Another is _calcium_. United to oxygen and carbonic acid, it makes carbonate of lime, the chief substance in limestone; so all limestones belong to the third class of Calcareous or Lime Rocks. Other important materials may be mentioned, such as _magnesium, potassium, sodium, iron, carbon, sulphur, hydrogen, chlorine, nitrogen_. These, with many more, not so common, make up the remaining quarter of the earth-crust. Carbon plays as important a part in animal and vegetable life as silicon in rocks. Carbon is most commonly seen in three distinct forms--as charcoal, as black-lead, and as the pure brilliant diamond. Carbon united, in a particular proportion, to oxygen, forms carbonic acid; and carbonic acid united, in a particular proportion, to lime, forms limestone. _Hydrogen_ united to oxygen forms water. Each of these two gases is invisible alone, but when they meet and mingle they form a liquid. _Nitrogen_ united to oxygen and to a small quantity of carbonic acid gas forms our atmosphere. Rocks of pure flint, pure clay, or pure lime, are rarely or never met with. Most rocks are made up of several different substances melted together. * * * * * In the fire-built rocks no remains of animals are found, though in water-built rocks they abound. Water-built rocks are sometimes divided into two classes--those which only contain occasional animal remains, and those which are more or less built up of the skeletons of animals. [Illustration: AMIBA PRINCEPS, ONE OF THE MANY ORDERS OF THE RHIZOPODA CLASS, MAGNIFIED ONE HUNDRED TIMES.] There are some exceedingly tiny creatures inhabiting the ocean, called Rhizopods. They live in minute shells, the largest of which may be almost the size of a grain of wheat, but by far the greater number are invisible as shells without a microscope, and merely show as fine dust. The rhizopods are of different shapes, sometimes round, sometimes spiral, sometimes having only one cell, sometimes having several cells. In the latter case a separate animal lives in each cell. The animal is of the very simplest as well as the smallest kind. He has not even a mouth or a stomach but can take in food at any part of his body. [Illustration: RHIZOPODS (MAGNIFIED).] These rhizopods live in the oceans in enormous numbers. Tens of millions are ever coming into existence, living out their tiny lives, dying, and sinking to the bottom. There upon the ocean-floor gather their remains, a heaped-up multitude of minute skeletons or shells, layer forming over layer. It was long suspected that the white chalk cliffs of England were built up in some such manner as this through past ages. And now at length proof has been found, in the shape of mud dredged up from the ocean-bottom--mud entirely composed of countless multitudes of these little shells, dropping there by myriads, and becoming slowly joined together in one mass.