170 THK KARTH AS MAN'S ABODE fossils. Otherwise fungi, bacteria, or animals soon destroy them. Only a few fossils have been formed from soft-bodied organisms. Carcasses of woolly mammoths have been found frozen in Siberia, so well preserved that their flesh was still edible after thousands of years. Fossil remains consist of the shells, skeletons, and other hard parts of living organisms which contain enough mineral matter to enable them to resist decomposition long enough for them to be covered by deposits which have preserved them in stratified rock until the present time. Fig. 53. Uncovering Brontosaurus bonet, l; hl Cal)in (Juarry, Wyoming. (Courtesy of the American Museum of Natural History.) Often pseudomorphs are found in the older rocks. A pseudomorph is a fossil which has been preserved by the substitution of the original material by mineral matter in such a way as to preserve the original shape and features of the specimen. Petrified trees are pseudomorphs. Frequently fossil molds or casts are found. A mold is a cavity left in a rock by the complete decay or solution of a dead organism, leaving its form in the sediment. If a mold is later filled with mineral matter which retains the shape of the mold, the fossil is called a cast. Tracks, trails, and furrows have also been preserved in the rocks. Peat bogs and asphalt pits have proven to be especially good sources of fossils of animals which have been trapped in them. Coal deposits
THE HISTORY OF THE EARTH 171 often show patterns of leaves, while volcanic ash and lava yield fossil remains that have been caught and petrified in these deposits. In the Mojave Desert there is a thick stratum of volcanic ash that probably settled in a lake and covered over branches and trunks of trees, which were then petrified as they lay in this sediment at the bottom of the lake. The fact that the types of fossils in different rock layers vary from simple one-celled organisms in the lowest rock layers to the complex mammals in the top layers indicates that life has not always existed Fig. 54. Dinosaur tracks in sandstone in the Peace River Canyon, British Columbia. (Courtesy of the Department of Mines and Resources, Canada.) on the earth in its present forms but has passed through an evolutionary process. Repeated observations have shown that rocks of a given period contain fossils peculiar to that period. Geologists who specialize in the study of fossils are called paleontologists ; they are able to date a given stratum by an examination of the microscopic and other fossils which it contains. This identification of strata is very important in petroleum geology, because it enables the geologist to examine oilwell cores and tell what stratum is being drilled and when a stratum which usually contains oil may be expected to be reached.