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 which 

 aflbrds the support. Thus, 

 in Fig. 8., it will be seen 



Fossil GryphcBa, covered both on the outside and 

 inside with fossil serpnlcs. 



