change or decomposition, the hydrogen and oxygen mostly disappear, leaving much of the carbon with original structures often remarkably preserved. Fine examples are fossil plants in the great coal-bearing strata. Much more rarely entire organisms have been preserved either by freezing or by natural embalmment. Most remarkable are the species of mammoths and rhinoceroses, extinct for thousands of years, bodies of which, with flesh, hide, and hair still intact, have been held in cold storage in the frozen soils of Siberia, or other cases. Insects have been perfectly preserved in amber, as, for example, in the Baltic region. This amber is a hardened resin in which the insects were caught while it was still soft and exuding from the trees. Finally, we should mention the preservation of tracks and trails of land and water animals. Thousands of tracks of long-extinct great reptiles occur in the sandstones and shales of the Connecticut Valley of Massachusetts. The footprints were made in soft sandy mud which hardened and then became covered with more sediment. Few fossils occur in other than the sedimentary rocks. Most numerous, by far, are fossils in rocks of marine origin, because on relatively shallow sea bottoms, where sediments of the geologic ages have largely accumulated, the conditions for fossilization have been most favorable. Among the many conditions which have produced great diversity in numbers and distribution of marine organisms during geologic time are temperature, depth of water, clearness of water, nature of sea bottom, degree of salinity, and food supply. River and lake deposits also not uncommonly contain remains of organisms which inhabited the waters, but also others which were carried in. "Surrounding trees drop their leaves, flowers, and fruit upon the mud flats, insects fall into the quiet waters, while quadrupeds are mired in mud or quicksand and soon buried out of sight. Flooded streams bring in quantities of vegetable debris, together with carcasses of land animals drowned by the sudden rise of the flood" (W. B. Scott).