. Elements of geology. Geology. 36 LYELL'S ELEMENTS OF GEOLOGY. Gradual Deposition indicated by Fossils. Some limestones consist almost exclusively of corals, and their position has evidently been determined by the manner in which the zoophytes grew; for if the stratum be horizontal, the round spherical head of certain species is ^uppermost, and the point of attachment directed downwards. This arrangement is sometimes repeated throughout a great succession of strata. From what we know of the growth of similar zoophytes in modern reefs, we infer that the rate of increase was extremely slow, and
. Elements of geology. Geology. 36 LYELL'S ELEMENTS OF GEOLOGY. Gradual Deposition indicated by Fossils. Some limestones consist almost exclusively of corals, and their position has evidently been determined by the manner in which the zoophytes grew; for if the stratum be horizontal, the round spherical head of certain species is ^uppermost, and the point of attachment directed downwards. This arrangement is sometimes repeated throughout a great succession of strata. From what we know of the growth of similar zoophytes in modern reefs, we infer that the rate of increase was extremely slow, and some of the fossils must have flourished for ages like forest trees, before they attained so large a size. During these ages, the water re- mained clear and transparent, for such zoophytes cannot live in turbid water. In like manner, when we see thousands of full-grown shells dispersed every where throughout a long series of strata, we can- not doubt that time was required for the multiplication of suc- cessive generations; and the evidence of slow accumulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of the ocean after death, before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with serpula?, acorn-shells, corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried. Fig. 8. in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear wa- ter, when the testacea, now adhering to it, grew from an embryo state to full ma- turity. Attached shells, which are merely external, like some of the serpulss in the annexed figure, (Fig. 8.) may often have grown Upon an oyster, or other shell, while the animal with- in was still living; but if they are found on the in- side, it could only happen after the death of the in- habitant of the shell whic
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