90. _Fossiliferous Strata of Different Ages._--Another fact would also gradually dawn upon us--this, namely, that in certain rocks the fossils depart much more widely from analogous living forms, than the organic remains in certain other rocks do. The cause of this lies in the fact that the fossiliferous strata are of different ages; they have not all been formed at approximately the same time. On the contrary, they have been slowly amassed, as we have seen, during a long succession of eras. While they have been accumulating, great vicissitudes in the distribution of land and sea have taken place, climates have frequently altered, and the whole organic life of the globe has slowly changed again and again--successive races of plants and animals flourishing each for its allotted period, and then becoming extinct for ever.[G] Thus, strata formed at approximately the same time contain generally the same fossils; while, on the other hand, sedimentary deposits accumulated at different periods are charged with different fossils. Fossils in this way become invaluable to the geologist. They enable him to identify formations in separate districts, and to assign to them their relative antiquity.[H] If, for example, we have a series of formations, A, B, C, piled one on the top of the other, A being the lowest, and C the highest, and each charged with its own peculiar fossils, we may compare the fossils met with in other sets of strata with the organic remains found in A, B, C. Should the former be found to correspond with the fossil contents of B, we conclude that the rocks in which they occur are approximately of contemporaneous origin with B, even although the equivalents of the formations A and C should be entirely wanting. Further, we soon learn that the order of the series A, B, C, is never inverted. If A be the lowest, and C the highest stratum in one place, it is quite certain that the same order of succession will obtain wherever the equivalents of these strata happen to occur together. But the succession of strata is not invariably the same all the world over; in some countries, we may have dozens of separate formations piled one on the top of the other; in other countries, many members of the series are absent; in brief, _blanks in the succession_ are of constant occurrence. But by dovetailing, as it were, all the formations known to us, we are enabled to form a more or less complete series of rocks arranged in the order of their age. A little reflection will serve to shew that the partial mode in which the strata are distributed over the globe arises chiefly from two causes. We have to remember, _first_, that the deposits themselves were laid down only here and there in irregular spreads and patches--opposite the mouths of rivers, at various points along the ancient coast-lines, and over certain areas in the deeper abysses of the ocean--the coarser accumulations being of much less extent than those formed of finer materials.