There are varieties of marble without end. The purest and daintiest is the white of which statues are carved; but there are black, red, yellow, gray, blue, green, pink, and orange in all shades. Many are beautifully marked. The inner walls of buildings are sometimes covered with thin slabs of marble. These are often carefully split, and the two pieces put up side by side, so that the pattern on one is reversed on the other. Certain kinds of marble contain fossils or remains of coral and other animals that lived hundreds of thousands of years ago. In some marbles there are so many that the stone seems to be almost made of them. When a slab is cut and polished, the fossils are of course cut into; but even then we can sometimes see their shape. One of the most common is the crinoid. This was really an animal, but it looked somewhat like a closed pond lily with a long stem, and people used to call it the stone lily. This stem is made up of little flat rings looking like bits of a pipestem. The stems are often broken up and these bits are scattered through the marble. The animals whose shells help to make marble lived in the ocean, and when they died sank to the bottom. Many of the shells were broken by the beating of the waves, but both broken shells and whole ones became united and hardened into limestone, one kind of which we call marble. Common chalk is another kind. Blackboard crayons are made of this: so are whitewash and whiting for cleaning silver and making putty. Another stone that builders would be sorry to do without is slate. This, too, was formed at the bottom of the sea. Rivers brought down fine particles of clay, which settled, were covered by other matter, and finally became stone. It was formed in layers, of course, but, queerly enough, it splits at right angles to its bottom line. Just why it does this is not quite certain, but the action is thought to be due to heat and long, slow pressure, which will do wonderful things, as in the case of coal. This splitting is a great convenience for the people who want to use it for roofing and for blackboards. Blocks of slate are loosened by blasting, and are taken to the splitting-shed.