Fossils. nature of fossils and the modes of fossilization are described in the article sil_ all animals and plants of past time have been preserved to us as fossils. .\ large number of them were of such n ti at they were hardly likely to leave traces of their existence in the rocks. It is also known that many formations which originally e1ut:tined fos sils have suffered so great metamorphism that their organic contents have been wholly or par tially destroyed, tither formations have been upheaved above the level of the ocean to form part of the land and have been subjected to ero sion, with the result that their materials have been carried down to lower levels into sea, there to build up deposits of age. Yet. in spite of the many gaps which w ill ably never be tilled. the history of org.it is is being rapidly compiled with an increasing degree of continuity in the series of life epoch,• the hues of descent of many raves of animils and plant; have been established, ?nd the sequence oft lose events in geological history that upon the conditions existence of extinct faunas has 1;4.11 worked, out for several portiors of the surface. and with a surprising degree of tail.
Through observation of tl e true order of position. the rocks of the crust h lye be arranged by eeolo!dsts into a series of systems, and formations, the loWern ost the oldest and the uppermost the youngest, and the relative succession of the individual of this series has been Ile ti ruined to be the same in all parts of the world. The fossils of ti e different of the serif have been d and scribed. and it has been ascertained that the sembhore of fossils found in one formation differs I tore or less from the assemblage of fi found in the overlying and underlyin_ amd furthermore that the difference is avecptit I the distance between the formations is inere.ise 1.
FOT table of geological formations, see I :E111_0( v.
The fossils contained in a rock for mation arc in general indicative of the animal and vegetable life that lived during the period of time in which that rock was deposited. Suc cessive faunas and floras of variable expro*sion have succeeded each other in] the earth and hare left their remains in the rocks that were forming during the periods of their existence. The large divisions of time are distinguished by the pre dominant types of life, as the Paleozoic era by invertebrates. the :Mesozoic era by reptiles, etc., and the smaller periods of time, represented by the stages and substages, arc characterized by particular genera and species. Thus the Helder bergian series is identifiable by the brachiopod genus Itypidula, and a division of that series, the Coeymans limestone, is recognizable by the pres ence of (Lypidula yulryi to a 1\.11-marked species that serves tar the blentifieation of that lywizon in many parts of America. Such a characteristic genus fir species, which can be relied upon for the recognition of geological horizons or zones, is called an index fossil, and its use is the result primarily of careful observation in the field. This empirical usage of index fossils is that adopted by geologists who have not been trained in the methods of paleontological research. There is a second and more important usage of index fossils based upon a knowledge of the phylogeny of races of animals and plants. liy recognition of the phyletie position of a fossil the expert pale ontologist can determine within close limits the relative age of the rock from which the fossil was obtained.