeeding diatoms get smaller very gradually, and when the offspring becomes reduced to about half the original size, the plant then casts off its flinty case, which sinks to the bottom of the ocean or lake. These diatoms are not necessarily marine as are the Foraminifera and the Radiolaria, indeed, we may find diatoms in the ordinary mud of any pond. Many of these beautiful diatoms are quite invisible to the unaided eye, and yet there are thousands of varieties. They are found in the oceans, lakes, and rivers of every part of the world.

It will be observed that the life history of the diatom, being a plant, is different from that of the other tiny creatures which go to form these deposits which ultimately become rock. The diatom plant does not lose its flinty coat merely at death; millions of diatoms are constantly shedding their coats, then growing larger, forming new flinty cases, again subdividing, and so on.

But for the microscope we should never have discovered the history of these different varieties of rock. Examine any one of those photographs of Foraminifera, Radiolaria or diatomacea; and try to realise that all that mass of detailed structure is contained in the tiniest speck which you can detect with the unaided vision.

Of course, we must not run away with the idea that all rocks have been formed in this fashion. Such sedimentary rocks could only be formed beneath an ocean, or in the case of the diatoms beneath an ocean or lake. We know that the solid rock mantle of the planet must have been formed before the oceans could exist. We know that this great globe was a molten mass at one time. We picture the heavier metallic substance sinking to form a core, and the stony material rising to form an outer crust. It is apparent that this solid rock mantle was formed by the solidification of molten matter. We speak of these rocks as "primary," because they were formed first. Another name we give them is "igneous," because they are fire-hardened.

When the amateur chemist makes a hot solution of some salts, and finds that in cooling there are beautiful crystals formed, the action is not unlike that which took place in the formation of some of the primary rocks, such as granite.

This hard rock mantle was subjected to atmospheric action, changes of temperature, and the action of water. Fragments were broken off and washed down by the running water, and in time these deposits formed sandstone and such-like "secondary rocks." And so we see that the rocks formed by the shells of the microscopic creatures and plants, already described, constitute only a part of the sedimentary or secondary rocks. All these deposited rocks occur in layers, and thus differ from the original unstratified rocks.

It goes without saying that the fossil shells, the fossil fish, and all fossils are found only in the sedimentary rocks, or in the deposits of gravel and other material on their surface. No shells, plants, or animals existed at the time when the fire-hardened rocks were formed.

The primary rock mantle is not hidden entirely by the secondary or deposited rocks. We see the hard, unstratified rock sticking up through the crust of the Earth in mountain ranges. Thus in the Highlands of Scotland there are many huge masses of primary rock.

There is no intention of considering in detail the geology of different classes of rocks, but one well-known class cannot be passed over, as its formation is so distinctly different from those already described. We are very familiar with that soft black rock which we call "coal," and the method of its formation is well known to all. But how do we know that the coal-fields are the mineralised vegetation of a far distant age? The geologist has no difficulty in convincing us of this fact. In the rocks which enclose the coal-beds he can show us innumerable fossils of ferns and trees. Many of us made collections of these during our school days. The geologist can show us the whole trunk of a tree fossilised with its roots still in the underclay. He may point to the peat-bogs of the present day and suggest that if these beds of peat were sunk gradually beneath the ocean, and in the process of time became covered over with deposits of sand and limestone, the peat would in due time become mineral coal. But better than all these arguments, he can give us direct evidence from the coal itself. By cutting thin slices of coal and examining them in a microscope, we can see even what particular kinds of plants went to form the coal. We can tell whether the plants belonged to the family of cone-bearing trees, to ferns, to horse-tail plants, or to club-mosses.

So it has been discovered that our coal-fields are due to the decayed vegetation of great forests; forests of fern-trees, cone-bearing trees, horse-tails, and club-mosses. We find that these grew to gigantic proportions in those far-off days. They varied from sixty to eighty or even one hundred feet in height, and the foliage was exceedingly rich. It is impossible to discover the exact condition of the Earth at that time, but the general evidence is that these great forests grew in a warm, moist atmosphere, and that the ground was soft and slimy, and thus well able to deal with the falling vegetation.

The coal-beds occur, one seam above another, with great deposits of clay and other sedimentary rocks between. Here we have another proof that these parts of the Earth have had their ups and downs. Each of these coal-seams was at one time on the surface of the Earth, so when we find a dozen or more different coal-seams in one mine, we know that that particular part of the globe was a dozen or more times covered by forests or swamps.

These great coal-seams were laid down a very long time ago, but even before that time the first mountains appeared on the surface of the Earth. There is no doubt that these upheavals were due to the Earth's crust trying to adjust the pressure of its plastic centre. One reasonable explanation is that the rivers, having been busy for millions of years transferring matter from the land to the ocean-beds, the balance of pressure was upset, and in the readjustment there occurred great uprisings and depressions of land. Some of our great rivers of to-day carry down hundreds of millions of tons of matter in a single year.

Long after the formation of these first mountains there was a further upset of the balance between the crust and the interior, due to the formation of the great masses of sedimentary rock built up by the microscopic creatures as already described. The weight of these immense deposits of limestone forced mountain ranges to appear very gradually but on a larger scale than the earlier mountains. This mountain building continued, up to so recent a period as two million years ago, long after there was animal life upon the planet.

During all these geologic changes we think of the Earth cooling very gradually, but we do not picture its surface as a tropical climate becoming more temperate. We know that even to-day there is a great variation of climate at different places, and that the Earth's polar regions are covered with ice. In the long ago, our own part of the world was covered with ice, not only once but possibly several times. Although we have discovered, from evidences in the Earth's crust, that great glaciers did exist in these parts, leaving their marks behind, and that parts of both Europe and America were covered with an enormous thickness of ice, yet we have failed to discover exactly how these conditions came about, or exactly when these ice ages occurred. Some geologists believe they have evidence that the whole ice ages occurred some twenty to sixty thousand years ago. Others see traces which they say take us back a few hundred thousand years, while they put down the disappearance of the last ice age to so long ago as eighty thousand years, and others seek to push its date still farther back.

In the present chapter we have seen how the solid crust of the Earth has been formed. It will be of interest to examine something of the history of the Earth which has been locked up in these gradual deposits. In the succeeding chapter we shall confine our attention to those discoveries which have special reference to the animal life of the Earth of long ago.