Fig. 641.—Cliffs showing strata.
Fig. 642.—Limestone with encrinites.
You will find the remains of animals—that is, shells and tiny bits of coral packed close together. Under the microscope they will become more separated, and the grains will be distinct fragments of shells, etc. If this little bit of chalk be composed of marine animals’ shells, of course the whole cliff is composed of the same kind of material. But how did the shells get into the chalk? Shells are chalk—carbonate of lime; lime was deposited at the bottom of the sea, and the infinite millions of minute animals formed themselves shells, and left them to be piled up by Nature’s forces into cliffs during countless ages.
Yes, but how did the lime get into the water to make the shells? We will endeavour to explain. Rain, when falling, takes some carbonic acid from the air, which we know contains it. This acts upon the lime in the rocks (lime is oxide of calcium, and calcium is an element in the earth), so we get a bi-carbonate of lime (soluble in water), which rises from the rocks in springs. These springs and their streams deposit lime, as we can see in caverns where we find stalactites and stalagmites. The lime is transmitted to the ocean, and absorbed by the crinoideans and molluscs which produced shells. These shells hardened and crystallized became limestone, and whole mountains are formed of this “organic” rock, which is used for so many purposes.
Fig. 643.—Chalk cliff.
We have spoken of Organic Rocks, but there are others, and we ought, perhaps, to have spoken of that kind before the chalk put them aside. Let us go back to our sandy shore again and look at the Sedimentary Rocks, which are the very first formation. We have all seen sandstone, and visitors to the South Devon Coast will remember the red cliffs near Dawlish and Teignmouth. These are red sandstone—not the very “old red” so pleasantly written of by Hugh Miller, but at any rate sandstone, and composed of grains of sand. When we were at Dawlish last year a piece of the sandstone had fallen on to the beach, and when the waves came up that stone was no doubt gradually washed away into sand, and then fell to the earth as sediment.
We said something a few pages back about the wear and tear which is always going on: the mountain is worn away—a mass falls, it is broken into smaller pieces; these are carried by a river; the mud is deposited, and the finer particles are ground and rounded into gravel, and finally sand. Beneath the current of the river, and at the bottom of a lake or sea, these sediments (mud, etc.) accumulate one on the top of the other in regular series called strata, and then the weight and pressure acting with the soluble mineral deposits always washing down, consolidate and bind the loose sand-grains into stone, which, in the course of ages, hardens. The stones thus formed from sediments such as gravel, mud, and sand, are termed Sedimentary Rocks; they have become rocks by enormous and continuous pressure. Thus:—