1. _Original composition not completely changed._—Extinct elephants have been found frozen in the river-bluffs of Siberia so perfectly preserved that dogs and wolves ate their flesh. The bodies of animals are also found well preserved in peat-bogs. All coal is simply fossil vegetation retaining in a large degree the original composition; and the same is true of ferns, etc., preserved as black impressions in the rocks. All bones and shells consist of mineral matter which makes them hard, and animal matter which makes them tough and strong. In very many cases, especially in the newer formations, the animal matter is still partially, and the mineral matter almost wholly, intact. 2. _Original composition completely changed, but form and structure preserved._—All kinds of fossils are commonly called petrifactions, but only those preserved in this second way are truly petrified, _i.e._, turned to stone. “Petrified wood is the best illustration, and in a good specimen not only the external form of the wood, not only its general structure—bark, wood, radiating silver-grain, and concentric rings of growth—are discernible, but even the microscopic cellular structure of the wood, and the exquisite sculpturing of the cell-walls, are perfectly preserved, so that the kind of wood may often be determined by the microscope with the utmost certainty. Yet not one particle of the organic matter of the wood remains. It has been entirely replaced by mineral matter; usually by some form of silica. The same is true of the shells and bones of animals.”—LE CONTE. 3. _Original composition and structure both obliterated, and form alone preserved._—This occurs most commonly with shells, although fossil trees are also often good illustrations. The general result is accomplished in several ways: (_a_) The shell after being buried in the sediment may be removed by solution, leaving a _mould_ of its external form, (_b_) This mould may subsequently be filled by the infiltration of finer sediment, forming a _cast_ of the exterior of the shell. (_c_) The shell, before its solution, may have been filled with mud; and if the shell itself is then dissolved, we have a cast of its interior in a mould of its exterior. TIME REQUIRED FOR THE FORMATION OF STRATIFIED ROCKS.—Many attempts have been made to determine the time required for the deposition of any given thickness of stratified rocks. Of course, only roughly approximate results can be hoped for in most cases; but these are at least sufficient to make it certain that geological time is very long. The average relative rate of growth of different kinds of sediment is, however, less open to doubt, for we have already seen that coarse sediments like gravel and sand accumulate much more rapidly than finer sediments like clay and limestone; and we are sometimes able to compare these two classes of rocks on a very large scale.