22 



GRADUAL DEPOSITION 



[Ch. m. 



almost exclusively of corals, and in many cases it is evident that the present 

 position of each fossil zoophyte has been determined by the manner in 

 which it- grew originally. The axis of the coral, for example, if its nat- 

 ural growth is erect, still remains at right angles to the plane of stratifi- 

 cation. If the stratum be now horizontal, the round spherical heads of 

 certain species continue uppermost, and their points of attachment are 

 directed downwards. This arrangement is sometimes repeated through- 

 out 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 remained clear and transparent, for such corals cannot live in tur- 

 bid water. 



In like manner, when we see thousands of full-grown shells dispersed 

 everywhere throughout a long series of strata, we cannot doubt that 

 time was required for the multiplication of successive 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 serpulae, or barnacles (acorn-shells), or corals, and other creatures, 

 attached to the inside of the valves, so that the mollusk was certainly not 

 buried in argillaceous mud the moment it died. There must have been 

 an interval during which it was still surrounded with clear water, when 

 the creatures whose remains now adhere to it, grew from an embryo to a 

 mature state. Attached shells which are merely external, like some of the 

 serpulae (a) in the annexed figure (fig. 10), may often have grown upon 

 FJ g- 10. an oyster or other shell while the an- 



imal within was still living; but if 

 they are found on the inside, it could 

 only happen after the death of the 

 " inhabitant of the shell which affords 

 the support. Thus, in fig. 10, it will 

 be seen that two serpulae have grown 

 on the interior, one of them exactly 

 on the place where the adductor mus- 

 cle of the Grypliwa (a kind of oys- 

 ter) was fixed. 



Some fossil shells, even if simply 

 attached to the outside of others, bear 

 full testimony to the conclusion above 

 alluded to, namely, that an interval 

 elapsed between the death of the 

 creature to whose shell they adhere, 

 and the burial of the same in mud or 

 sand. The sea-urchins or Echini, so 



Fossil Gryphoea, covered both on the outside ■, -, , • -i •, in re i ^.^j 



and inside with fossil serpnire. abundant in white chalk, attorn a good 



