Page:Popular Science Monthly Volume 15.djvu/787

Rh mixture of these with a fine granular substance. Other similar deposits appear in many parts of the world.

The many silicic clays owe their peculiar characters to the microscopic fossils they have contained. The sands and mergel of Sienna and Coroncina, in Italy, imbed great quantities. The lanceolets, the lowest of vertebrates, the mud-eating fishes, and the dirt-eaters among men, subsist chiefly from these tiny organisms, for the so-called "edible earths" and "infusorial earths" are made up largely of, and owe their nutritive qualities to, the remains of microscopic animals. These earths are eaten in times of need by the Lapps and Tungusians. They are likewise used in South America, in New Caledonia, Kurdistan, in China, and in some of our own Southern States. The "bread-stone" of China belongs to this kind of food. On the shores of a lake near Uranea, in Sweden, there is a large deposit of infusorial powder called Bergmehl (mountain-meal), which is mixed with flour and eaten. It consists almost entirely of microscopic shells.

The animalcules of plasma without a cover of cell-membrane are known as rhizopods, or root-footed animals. These have been of the greatest benefit in geological history. Those which have a central spore case are called radiolarians, and generally bear beautiful spheroid, radiate, silicic frames, which have assisted largely in producing great flinty deposits in the depths of the sea, constructing extensive masses of rock. There is no doubt that they helped greatly in the formation of the silicic rocks of Virginia, the Nicobar Islands, Sicily, Barbadoes, etc. Indeed, this latter island consists almost entirely of their remains, and two hundred and thirty-two kinds have been described on it alone. Also the barren rocks, twenty feet thick, on which the city of Richmond, Virginia, stands, consist mainly of their discoid shells.

Other rhizopods, called foraminifers, produce porous, calcareous incasements for themselves and help form limestone rocks.

Surprising as it may sound, it is nevertheless true that substantially the rhizopods built the temples and mammoth pyramids of Egypt and the stone walls of Vienna and Paris, for the very rocks of these structures, as well as those which surround the Mediterranean Sea and extend thence to the Himalayan Mountains, are chiefly built up by their infinitely numerous perforated shells. There are extensive limestone formations, which have resulted mainly from their remains, and some of these bear their names, as the miliolithic, of the Paris basin; that of the Vienna basin; the alveolithic, of western France; and the nummulithic, of the Mediterranean. Limestones composed chiefly, sometimes entirely, of their shells, appear in the Grob-Kalk of Gentilly and in very many other localities, also forming a broad belt along both sides of the Mediterranean Sea and eastward therefrom, sometimes hundreds of feet in thickness.

But what is still more astonishing is the fact that the whole geological formation known as the cretaceous or chalk has been produced