(From The World's Foundations.)
By AGNES GIBERNE.
"Stand still and consider the wondrous works of God."
What is the earth made of—this round earth upon which we human beings live and move?
A question more easily asked than answered, as regards a very large portion of it. For the earth is a huge ball nearly eight thousand miles in diameter, and we who dwell on the outside have no means of getting down more than a very little way below the surface. So it is quite impossible for us to speak positively as to the inside of the earth, and what it is made of. Some people believe the earth's inside to be hard and solid, while others believe it to be one enormous lake or furnace of fiery melted rock. But nobody really knows.
This outside crust has been reckoned to be of many different thicknesses. One man will say it is ten miles thick, and another will rate it at four hundred miles. So far as regards man's knowledge of it, gained from mining, from boring, from examination of rocks, and from reasoning out all that may be learned from these observations, we shall allow an ample margin if we count the field of geology to extend some twenty miles downwards from the highest mountain-tops. Beyond this we find ourselves in a land of darkness and conjecture.
Twenty miles is only one four-hundredth part of the earth's diameter—a mere thin shell over a massive globe. If the earth were brought down in size to an ordinary large school globe, a piece of rough brown paper covering it might well represent the thickness of this earth-crust, with which the science of geology has to do. And the whole of the globe, this earth of ours, is but one tiny planet in the great Solar System. And the centre of that Solar System, the blazing sun, though equal in size to more than a million earths, is yet himself but one star amid millions of twinkling stars, scattered broadcast through the universe. So it would seem at first sight that the field of geology is a small field compared with that of astronomy....
With regard to the great bulk of the globe little can be said. Very probably it is formed through and through of the same materials as the crust. This we do not know. Neither can we tell, even if it be so formed, whether the said materials are solid and cold like the outside crust, or whether they are liquid with heat. The belief has been long and widely held that the whole inside of the earth is one vast lake or furnace of melted fiery-hot material, with only a thin cooled crust covering it. Some in the present day are inclined to question this, and hold rather that the earth is solid and cold throughout, though with large lakes of liquid fire here and there, under or in her crust, from which our volcanoes are fed....
The materials of which the crust is made are many and various; yet, generally speaking, they may all be classed under one simple word, and that word is—Rock.
It must be understood that, when we talk of rock in this geological sense, we do not only mean hard and solid stone, as in common conversation. Rock may be changed by heat into a liquid or "molten" state, as ice is changed by heat to water. Liquid rock may be changed by yet greater heat to vapor, as water is changed to steam, only we have in a common way no such heat at command as would be needed to effect this. Rock may be hard or soft. Rock maybe chalky, clayey, or sandy. Rock may be so close-grained that strong force is needed to break it; or it may be so porous—so full of tiny holes—that water will drain through it; or it may be crushed and crumbled into loose grains, among which you can pass your fingers.
The cliffs above our beaches are rock; the sand upon our seashore is rock; the clay used in brick-making is rock; the limestone of the quarry is rock; the marble of which our mantel-pieces are made is rock. The soft sandstone of South Devon, and the hard granite of the north of Scotland, are alike rock. The pebbles in the road are rock; the very mould in our gardens is largely composed of crumbled rock. So the word in its geological sense is a word of wide meaning.
Now the business of the geologist is to read the history of the past in these rocks of which the earth's crust is made. This may seem a singular thing to do, and I can assure you it is not an easy task.
For, to begin with, the history itself is written in a strange language, a language which man is only just beginning to spell out and understand. And this is only half the difficulty with which we have to struggle.
If a large and learned book were put before you and you were set to read it through, you would perhaps, have no insurmountable difficulty, with patience and perseverance, in mastering its meaning.
But how if the book were first chopped up into pieces, if part of it were flung away out of reach, if part of it were crushed into a pulp, if the numbering of the pages were in many places lost, if the whole were mixed up in confusion, and if then you were desired to sort, and arrange, and study the volume?
Picture to yourself what sort of a task this would be, and you will have some idea of the labors of the patient geologist.
Rocks may be divided into several kinds or classes. For the present moment it will be enough to consider the two grand divisions—Stratified rocks and Unstratified rocks.
Unstratified rocks are those which were once, at a time more or less distant, in a melted state from intense heat, and which have since cooled into a half crystallized state; much the same as water, when growing colder, cools and crystallizes into ice. Strictly speaking, ice is rock, just as much as granite and sandstone are rock. Water itself is of the nature of rock, only as we commonly know it in the liquid state we do not commonly call it so.
"Crystallization" means those particular forms or shapes in which the particles of a liquid arrange themselves, as that liquid hardens into a solid—in other words, as it freezes. Granite, iron, marble, are frozen substances, just as truly as ice is a frozen substance; for with greater heat they would all become liquid like water. When a liquid freezes, there are always crystals formed, though these are not always visible without the help of a microscope. Also the crystals are of different shapes with different substances.
If you examine the surface of a puddle or pond, when a thin covering of ice is beginning to form, you will be able to see plainly the delicate sharp needle-like forms of the ice crystals. Break a piece of ice, and you will find that it will not easily break just in any way that you may choose, but it will only split along the lines of these needle-like crystals. This particular mode of splitting in a crystallized rock is called the cleavage of that rock.
Crystallization may take place either slowly or rapidly, and either in the open air or far below ground. The lava from a volcano is an example of rock which has crystallized rapidly in the open air; and granite is an example of rock which has crystallized slowly underground beneath great pressure.
Stratified rocks, on the contrary, which make up a very large part of the earth's crust, are not crystallized. Instead of having cooled from a liquid into a solid state, they have been slowly built up, bit by bit and grain upon grain, into their present form, through long ages of the world's history. The materials of which they are made were probably once, long, long ago, the crumblings from granite and other crystallized rocks, but they show now no signs of crystallization.
a. Conglomerate. b. Pebbly Sandstone, c. Thin-bedded Sandstone, d. Shelly Sandstone, e. Shale. f. Limestone.
They are called "stratified" because they are in themselves made up of distinct layers, and also because they lie thus one upon another in layers, or strata, just as the leaves of a book lie, or as the bricks of a house are placed.
Throughout the greater part of Europe, of Asia, of Africa, of North and South America, of Australia, these rocks are to be found, stretching over hundreds of miles together, north, south, east, and west, extending up to the tops of some of the earth's highest mountains, reaching down deep into the earth's crust. In many parts if you could dig straight downwards through the earth for thousands of feet, you would come to layer after layer of these stratified rocks, one kind below another, some layers thick, some layers thin, here a stratum of gravel, there a stratum of sandstone, here a stratum of coal, there a stratum of clay.
But how, when, where, did the building up of all these rock-layers take place?
THE BEACH IN THE FOREGROUND IS A ROCKY SHELF, THE REMNANT OF THE CLIFF WHICH ONCE EXTENDED OUT TO THE ISLAND.
People are rather apt to think of land and water on the earth as if they were fixed in one changeless form,—as if every continent and every island were of exactly the same shape and size now that it always has been and always will be.
Yet nothing can be further from the truth. The earth-crust is a scene of perpetual change, of perpetual struggle, of perpetual building up, of perpetual wearing away.
The work may go on slowly, but it does go on. The sea is always fighting against the land, beating down her cliffs, eating into her shores, swallowing bit by bit of solid earth; and rain and frost and inland streams are always busily at work, helping the ocean in her work of destruction. Year by year and century by century it continues. Not a country in the world which is bordered by the open sea has precisely the same coast-line that it had one hundred years ago; not a land in the world but parts each century with masses of its material, washed piecemeal away into the ocean.
Is this hard to believe? Look at the crumbling cliffs around old England's shores. See the effect upon the beach of one night's fierce storm. Mark the pathway on the cliff, how it seems to have crept so near the edge that here and there it is scarcely safe to tread; and very soon, as we know, it will become impassable. Just from a mere accident, of course,—the breaking away of some of the earth, loosened by rain and frost and wind. But this is an accident which happens daily in hundreds of places around the shores.
Leaving the ocean, look now at this river in our neighborhood, and see the slight muddiness which seems to color its waters. What from? Only a little earth and sand carried off from the banks as it flowed,—very unimportant and small in quantity, doubtless, just at this moment and just at this spot. But what of that little going on week after week, and century after century, throughout the whole course of the river, and throughout the whole course of every river and rivulet in our whole country and in every other country. A vast amount of material must every year be thus torn from the land and given to the ocean. For the land's loss here is the ocean's gain.
And, strange to say, we shall find that this same ocean, so busily engaged with the help of its tributary rivers in pulling down land, is no less busily engaged with their help in building it up.
You have sometimes seen directions upon a vial of medicine to "shake" before taking the dose. When you have so shaken the bottle the clear liquid grows thick; and if you let it stand for awhile the thickness goes off, and a fine grain-like or dust-like substance settles down at the bottom—the settlement or sediment of the medicine. The finer this sediment, the slower it is in settling. If you were to keep the liquid in gentle motion, the fine sediment would not settle down at the bottom. With coarser and heavier grains the motion would have to be quicker to keep them supported in the water.
Now it is just the same thing with our rivers and streams. Running water can support and carry along sand and earth, which in still water would quickly sink to the bottom; and the more rapid the movement of the water, the greater is the weight it is able to bear.
This is plainly to be seen in the case of a mountain torrent. As it foams fiercely through its rocky bed it bears along, not only mud and sand and gravel, but stones and even small rocks, grinding the latter roughly together till they are gradually worn away, first to rounded pebbles, then to sand, and finally to mud. The material thus swept away by a stream, ground fine, and carried out to sea—part being dropped by the way on the river-bed—is called detritus, which simply means worn-out material.
The tremendous carrying-power of a mountain torrent can scarcely be realized by those who have not observed it for themselves. I have seen a little mountain-stream swell in the course of a heavy thunderstorm to such a torrent, brown and turbid with earth torn from the mountainside, and sweeping resistlessly along in its career a shower of stones and rock-fragments. That which happens thus occasionally with many streams is more or less the work all the year round of many more.
As the torrent grows less rapid, lower down in its course, it ceases to carry rocks and stones, though the grinding and wearing away of stones upon the rocky bed continues, and coarse gravel is borne still upon its waters. Presently the widening stream, flowing yet more calmly, drops upon its bed all such coarser gravel as is not worn away to fine earth, but still bears on the lighter grains of sand. Next the slackening speed makes even the sand too heavy a weight, and that in turn falls to line the river-bed, while the now broad and placid stream carries only the finer particles of mud suspended in its waters. Soon it reaches the ocean, and the flow being there checked by the incoming ocean-tide, even the mud can no longer be held up, and it also sinks slowly in the shallows near the shore, forming sometimes broad mud-banks dangerous to the mariner.
This is the case only with smaller rivers. Where the stream is stronger, the mud-banks are often formed much farther out at sea; and more often still the river-detritus is carried away and shed over the ocean-bed, beyond the reach of our ken. The powerful rush of water in earth's greater streams bears enormous masses of sand and mud each year far out into the ocean, there dropping quietly the gravel, sand, and earth, layer upon layer at the bottom of the sea. Thus pulling down and building up go on ever side by side; and while land is the theatre oftentimes of decay and loss, ocean is the theatre oftentimes of renewal and gain.
Did you notice the word "sediment" used a few pages back about the settlement at the bottom of a medicine-vial?
There is a second name given to the Stratified Rocks, of which the earth's crust is so largely made up. They are called also Sedimentary Rocks.
The reason is simply this. The Stratified Rocks of the present day were once upon a time made up out of the sediment stolen first from land and then allowed to settle down on the sea-bottom.
Long, long ago, the rivers, the streams, the ocean, were at work, as they are now, carrying away rock and gravel, sand and earth. Then, as now, all this material, borne upon the rivers, washed to and fro by the ocean, settled down at the mouths of rivers or at the bottom of the sea, into a sediment, one layer forming over another, gradually built up through long ages. At first it was only a soft, loose, sandy or muddy sediment, such as you may see on the seashore, or in a mud-bank. But as the thickness of the sediment increased, the weight of the layers above gradually pressed the lower layers into firm hard rocks; and still, as the work of building went on, these layers were, in their turn, made solid by the increasing weight over them. Certain chemical changes had also a share in the transformation from soft mud to hard rock, which need not be here considered.
All this has through thousands of years been going on. The land is perpetually crumbling away; and fresh land under the sea is being perpetually built up, from the very same materials which the sea and the rivers have so mercilessly stolen from continents and islands. This is the way, if geologists rightly judge, in which a very large part of the enormous formations of Stratified or Sedimentary Rocks have been made.
So far is clear. But now we come to a difficulty.
The Stratified Rocks, of which a very large part of the continents is made, appear to have been built up slowly, layer upon layer, out of the gravel, sand, and mud, washed away from the land and dropped on the shore of the ocean.
You may see these layers for yourself as you walk out into the country. Look at the first piece of bluff rock you come near, and observe the clear pencil-like markings of layer above layer—not often indeed lying flat, one over another, and this must be explained later, but however irregularly slanting, still plainly visible. You can examine these lines of stratification on the nearest cliff, the nearest quarry, the nearest bare headland, in your neighborhood.
But how can this be? If all these stratified rocks are built on the floor of the ocean out of material taken from the land, how can we by any possibility find such rocks upon the land? In the beds of rivers we might indeed expect to see them, but surely nowhere else save under ocean waters.
Yet find them we do. Through England, through the two great world-continents, they abound on every side. Thousands of miles in unbroken succession are composed of such rocks.
Stand with me near the seashore, and let us look around. Those white chalk cliffs—they, at least, are not formed of sand or earth. True, and the lines of stratification are in them very indistinct, if seen at all; yet they too are built up of sediment of a different kind, dropping upon ocean's floor. See, however, in the rough sides of yonder bluff the markings spoken of, fine lines running alongside of one another, sometimes flat, sometimes bent or slanting, but always giving the impression of layer piled upon layer. Yet how can one for a moment suppose that the ocean-waters ever rose so high?
Stay a moment. Look again at yonder white chalk cliff, and observe a little way below the top a singular band of shingles, squeezed into the cliff, as it were, with chalk below and earth above.
That is believed to be an old sea-beach. Once upon a time the waters of the sea are supposed to have washed those shingles, as now they wash the shore near which we stand, and all the white cliff must have lain then beneath the ocean.
Geologists were for a long while sorely puzzled to account for these old sea-beaches, found high up in the cliffs around our land in many different places.
They had at first a theory that the sea must once, in far back ages, have been a great deal higher than it is now. But this explanation only brought about fresh difficulties. It is quite impossible that the level of the sea should be higher in one part of the world than in another. If the sea around England were then one or two hundred feet higher than it is now, it must have been one or two hundred feet higher in every part of the world where the ocean-waters have free flow. One is rather puzzled to know where all the water could have come from, for such a tremendous additional amount. Besides, in some places remains of sea-animals are found in mountain heights, as much as two or three thousand feet above the sea-level—as, for instance, in Corsica. This very much increases the difficulty of the above explanation.
So another theory was started instead, and this is now generally supposed to be the true one. What if instead of the whole ocean having been higher, parts of the land were lower? England at one time, parts of Europe at another time, parts of Asia and America at other times, may have slowly sunk beneath the ocean, and after long remaining there have slowly risen again.
This is by no means so wild a supposition as it may seem when first heard, and as it doubtless did seem when first proposed. For even in the present day these movements of the solid crust of our earth are going on. The coasts of Sweden and Finland have long been slowly and steadily rising out of the sea, so that the waves can no longer reach so high upon those shores as in years gone by they used to reach. In Greenland, on the contrary, land has long been slowly and steadily sinking, so that what used to be the shore now lies under the sea. Other such risings and sinkings might be mentioned, as also many more in connection with volcanoes and earthquakes, which are neither slow nor steady, but sudden and violent.
So it becomes no impossible matter to believe that, in the course of ages past, all those wide reaches of our continents and islands, where sedimentary rocks are to be found, were each in turn, at one time or another, during long periods, beneath the rolling waters of the ocean....
These built-up rocks are not only called "Stratified," and "Sedimentary." They have also the name of Aqueous Rock, from the Latin word aqua, water; because they are believed to have been formed by the action of the water.
They have yet another and fourth title, which is, Fossiliferous Rocks.
Fossils are the hardened remains of animals and vegetables found in rocks. They are rarely, if ever, seen in unstratified rocks; but many layers of stratified rocks abound in these remains. Whole skeletons as well as single bones, whole tree-trunks as well as single leaves, are found thus embedded in rock-layers, where in ages past the animal or plant died and found a grave. They exist by thousands in many parts of the world, varying in size from the huge skeleton of the elephant to the tiny shell of the microscopic animalcule.
Fossils differ greatly in kind. Sometimes the entire shell or bone is changed into stone, losing all its animal substance, but retaining its old outline and its natural markings. Sometimes the fossil is merely the hardened impress of the outside of a shell or leaf, which has dented its picture on soft clay, and has itself disappeared, while the soft clay has become rock, and the indented picture remains fixed through after-centuries. Sometimes the fossil is the cast of the inside of a shell; the said shell having been filled with soft mud, which has taken its exact shape and hardened, while the shell itself has vanished. The most complete description of fossil is the first of these three kinds. It is wonderfully shown sometimes in fossil wood, where all the tiny cells and delicate fibres remain distinctly marked as of old, only the whole woody substance has changed into hard stone.
But although the fossil remains of quadrupeds and other land-animals are found in large quantities, their number is small compared with the enormous number of fossil sea-shells and sea-animals.
Land-animals can, as a rule, have been so preserved, only when they have been drowned in ponds or rivers, or mired in bogs and swamps, or overtaken by frost, or swept out to sea.
Sea-animals, on the contrary, have been so preserved on land whenever that land has been under the sea; and this appears to have been the case, at one or another past age, with the greater part of our present continents. These fossil remains of sea-animals are discovered in all quarters of the world, not only on the seashore but also far inland, not only deep down underground but also high up on the tops of lofty mountains—a plain proof that over the summits of those mountains the ocean must once have rolled, and this not for a brief space only, but through long periods of time. And not on the mountain-summit only are these fossils known to abound, but sometimes in layer below layer of the mountain, from top to bottom, through thousands of feet of rock.
This may well seem puzzling at first sight. Fossils of sea-creatures on a mountain-top are startling enough; yet hardly so startling as the thought of fossils inside that mountain. How could they have found their way thither?
The difficulty soon vanishes, if once we clearly understand that all these thousands of feet of rock were built up slowly, layer after layer, when portions of the land lay deep under the sea. Thus each separate layer of mud or sand or other material became in its turn the top layer, and was for the time the floor of the ocean, until further droppings of material out of the waters made a fresh layer, covering up the one below.
While each layer was thus in succession the top layer of the building, and at the same time the floor of the ocean, animals lived and died in the ocean, and their remains sank to the bottom, resting upon the sediment floor. Thousands of such dead remains disappeared, crumbling into fine dust and mingling with the waters, but here and there one was caught captive by the half-liquid mud, and was quickly covered and preserved from decay. And still the building went on, and still layer after layer was placed, till many fossils lay deep down beneath the later-formed layers; and when at length, by slow or quick upheaval of the ground, this sea-bottom became a mountain, the little fossils were buried within the body of that mountain. So wondrously the matter appears to have come about.
Another difficulty with respect to the stratified rocks has to be thought of. All these layers or deposits of gravel, sand, or earth, on the floor of the ocean, would naturally be horizontal—that is, would lie flat, one upon another. In places the ocean-floor might slant, or a crevice or valley or ridge might break the smoothness of the deposit. But though the layers might partake of the slant, though the valley might have to be filled, though the ridge might have to be surmounted, still the general tendency of the waves would be to level the dropping deposits into flat layers.
Then how is it that when we examine the strata of rocks in our neighborhood, wherever that neighborhood may be, we do not find them so arranged? Here, it is true, the lines for a space are nearly horizontal, but there, a little way farther on, they are perpendicular; here they are bent, and there curved; here they are slanting, and there crushed and broken.
This only bears out what has been already said about the Book of Geology. It has been bent and disturbed, crushed and broken.
Great powers have been at work in this crust of our earth. Continents have been raised, mountains have been upheaved, vast masses of rock have been scattered into fragments. Here or there we may find the layers arranged as they were first laid down; but far more often we discover signs of later disturbance, either slow or sudden, varying from a mere quiet tilting to a violent overturn.
So the Book of Geology is a torn and disorganized volume, not easy to read.
Yet, on the other hand, these very changes which have taken place are a help to the geologist.
It may seem at first sight as if we should have an easier task, if the strata were all left lying just as they were first formed, in smooth level layers, one above another. But if it were so, we could know very little about the lower layers.
We might indeed feel sure, as we do now, that the lowest layers were the oldest and the top layers the newest, and that any fossils found in the lower layers must belong to an age farther back than any fossils found in the upper layers.
So much would be clear. And we might dig also and burrow a little way down, through a few different kinds of rock, where they were not too thick. But that would be all. There our powers would cease.
Now how different. Through the heavings and tiltings of the earth's crust, the lower layers are often pushed quite up to the surface, so that we are able to examine them and their fossils without the least difficulty, and very often without digging underground at all.
You must not suppose that the real order of the rocks is changed by these movements, for generally speaking it is not. The lower kinds are rarely if ever found placed over the upper kinds; only the ends of them are seen peeping out above ground.
It is as if you had a pile of copy-books lying flat one upon another, and were to put your finger under the lowest and push it up. All those above would be pushed up also, and perhaps they would slip a little way down, so that you would have a row of edges showing side by side, at very much the same height. The arrangement of the copy-books would not be changed, for the lowest would still be the lowest in actual position; but a general tilting or upheaval would have taken place.
Just such a tilting or upheaval has taken place again and again with the rocks forming our earth-crust. The edges of the lower rocks often show side by side with those of higher layers.
But geologists know them apart. They are able to tell confidently whether such and such a rock, peeping out at the earth's surface, belongs really to a lower or a higher kind. For there is a certain sort of order followed in the arrangement of rock-layers all over the earth, and it is well known that some rocks are never found below some other rocks, that certain particular kinds are never placed above certain other kinds. Thus it follows that the fossils found in one description of rock, must be the fossils of animals which lived and died before the animals whose fossil remains are found in another neighboring rock, just because this last rock-layer was built upon the ocean-floor above and therefore later than the other.
All this is part of the foreign language of geology—part of the piecing and arranging of the torn volume. Many mistakes are made; many blunders are possible; but the mistakes and blunders are being gradually corrected, and certain rules by which to read and understand are becoming more and more clear.
It has been already said that unstratified rocks are those which have been at some period, whether lately or very long ago, in a liquid state from intense heat, and which have since cooled, either quickly or slowly, crystallizing as they cooled.
Unstratified Rocks may be divided into two distinct classes.
First.—Volcanic Rocks, such as lava. These have been quickly cooled at the surface of the earth, or not far below it.
Secondly.—Plutonic Rocks, such as granite. These have been slowly cooled deep down in the earth under heavy pressure.
There is also a class of rocks, called metamorphic rocks, including some kinds of marble. These are, strictly speaking, crystalline rocks, and yet they are arranged in something like layers. The word "metamorphic" simply means "transformed." They are believed to have been once stratified rocks, perhaps containing often the remains of animals; but intense heat has later transformed them into crystalline rocks, and the animal remains have almost or quite vanished.
Just as the different kinds of Stratified Rocks are often called Aqueous Rocks, or rocks formed by the action of water—so these different kinds of Unstratified Rocks are often called Igneous Rocks, or rocks formed by the action of fire—the name being taken from the Latin word for fire. The Metamorphic Rocks are sometimes described as "Aqueo-igneous," since both water and fire helped in the forming of them.
It was at one time believed, as a matter of certainty, that granite and such rocks belonged to a period much farther back than the periods of the stratified rocks. That is to say, it was supposed that fire-action had come first and water-action second; that the fire-made rocks were all formed in very early ages, and that only water-made rocks still continued to be formed. So the name of Primary Rocks, or First Rocks, was given to the granites and other such rocks, and the name of Secondary Rocks to all water-built rocks; while those of the third class were called Transition Rocks, because they seemed to be a kind of link or stepping-stone in the change from the First to the Second Rocks.
The chief reason for the general belief that fire-built rocks were older than water-built ones was, that the former are as a rule found to lie lower than the latter. They form, as it were, the basement of the building, while the top-stories are made of water-built rocks.
Many still believe that there is much truth in the thought. It is most probable, so far as we are able to judge, that the first-formed crust of rocks all over the earth was of cooled and crystallized material. As these rocks were crumbled and wasted by the ocean, materials would have been supplied for the building-up of rocks, layer upon layer.
But this is conjecture. We cannot know with any certainty the course of events so far back in the past. And geologists are now able to state with tolerable confidence that, however old many of the granites may be, yet a large amount of the fire-built rocks are no older than the water-built rocks which lie over them.
So by many geologists the names of Primary, Transition, and Secondary Formations are pretty well given up. It has been proposed to give instead to the crystallized rocks of all kinds the name of Underlying Rocks (Hypogene Rocks).
But if they really do lie under, how can they possibly be of the same age? One would scarcely venture to suppose, in looking at a building, that the cellars had not been finished before the upper floors.
True. In the first instance doubtless the cellars were first made, then the ground-floor, then the upper stories.
When, however, the house was so built, alterations and improvements might be very widely carried on above and below. While one set of workmen were engaged in remodelling the roof, another set of workmen might be engaged in remodelling the kitchens and first floor, pulling down, propping up, and actually rebuilding parts of the lower walls.
This is precisely what the two great fellow-workmen, Fire and Water, are ever doing in the crust of our earth. And if it be objected that such alterations too widely undertaken might result in slips, cracks, and slidings, of ceilings and walls in the upper stories, I can only say that such catastrophes have been the result of underground alterations in that great building, the earth's crust....
We see therefore clearly that, although the earliest fire-made rocks may very likely date farther back than the earliest water-made rocks, yet the making of the two kinds has gone on side by side, one below and the other above ground, through all ages up to the present moment.
And just as in the present day water continues its busy work above ground of pulling down and building up, so also fire continues its busy work underground of melting rocks which afterwards cool into new forms, and also of shattering and upheaving parts of the earth-crust.
For there can be no doubt that fiery heat does exist as a mighty power within our earth, though to what extent we are not able to say.
These two fellow-workers in nature have different modes of working. One we can see on all sides, quietly progressing, demolishing land patiently bit by bit, building up land steadily grain by grain. The other, though more commonly hidden from sight, is fierce and tumultuous in character, and shows his power in occasional terrific outbursts.
We can scarcely realize what the power is of the imprisoned fiery forces underground, though even we are not without some witness of their existence. From time to time even our firm land has been felt to tremble with a thrill from some far-off shock; and even in our country is seen the marvel of scalding water pouring unceasingly from deep underground....
Think of the tremendous eruptions of Vesuvius, of Etna, of Hecla, of Mauna Loa. Think of whole towns crushed and buried, with their thousands of living inhabitants. Think of rivers of glowing lava streaming up from regions below ground, and pouring along the surface for a distance of forty, fifty, and even sixty miles, as in Iceland and Hawaii. Think of red-hot cinders flung from a volcano-crater to a height of ten thousand feet. Think of lakes of liquid fire in other craters, five hundred to a thousand feet across, huge cauldrons of boiling rock. Think of showers of ashes from the furnace below of yet another, borne so high aloft as to be carried seven hundred miles before they sank to earth again. Think of millions of red-hot stones flung out in one eruption of Vesuvius. Think of a mass of rock, one hundred cubic yards in size, hurled to a distance of eight miles or more out of the crater of Cotopaxi.
Think also of earthquake-shocks felt through twelve hundred miles of country. Think of fierce tremblings and heavings lasting in constant succession through days and weeks of terror. Think of hundreds of miles of land raised several feet in one great upheaval. Think of the earth opening in scores of wide-lipped cracks, to swallow men and beasts. Think of hot mud, boiling water, scalding stream, liquid rock, bursting from such cracks, or pouring from rents in a mountain-side.
Truly these are signs of a state of things in or below the solid crust on which we live, that may make us doubt the absolute security of "Mother Earth."
Different explanations have been put forward to explain this seemingly fiery state of things underground.
Until lately the belief was widely held that our earth was one huge globe of liquid fire, with only a slender cooled crust covering her, a few miles in thickness.
This view was supported by the fact that heat is found to increase as men descend into the earth. Measurements of such heat-increase have been taken, both in mines and in borings for wells. The usual rate is about one degree more of heat, of our common thermometer, for every fifty or sixty feet of descent. If this were steadily continued, water would boil at a depth of eight thousand feet below the surface; iron would melt at a depth of twenty-eight miles; while at a depth of forty or fifty miles no known substance upon earth could remain solid.
The force of this proof is, however, weakened by the fact that the rate at which the heat increases differs very much in different places. Also it is now generally supposed that such a tremendous furnace of heat—a furnace nearly eight thousand miles in diameter—could not fail to break up and melt so slight a covering shell.
Many believe, therefore, not that the whole interior of the earth is liquid with heat, but that enormous fire-seas or lakes of melted rock exist here and there, under or in the earth-crust. From these lakes the volcanoes would be fed, and they would be the cause of earthquakes and land-upheavals or land-sinkings. There are strong reasons for supposing that the earth was once a fiery liquid body, and that she has slowly cooled through long ages. Some hold that her centre probably grew solid first from tremendous pressure; that her crust afterwards became gradually cold; and that between the solid crust and the solid inside or "nucleus," a sea of melted rock long existed, the remains of which are still to be found in these tremendous fiery reservoirs.
The idea accords well with the fact that large numbers of extinct or dead volcanoes are scattered through many parts of the earth. If the above explanation be the right one, doubtless the fire-seas in the crust extended once upon a time beneath such volcanoes, but have since died out or smouldered low in those parts.
A somewhat curious calculation has been made, to illustrate the different modes of working of these two mighty powers—Fire and Water.
The amount of land swept away each year in mud, and borne to the ocean by the River Ganges, was roughly reckoned, and also the amount of land believed to have been upheaved several feet in the great Chilian earthquake.
It was found that the river, steadily working month by month, would require some four hundred years to carry to the sea the same weight of material, which in one tremendous effort was upheaved by the fiery underground forces.
Yet we must not carry this distinction too far. Fire does not always work suddenly, or water slowly; witness the slow rising and sinking of land in parts of the earth, continuing through centuries; and witness also the effects of great floods and storms.
The crust of the earth is made of rock. But what is rock made of?
Certain leading divisions of rocks have been already considered:
The Water-made Rocks;
The Fire-made Rocks, both Plutonic and Volcanic;
The Water-and-Fire-made Rocks.
The first of these—Water-made Rocks—may be subdivided into three classes. These are,—
I. Flint Rocks; II. Clay Rocks; III. Lime Rocks.
This is not a book in which it would be wise to go closely into the mineral nature of rocks. Two or three leading thoughts may, however, be given.
Does it not seem strange that the hard and solid rocks should be in great measure formed of the same substances which form the thin invisible air floating around us?
Yet so it is. There is a certain gas called Oxygen Gas. Without that gas you could not live many minutes. Banish it from the room in which you are sitting, and in a few minutes you will die.
This gas makes up nearly one-quarter by weight of the atmosphere round the whole earth.
The same gas plays an important part in the ocean; for more than three-quarters of water is oxygen.
It plays also an important part in rocks; for about half the material of the entire earth's crust is oxygen.
Another chief material in rocks is silicon. This makes up one-quarter of the crust, leaving only one-quarter to be accounted for. Silicon mixed with oxygen makes silica or quartz. There are few rocks which have not a large amount of quartz in them. Common flint, sandstones, and the sand of our shores, are made of quartz, and therefore belong to the first class of Silicious or Flint Rocks. Granites and lavas are about one-half quartz. The beautiful stones, amethyst, agate, chalcedony, and jasper, are all different kinds of quartz.
Another chief material in rocks is a white metal called aluminium. United to oxygen it becomes alumina, the chief substance in clay. Rocks of this kind—such as clays, and also the lovely blue gem, sapphire—are called Argillaceous Rocks, from the Latin word for clay, and belong to the second class. Such rocks keep fossils well.
Another is calcium. United to oxygen and carbonic acid, it makes carbonate of lime, the chief substance in limestone; so all limestones belong to the third class of Calcareous or Lime Rocks.
Other important materials may be mentioned, such as magnesium, potassium, sodium, iron, carbon, sulphur, hydrogen, chlorine, nitrogen. These, with many more, not so common, make up the remaining quarter of the earth-crust.
Carbon plays as important a part in animal and vegetable life as silicon in rocks. Carbon is most commonly seen in three distinct forms—as charcoal, as black-lead, and as the pure brilliant diamond. Carbon united, in a particular proportion, to oxygen, forms carbonic acid; and carbonic acid united, in a particular proportion, to lime, forms limestone.
Hydrogen united to oxygen forms water. Each of these two gases is invisible alone, but when they meet and mingle they form a liquid.
Nitrogen united to oxygen and to a small quantity of carbonic acid gas forms our atmosphere.
Rocks of pure flint, pure clay, or pure lime, are rarely or never met with. Most rocks are made up of several different substances melted together.
In the fire-built rocks no remains of animals are found, though in water-built rocks they abound. Water-built rocks are sometimes divided into two classes—those which only contain occasional animal remains, and those which are more or less built up of the skeletons of animals.
AMIBA PRINCEPS, ONE OF THE MANY ORDERS OF THE RHIZOPODA CLASS, MAGNIFIED ONE HUNDRED TIMES.
There are some exceedingly tiny creatures inhabiting the ocean, called Rhizopods. They live in minute shells, the largest of which may be almost the size of a grain of wheat, but by far the greater number are invisible as shells without a microscope, and merely show as fine dust. The rhizopods are of different shapes, sometimes round, sometimes spiral, sometimes having only one cell, sometimes having several cells. In the latter case a separate animal lives in each cell. The animal is of the very simplest as well as the smallest kind. He has not even a mouth or a stomach but can take in food at any part of his body.
These rhizopods live in the oceans in enormous numbers. Tens of millions are ever coming into existence, living out their tiny lives, dying, and sinking to the bottom.
There upon the ocean-floor gather their remains, a heaped-up multitude of minute skeletons or shells, layer forming over layer.
It was long suspected that the white chalk cliffs of England were built up in some such manner as this through past ages. And now at length proof has been found, in the shape of mud dredged up from the ocean-bottom—mud entirely composed of countless multitudes of these little shells, dropping there by myriads, and becoming slowly joined together in one mass.
Just so, it is believed, were the white chalk cliffs built—gradually prepared on the ocean-floor, and then slowly or suddenly upheaved, so as to become a part of the dry land.
Think what the enormous numbers must have been of tiny living creatures, out of whose shells the wide reaches of white chalk cliffs have been made. Chalk cliffs and chalk layers extend from Ireland, through England and France, as far as to the Crimea. In the south of Russia they are said to be six hundred feet thick. Yet one cubic inch of chalk is calculated to hold the remains of more than one million rhizopods. How many countless millions upon millions must have gone to the whole structure! How long must the work of building up have lasted!
These little shells do not always drop softly and evenly to the ocean-floor, to become quietly part of a mass of shells. Sometimes, where the ocean is shallow enough for the waves to have power below, or where land currents can reach, they are washed about, and thrown one against another, and ground into fine powder; and the fine powder becomes in time, through different causes, solid rock.
Limestone is made in another way also. In the warm waters of the South Pacific Ocean there are many islands, large and small, which have been formed in a wonderful manner by tiny living workers. The workers are soft jelly-like creatures, called polyps, who labor together in building up great walls and masses of coral.
They never carry on their work above the surface of the water, for in the air they would die. But the waves break the coral, and heap it up above high-water mark, and carry earth and seeds to drop there till at length a small low-lying island is formed.
The waves not only heap up broken coral, but they grind the coral into fine powder, and from this powder limestone rock is made, just as it is from the powdered shells of rhizopods. The material used by the polyps in building the coral is chiefly lime, which they have the power of gathering out of the water, and the fine coral-powder, sinking to the bottom, makes large quantities of hard limestone. Soft chalk is rarely, if ever, found near the coral islands.
Limestones are formed in the same manner from the grinding up of other sea-shells and fossils, various in kind; the powder becoming gradually united into solid rock.
There is yet another way in which limestone is made, quite different from all these. Sometimes streams of water have a large quantity of lime in them; and these as they flow will drop layers of lime which harden into rock. Or a lime-laden spring, making its way through the roof of an underground cavern, will leave all kinds of fantastic arrangements of limestone wherever its waters can trickle and drip. Such a cavern is called a "stalactite cave."
So there are different kinds of fossil rock-making. There may be rocks made of other materials, with fossil simply buried in them. There may be rocks made entirely of fossils, which have gathered in masses as they sank to the sea-bottom, and have there become simply and lightly joined together. There may be rocks made of the ground-up powder of fossils, pressed into a solid substance or united by some other substance.
Rocks are also often formed of whole fossils, or stones, or shells, bound into one by some natural soft sticky cement, which has gathered round them and afterwards grown hard, like the cement which holds together the stones in a wall.
The tiny rhizopods (meaning root foot) which have so large a share in chalk and limestone making, are among the smallest and simplest known kinds of animal life.
There are also some very minute forms of vegetable life, which exist in equally vast numbers, called Diatoms. For a long while they were believed to be living animals, like the rhizopods. Scientific men are now, however, pretty well agreed that they really are only vegetables or plants.
The diatoms have each one a tiny shell or shield, not made of lime like the rhizopod-shells, but of flint. Some think that common flint may be formed of these tiny shells.
Again, there is a kind of rock called Mountain Meal, which is entirely made up of the remains of diatoms. Examined under the microscope, thousands of minute flint shields of various shapes are seen. This rock, or earth, is very abundant in many places, and is sometimes used as a polishing powder. In Bohemia there is a layer of it no less than fourteen feet thick. Yet so minute are the shells of which it is composed, that one square inch of rock is said to contain about four thousand millions of them. Each one of these millions is a separate distinct fossil....
SUCCESSION OF BURIED COAL-GROWTHS AND ERECT TREE-STUMPS. SYDNEY, CAPE BRETON.
a. Sandstone, b. Shales, c. Coal-seams, d. Bed containing Roots and Stumps in situ.
If you examine carefully a piece of coal, you will find, more or less clearly, markings like those which are seen in a piece of wood. Sometimes they are very distinct indeed. Coal abounds in impressions of leaves, ferns, and stems, and fossil remains of plants and tree-trunks are found in numbers in coal-seams.
Coal is a vegetable substance. The wide coal-fields of Britain and other lands are the fossil remains of vast forests.
Long ages ago, as it seems, broad and luxuriant forests flourished over the earth. In many parts generation after generation of trees lived and died and decayed, leaving no trace of their existence, beyond a little layer of black mould, soon to be carried away by wind and water. Coal could only be formed where there were bogs and quagmires.
But in bogs and quagmires, and in shallow lakes of low-lying lands, there were great gatherings of slowly-decaying vegetable remains, trees, plants, and ferns all mingling together. Then after a while the low lands would sink and the ocean pouring in would cover them with layers of protecting sand or mud; and sometimes the land would rise again, and fresh forests would spring into life, only to be in their turn overwhelmed anew, and covered by fresh sandy or earthy deposits.
These buried forests lay through the ages following, slowly hardening into the black and shining coal, so useful now to man.
The coal is found thus in thin or thick seams, with other rock-layers between, telling each its history of centuries long past. In one place no less than sixteen such beds of coal are found, one below another, each divided from the next above and the next underneath by beds of clay or sand or shale. The forests could not have grown in the sea, and the earth-layers could not have been formed on land, therefore many land-risings and sinkings must have taken place. Each bed probably tells the tale of a succession of forests....
Before going on to a sketch of the early ages of the Earth's history—ages stretching back long long before the time of Adam—it is needful to think yet for a little longer about the manner in which that history is written, and the way in which it has to be read.
For the record is one difficult to make out, and its style of expression is often dark and mysterious. There is scarcely any other volume in the great Book of Nature, which the student is so likely to misread as this one. It is very needful, therefore, to hold the conclusions of geologists with a light grasp, guarding each with a "perhaps" or a "may be." Many an imposing edifice has been built, in geology, upon a rickety foundation which has speedily given way.
In all ages of the world's history up to the present day, rock-making has taken place—fire-made rocks being fashioned underground, and water-made rocks being fashioned above ground though under water.
Also in all ages different kinds of rocks have been fashioned side by side—limestone in one part of the world, sandstone in another, chalk in another, clay in another, and so on. There have, it is true, been ages when one kind seems to have been the chief kind—an age of limestone, or an age of chalk. But even then there were doubtless more rock-buildings going on, though not to so great an extent. On the other hand, there may have been ages during which no limestone was made, or no chalk, or no clay. As a general rule, however, the various sorts of rock-building have probably gone on together. This was not so well understood by early geologists as it is now.
The difficulty is often great of disentangling the different strata, and saying which was earlier and which later formed.
Still, by close and careful study of the rocks which compose the earth's crust, a certain kind of order is found to exist, more or less followed out in all parts of the world. When each layer was formed in England or in America, the geologist cannot possibly say. He can, however, assert, in either place, that a certain mass of rock was formed before a certain other mass in that same place, even though the two may seem to lie side by side; for he knows that they were so placed only by upheaval, and that once upon a time the one lay beneath the other.
The geologist can go further. He can often declare that a certain mass of rock in America and a certain mass of rock in England, quite different in kind, were probably built up at about the same time. How long ago that time was he would be rash to attempt to say; but that the two belong to the same age he has good reason for supposing.
We find rocks piled upon rocks in a certain order, so that we may generally be pretty confident that the lower rocks were first made, and the upper rocks the latest built. Further than this, we find in all the said layers of water-built rocks signs of past life.
As already stated, much of this life was ocean-life, though not all.
Below the sea, as the rock-layers were being formed, bit by bit, of earth dropping from the ocean to the ocean's floor, sea-creatures lived out their lives and died by thousands, to sink to that same floor. Millions passed away, dissolving and leaving no trace behind; but thousands were preserved—shells often, animals sometimes.
Nor was this all. For now and again some part of the sea-bottom was upheaved, slowly or quickly, till it became dry land. On this dry land animals lived again, and thousands of them, too, died, and their bones crumbled into dust. But here and there one was caught in bog or frost, and his remains were preserved till, through lapse of ages, they turned to stone.
Yet again that land would sink, and over it fresh layers were formed by the ocean-waters, with fresh remains of sea-animals buried in with the layers of sand or lime; and once more the sea-bottom would rise, perhaps then to continue as dry land, until the day when man should discover and handle these hidden remains.
Now note a remarkable fact as to these fossils, scattered far and wide through the layers of stratified rock.
In the uppermost and latest built rocks the animals found are the same, in great measure, as those which now exist upon the earth.
Leaving the uppermost rocks, and examining those which lie a little way below, we find a difference. Some are still the same, and others, if not quite the same, are very much like what we have now; but here and there a creature of a different form appears.
Go deeper still, and the kinds of animals change further. Fewer and fewer resemble those which now range the earth; more and more belong to other species.
Descend through layer after layer till we come to rocks built in earliest ages and not one fossil shall we find precisely the same as one animal living now.
So not only are the rocks built in successive order, stratum after stratum belonging to age after age in the past, but fossil-remains also are found in successive order, kind after kind belonging to past age after age.
Although in the first instance the succession of fossils was understood by means of the succession of rock-layers, yet in the second place the arrangement of rock-layers is made more clear by the means of these very fossils.
A geologist, looking at the rocks in America, can say which there were first-formed, which second-formed, which third-formed. Also, looking at the rocks in England, he can say which there were first-formed, second-formed, third-formed. He would, however, find it very difficult, if not impossible, to say which among any of the American rocks was formed at about the same time as any particular one among the English rocks, were it not for the help afforded him by these fossils.
Just as the regular succession of rock-strata has been gradually learned, so the regular succession of different fossils is becoming more and more understood. It is now known that some kinds of fossils are always found in the oldest rocks, and in them only; that some kinds are always found in the newest rocks, and in them only; that some fossils are rarely or never found lower than certain layers; that some fossils are rarely or never found higher than certain other layers.
So this fossil arrangement is growing into quite a history of the past. And a geologist, looking at certain rocks, pushed up from underground, in England and in America, can say: "These are very different kinds of rocks, it is true, and it would be impossible to say how long the building up of the one might have taken place before or after the other. But I see that in both these rocks there are exactly the same kinds of fossil-remains, differing from those in the rocks above and below. I conclude therefore that the two rocks belong to about the same great age in the world's past history, when the same animals were living upon the earth."
Observing and reasoning thus, geologists have drawn up a general plan or order of strata; and the whole of the vast masses of water-built rocks throughout the world have been arranged in a regular succession of classes, rising step by step from earliest ages up to the present time.
