The conditions of the preservation of fossils are much more favourable to some kinds of organisms than to others. It is only under the rarest circumstances that soft, gelatinous animals, which (like jelly-fish) have no hard parts, can leave traces in the rocks. The vast majority of fossilized animals are those which have hard shells, scales, teeth, or bones; and of plants, those which contain a sufficient amount of woody tissue.

Again, the conditions under which organisms live have a great influence upon the chances of their preservation as fossils. Land animals and plants are much less favourably situated than are aquatic forms, and since the greater number of sedimentary rocks were laid down in the sea, marine organisms are much more common as fossils than are those of fresh water.

On land, fossils have been preserved, sometimes in astonishing numbers, under wind-made accumulations of sand, dust, or volcanic ash, and in the flood-plain deposits of rivers. Peat-bogs are excellent places for fossilization, and the coal seams have yielded great numbers of fossils, principally of plants. The remains of land animals and plants, especially of the latter, are sometimes swept out to sea, sink to the bottom, and are there covered up and preserved in the deposits; but such occurrences are relatively uncommon. Small lakes offer more favourable conditions for the preservation of terrestrial organisms. 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 the carcasses of land animals, drowned by the sudden rise of the flood.

The great series of fresh-water and volcanic-ash deposits, which for long ages were formed in various parts of our West, have proved to be a marvellous museum of the land and fresh-water life of that region. On the fine-grained shales are preserved innumerable insects and fishes, with multitudes of leaves, many fruits, and occasionally flowers, while in the sands, clays, and tuffs, are entombed the bones of the reptiles, mammals, and, more rarely, birds of the land, mingled with those, of the crocodiles, turtles, and fishes that lived in the water. Similar deposits are known in other continents.