Harris, William H., and Judith S. Levey, eds. The rock is not necessarily compressed in the same direction as the sedimentary layers were originally laid down, and because the compression crumples and deforms the original sedimentary layers, the planes of slaty cleavage usually cut through the old bedding planes. Its characteristic color is gray-blue. Slate is mined in Maine, Vermont, Pennsylvania, Georgea, Lake Superior, and the Rocky Mts. Harris, William H., and Judith S. Levey, eds.
Most diatoms exist singly, although some join to form colonies.
They are usually yellowish or brownish, and are found in fresh and salt water, in moist soil, and on the moist surface of other plants. They are most abundant in polar and other cold waters. Some 15,000 species are known. The living matter of each diatom is enclosed in a shell of siliceous material that it secretes. Some diatoms can move over moist surfaces by a streaming of the protoplasm, and some aquatic forms can move about in the water.