216 ON THE ORIGIN^ MATERIALS, COMPOSITION, 



may be accounted for by the circumstances of dif- 

 ference in respect to consolidation to which they have 

 been exposed, we should find among them a set of 

 alternations analogous to the sandstone, shale, and 

 limestone of the latest series. Such an analogy can 

 indeed be traced, nor is it very imperfect. It must 

 be obvious, that a great part of the differences that 

 do exist, is explained by admitting the effects of heat 

 on them ; and it may be supposed that there have 

 also been some differences in the rocks from which 

 the materials of these strata were originally deposited. 

 But, as to the whole mass, there is still one leading 

 difference remaining, of great importance, and on 

 which some light may at least be thrown, if it cannot 

 be fully explained. It consists in the very great dis- 

 proportion of limestone in the two series ; that rock 

 being abundant in the later, and comparatively rare 

 in the earlier strata. 



The formation of Coral islands proves that enor- 

 mous and solid masses of calcareous rock are the 

 produce of animals alone ; and when we reflect on the 

 magnitude of some of these, we have no reason to be 

 surprised at the extent of those rocks which, among 

 the secondary strata, are composed chiefly of shells. 

 Were we even to suppose that every particle of the 

 largest bed of limestone known, was originally part 

 of the body of a shell, we should, as far as the bulk of 

 the mass is concerned, assume nothing that would 

 not be countenanced by the magnitude of the great 

 coral reef of New Holland. If the most minute 

 animals of creation can thus, by their numbers, exe- 

 cute, unassisted, works of such enormous magnitude, 

 and, as navigators think, within spaces of time com- 

 paratively limited, it is far from unreasonable to be- 

 lieve that the succession, through unnumbered ages, 



