4. As many parts of the bed of the present seas, which are probably receiving detrital matter constantly, are unfavorable for the development of animal life, while other parts are highly favorable, it might be presumed that animal life would be equally scanty in particular localities while the earlier rocks were forming, and in other localities very abundant. Hence some strata, for hundreds of feet in thickness, are composed almost entirely of fossils, while other strata are nearly or quite destitute of them. The same member of a formation may in one place be full of fossils, and in another without them. _The distribution of fossils_ is therefore subject to no general law; at least, none of which we can avail ourselves, in the search for them. 5. The value of fossils in geology consists in the use which is made of them in determining _the origin and age of strata_. As the animal species which inhabit bodies of fresh water are always different from those found in the sea, their remains constitute the best means of determining whether a formation is of _fresh water_ or _marine origin_. In order to decide this point, it may, in some cases, be necessary to be acquainted with the habits of particular species. In most cases, however, it will be sufficient to remember that in fresh-water formations, first, there are no sponges, corals, or chambered shells; second, the univalves all have entire mouths (Fig. 44). Third, the bivalves are all bimuscular (Fig. 47). If, therefore, a formation is found to contain sponge, coral, a chambered shell, a univalve with a deeply notched mouth (Fig. 45), or a unimuscular bivalve (Fig. 46), it must be considered a marine formation. [Illustration: Fig. 44.] [Illustration: Fig. 45.] [Illustration: Fig. 46.] [Illustration: Fig. 47.] We have seen that the same formation, as exhibited in different places, differs in its thickness, composition and degree of solidification. If we could trace the strata through all the intermediate space, we might be certain of their being the same formation, notwithstanding the change in lithological characters. But this can seldom be done, even for a few miles in extent. Sections of the strata are obtained only occasionally, where rivers have cut through them, or where, over limited areas, the soil has been removed from the outcropping edges. It is also frequently the case that the strata are so much disturbed that their position will furnish no aid in determining their age. When folded axes occur (as here represented), the older strata are often the uppermost. There is an instance in the Alps in which strata of vast thickness have been inverted during the process of upheaval, and now rest on a bed of rock formed from the debris which they had supplied. [Illustration]