The Story of the Hills: A Book About Mountains for General Readers.

by H. N. Hutchinson

CHALK ROCKS, FLAMBOROUGH HEAD. From a Photograph by G. W. Wilson.

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.

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.

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:

	{	I. Sedimentary.	{	Clay, shale, slate, etc. Sandstones. Conglomerates.
Rocks of aqueous origin.		II. Organic.	{	Limestones. Flint. Coal.
		III. Chemical.	{	Rock salt. Gypsum, etc.
Rocks of igneous origin.	{	I. Volcanic	{	Lavas. Volcanic ashes, etc.
		II. Plutonic	{	Basalt. Granite.
Metamorphic rocks of aqueous and igneous origin.			{	Marbles. Various kinds of schists. 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.