New rocks have been formed not only by the breaking up and welding together of the original earth’s crust, but by animal and plant remains. In some places, in past ages, billions and billions of little shell-fish have lived in the waters, died there and their shells have fallen to the bottom of their watery home. Now we know their shell remains as chalk or if it has undergone great pressure, owing to changes in the earth’s surface, as limestone. There are rock masses also formed of the remains of countless Diatoms; sponge spicules, too, have played their part in rock formation.
It is clear from what we have written that, in the first place, the rocks of the earth form two great divisions, the rocks which formed the original crust of the earth, Primary rocks they are called, and the later-formed Secondary rocks. The Primary rocks are often glassy in appearance, they show unmistakable signs of having once been molten and they never contain animal or plant remains, fossils as they are called, for the reason we have already explained. The secondary rocks often betray their origin by occurring in layers, or strata to speak more scientifically. Always when we think of the rocks we must think in thousands of years, then it will be easier to understand the formation of these strata, each one of which may represent the work of hundreds and hundreds of years.
Now to return to our sand; we have explained briefly that sand is really the remains of broken up rocks. First we have the solid rock, boulders are broken from it as the boulders are acted upon by rain, frost and wind: they become more and more broken up and at the same time they are carried towards the sea where they take the form of shingle. The shingle, in the course of time, becomes broken up into sand, and the sand again becomes so finely divided that it forms mud. From this it is clear that the composition of the sand depends largely on the nature of the rocks in the neighbourhood.
By the courtesy of Messrs. F. Davidson & Co.
1. Foraminifera
Small sea shells. The inmates die and the shells, falling to the bottom of the sea, are gradually converted into chalk.
2. Diatoms
Small water plants which make beautiful objects for the microscope by reason of their remarkable shapes and sculpturing.