The rocks themselves must explain the many things which have happened during the course of millions of years, and this they do remarkably well when carefully studied, for many of the factors involved in their histories leave characteristic marks. Changing climates, the draining of seas, the uplifting of mountain ranges, all have ways of registering their occurrence which are as convincing and reliable as anything ever written by man. Piece by piece the story has been patched together through the efforts of thousands of investigators. Parts of the narrative remain buried at inaccessible depths, and whole chapters, no doubt, have been destroyed by the same forces that composed this tremendous record of prehistoric times. VARIETIES OF FOSSILS It would be a serious mistake to regard nature as divided into a number of distinct and independent schools of fossil making, each refusing to use the methods and devices of another. There are, however, certain features which stand out so prominently that a little classification becomes helpful. While this procedure brings out differences it should be understood that processes actually work together, several of them usually being involved in the production of any individual specimen. (1) Impressions of animals and plants, or parts of these, are frequently left in soft sand or mud which later becomes converted into more durable rock. This type of fossils is represented by animal foot-prints and the imprints of leaves, flowers, insects, and like objects which may be mingled with the finely ground materials of the common sedimentary rocks. (2) Parts of plants and animals may be gradually replaced by mineral matter with little or no change from original form and texture. Fossils of this class are said to be petrified or turned to stone. They are also known as replacements. The fleshy parts of animals do not petrify. (3) Many animals among the invertebrates use mineral substances for protective or supporting structures. Small plants of various kinds follow a similar practice. These structures, being produced in stony materials, are readily converted into fossils. The shells of mollusks are the best known illustrations in this field, and all that is required for a shell to become a fossil is the extinction of the species of animal that produced it. Fossils of this type are extremely abundant. (4) Preservative substances other than those which produce common rocks may be mentioned among fossil-making possibilities. Bones are known to have been preserved in asphalt, and insects in resins, but such cases are few in comparison with the products of other methods.