Page:South African Geology - Schwarz - 1912.djvu/52

 This means that the sodium chloride is not used up in the sea, but simply accumulates from year to year; the lime salts on the other hand are made use of by countless myriads of sea animals. On the coast we have shell fish and corals secreting lime to form their hard parts, and in the open ocean there are forms of microscopic life which do the same. The surface of the ocean swarms with animals and plants which are barely visible to the naked eye, but which show under the microscope many remarkable forms. The principal of these are the Foraminifera, of which Globigerina is the commonest type. They are animals consisting of little rounded masses of jelly, which are enclosed in a hard shell of calcium carbonate. With them, and affording a part at least of the food supply of the Foraminifera, are the microscopic plants called Coccoliths, also possessing hard parts made of calcium carbonate. These two sets of organisms multiply with great rapidity, and, when dead, sink to the bottom of the ocean in a continuous rain, so that the deep ocean becomes covered with a sediment, though of quite another type to that of the shore sediments: it is typically a limestone. The slimy matter not yet consolidated into rock is called an ooze; this substance is found adhering to the leads used for sounding in the deep ocean. When the ooze is examined it is seen to consist of the dead shells of the animals which may be caught living in a tow net on the surface. Consolidated ooze is chalk, and ordinary chalk will still show the original Globigerinas, though a large portion is made up of broken fragments.

The reason, then, why so little lime salts are found in sea salt is that the substances are constantly being abstracted by organisms.