Part 5

UPHEAVAL AND DEPRESSION.

These two words are so often used in books on geology that we shall not be able to get on without knowing their meaning. We have seen that the rocks have been formed in a certain way—some by heat, some by water, and some by dead forests—and that they lie over one another in pretty regular order. But this order has sometimes been disturbed and the layers have been tumbled about among one another very much. In some places the older rocks, such as granite, slate, and sandstone, have been pushed up through those above, and in others the coal has sunk down and been covered with thick layers of chalk, sand, and mud. When the force below pushes a layer up through the others it is called upheaval, and when a layer sinks down it is called depression, or subsidence. Both these actions are going on now in different parts of the Earth. A great part of Sweden, Norway, and Denmark, of Spitzbergen, Siberia, and the north of America, is being slowly raised higher above the sea, as we know by the height their old sea beaches now are above the water; while part of the shore of America opposite to Europe and also the south of Greenland is slowly sinking down, as we know by the remains of land animals and trees which are now covered by the tide; and at many places on the coast of India this subsidence is also going on. Nearer home, too, there is an example of it in the island of Guernsey. All round the coast of this island, like that of Jersey, are found tree trunks and other remains of old forest land beneath the water. Old histories refer to this as dry land; and if a map of it made in 1406 is correct, this land must have sunk about 150 feet since that time.

Thus we can see, even at the present time, the very same changes which have worked upon our Earth for innumerable ages. It is now easy to understand how the forests which must have grown above in the air have, after a long time, sunk down to a great depth, and been turned into coal, and covered with the sediment, sand, gravel, and chalk from the seas which afterwards flowed over the places where they grew.

Sometimes the rocks by the sea shore are cut into terraces or steps by the constant wear of the water, and when we see these water marks far above the present level of the sea we know that the land must have been lifted up gradually above the sea. There are many such terraces in Norway. To prove whether this is so marks have been cut upon rocks at a measured height above the sea, and after some years these marks have been noticed to have been raised much above the water by the “upheaval” of the earth at that place.

Generally this change of level has taken place gradually, and the greatest work in moving the layers of earth and displacing them has been very slow. But in some places violent and sudden shocks have happened, tearing up the rocks and piling them up in heaps; and now and then islands have suddenly appeared in the sea and vanished out of sight completely in a short time. Islands have thus come up in the Mediterranean Sea within the memory of man. In the year 1831 the island of Julia suddenly appeared near the coast of Sicily, and since the year 186 B.C. no less than three islands have started up in the bay of the island of Santorin. In this century islands have appeared among the Azores, the Indian Archipelago, the Philippines, the Moluccas, and on the coast of Kamtschatka and other places. Some of these have appeared suddenly, others slowly, and they no doubt have been raised by a great force from below.

You will see now how easy it is to account for the changes of the places of the layers of rock. The same thing is going on now which has been going on throughout all time, only perhaps with more energy formerly than now, making mountains, islands, and continents, raising up a large tract of land in one place and sinking an island or a sea shore in another.

These changes have been of great use to us too. Suppose all England had been covered with coal or slate, we should not have been able to grow anything! As it is we have sand and gravel in one county, chalk in another, slate or granite in another, and coal down below in several, and we can grow a great variety of plants on all these different soils. We have to thank “upheaval” and “depression” for this. The force which is always working below us has turned up the different soils like a gigantic plough, and brought some to the top and covered others, so that instead of having to dig down deeper than ever we have yet, we have only to go from one county to another to find the different rocks. We know that we could not get at the coal in Sussex without going down an unknown depth through the chalk and other earths, but we dig for it in the North of England, where we know its depth below the surface.

I will try now to give you some idea of the way in which the rocks come in their order, or the succession of formations as geologists call it. If we started to walk from Wales to London the rocks we should pass over would be—slate and flagstones in Wales, and going on towards London, limestone, old red sandstone, more limestones, coal beds, new red sandstone, oolite, greensand, chalk, and last London clay. We might not always find each of these near the surface, but they would be found to be the principal rocks on a line between Wales and London, the oldest being in Wales and the newest or most recent as we get nearer London. That word “oolite” which I used comes from two Greek words meaning “roe” and “stone,” because the rock is composed of little rounded grains of a chalky substance shaped like the hard roe of a fish, or like sago before it is cooked.

If you look at the following table you will see how the principal rocks are placed one upon the other, beginning at the lowest or oldest at the bottom and going up to the newest at the top of the table, and on the right hand side I have written the names of the principal fossils which each kind of earth contains.

TABLE OF THE SUCCESSION OF FORMATIONS.

TERTIARY, or Upper Rocks

Peat bogs and caves Fossil Man, with stone implements, River-mud and brickearth, &c., mammoth, gravels, and hippopotamus, rhinoceros, boulder clay (alluvium) Irish stag, cave lion, &c. Crag of Eastern Counties Numerous shell-fish, mastodon London clay, &c. Turtles, crocodiles, shell-fish

SECONDARY, or Middle Rocks

Cretaceous

Chalk (with and without Foraminifera, &c., sponges, flints) corals, sea-urchins, shell-fish Greensand and gault (Belemnites, Ammonites, Wealden clay, &c. &c.), fishes

Oolites

Portland stone Kimmeridge clay Coral rag Immense reptiles, the Ichthyosaurus, Oxford clay Plesiosaurus, Cornbrash and forest Megalosaurus, Pterodactyl, marble &c. Great oolite Fullers’ earth Animals allied to the opossum Lower oolite and kangaroo

Lias clay and limestone Cycads and other plants New red marl and sandstone

PRIMARY, or Lower Rocks

Coal Ferns, club-mosses, a few Millstone grit firs, calamites, &c., in Mountain limestone great abundance Old red sandstone Numerous corals, shell-fish, Silurian limestones and trilobites, fishes, &c. slates Cambrian slates The Laurentian rocks contain Laurentian rocks the oldest known fossil, the Eozöon (or “life-dawn animal”)

IGNEOUS, or Volcanic Rocks

Greenstone, basalt Of various ages (no fossils) Porphyry Granite, &c.

If you read this table upwards from the bottom you will notice that life began in a very small way with Eozöon (the “life-dawn animal”), that fishes appeared afterwards, that the wonderful forests of the coal period then grew and were covered up by other rocks and pressed into solid coal, that numbers of great crocodile-like animals lived all through the oolite time, how the deep wide beds of chalk were laid down by humble foraminifera, and when we get to the recent newest beds of gravel, mud, sand, clay, &c., the sweepings by water of the older rocks ground down by ages of wear and tear, we have the mammoth, mastodon, megatherium, and other great vegetable eaters, and lastly Man himself with his simple weapons of stone, bone, and horn—our early forefathers.

You must always keep in mind that the greatest of these changes have taken place very slowly. Mountains have been raised, and whole continents have been sunk by movements so slow that if the hands of a clock went only once round the dial in a year the hand would go faster than these mountains have risen or the continents sunk. Almost always whenever there has been sudden and violent action it has been near volcanoes or during earthquakes; but these things, terrible as they are to the people living near, disturb only a very small part of the surface, and such violence neither buried the coal beds nor raised the slate hills of Wales. Many of the small effects of the internal force of the earth have been sudden and violent, but the greatest and grandest have been slower than anything we can imagine.

If this had not been so, we should not find fossil shells just as they sank quietly to the bottom of ancient seas, quite undisturbed. We should not find delicate ferns and insects with all their breakable parts perfectly preserved, and as lightly laid as if you had put them away carefully in a cabinet upon cotton wool. Yet many of these have sunk down hundreds of feet below the open air where they must have lived. We find the ripple marks of the waves on old sandstones, and even the prints of the feet of birds and animals as they walked upon that rock when it was soft sand, and the little pits made by rain-drops on the moist earth. All this speaks of stillness, and gentle movement, no violence. So slowly and softly have these rocks settled down, that we can read in them the history of the life that was. But if there had been any sudden and rough movement all these fossils might have been broken up and we should have had nothing but fragments, and the “puzzle of life” could never have been put together. Nature’s forces are immense, but they work slowly, irresistibly, and majestically.

THE ICE AGE.

We have seen now what the principal rocks are made of and the way in which their places have been changed by upheaval and depression. Water, as we know, has been at work and has done great things in all ages of the World’s history. I have called it “one of Nature’s most powerful tools,” and when we look at the quantity of chalk alone that there is in the world, and remember that this was all laid down in water, and perhaps a great part of its lime carried down by rivers to the seas where it settled to the bottom, after the corals and small shell-fish had worked it into their bodies, we are right in thinking water a great Magician indeed. Why, even so small a river as the Thames carries down to the sea every year as much dissolved earth as would make a good large hill; and what must such rivers as the Nile, the Amazon, the Mississippi, and the great Chinese rivers do! There must have been gigantic rivers, too, in the old times, or else it would have been impossible that the deep sandstone and slate beds could have been formed; for these are all laid down by the washing away of earth in water.

Ice, which is only solid water, has also been a powerful tool in shaping the surface of the Earth, but it has not been always at work as water has. Ice now covers only a comparatively small part of the globe near the north and south poles, and mountains like those in Switzerland; but by watching what ice is doing now in these places we are able to be certain that there has been a time when it covered Scotland, Cumberland, Wales, Sweden and Norway, and nearly all North America. In watching the great “rivers of ice,” called glaciers, in the Alps, for instance, we see that they slip down from the mountains slowly, creeping on year by year, and bringing with them pieces of rock and stones. We see also where they have melted that they have been grinding the rocks beneath them with their great weight, and have cut grooves into, and scraped and polished the hardest granite. The stones underneath the glaciers have been pressed so heavily upon the rocks that they have left deep marks, and we find the same kinds of marks and heaps of stones in many mountains where there are no glaciers now. There are other things too which convince us that a great ice sheet spread over almost the whole of Great Britain. When the huge icebergs break away from the frozen shores of Greenland and North America, they often have frozen into their ice large blocks of rock, sand, gravel, &c., and when they drift into the warmer seas of the south they melt, and of course these blocks or “boulders,” as they are called, sink to the bottom. Just the same kind of boulders are found in many parts of the world, where icebergs never come now, and as they are of a different rock from that on which they lie, they must have been brought there somehow. We naturally suppose then that they were brought by icebergs. Sometimes boulders of granite have been found thus among clay, many miles from where there are any granite rocks on the surface, and there can be no doubt that they were originally frozen into an iceberg, which floated away with them and when it melted left them so far from their native place. In many of the midland and eastern counties once floated these icebergs, dropping the stones and boulders which they brought away from the Welsh, Cumberland, and Scottish mountains.