Page:South African Geology - Schwarz - 1912.djvu/54

 The same applies to the silica, which cannot be detected in sea salt, but which is there in minute quantities, and every molecule of which is in demand by organisms which have the power of making their hard parts of silica—the animals are called Radiolaria and the plants Diatoms. These live with the lime-secreting animals, though in lesser number, and are approximately of the same size. When the sea is very deep indeed, the calcareous shells become dissolved as they sink into the regions of great pressure, and only the fine insoluble residue eventually settles. This is known as red clay, which covers the floor of the very deep oceans in many parts of the world. The silica of the Radiolaria and Diatoms may similarly be left over by the solution of the calcareous shells of the Foraminifera and Coccoliths, and a siliceous slimy mass may result, which consolidates as chert, or if it forms lumps and nodules it is known as flint.

The iron enters in part into the living tissues of the organisms, but a great deal of it combines with the dissolved silica near the coast and forms a green silicate of iron, known as glauconite. Off the Agulhas Bank the Globigerina shells become filled with this glauconite and the shells subsequently dissolve, so that the sand dredged up consists of little rounded casts of the Globigerina shells in glauconite. Glauconite sand when consolidate is known as greensand, and forms a considerable portion of the rocks of Cambridgeshire, the Isle of Wight, and elsewhere.

The magnesia is not used up by the animals or plants, yet the comparison of the amount discharged by the rivers, and that existing in sea water, shows that it is being laid by somewhere. Now, no precipitation of the