Chapter III. Arrangement of Fossils in Strata.— Fresh-Water and Marine Fossils.

Successive Deposition indicated by Fossils. Limestones formed of Corals and Shells. Proofs of gradual Increase of Strata derived from Fossils. Serpula attached to Spatangus. Wood bored by Teredina. Tripoli formed of Infusoria. Chalk derived principally from Organic Bodies. Distinction of Fresh-water from Marine Formations. Genera of Fresh-water and Land Shells. Rules for recognising Marine Testacea. Gyrogonite and Chara. Fresh-water Fishes. Alternation of Marine and Fresh-water Deposits. Lym-Fiord.

Having in the last chapter considered the forms of stratification so far as they are determined by the arrangement of inorganic matter, we may now turn our attention to the manner in which organic remains are distributed through stratified deposits. We should often be unable to detect any signs of stratification or of successive deposition, if particular kinds of fossils did not occur here and there at certain depths in the mass. At one level, for example, univalve shells of some one or more species predominate; at another, bivalve shells; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating strata.

It may appear inconceivable to a beginner how mountains, several thousand feet thick, can have become full of fossils from top to bottom; but the difficulty is removed, when he reflects on the origin of stratification, as explained in the last chapter, and allows sufficient time for the accumulation of sediment. He must never lose sight of the fact that, during the process of deposition, each separate layer was once the uppermost, and immediately in contact with the water in which aquatic animals lived. Each stratum, in fact, however far it may now lie beneath the surface, was once in the state of shingle, or loose sand or soft mud at the bottom of the sea, in which shells and other bodies easily became enveloped.