220 ON THE ORIGIN, MATERIALS, COMPOSITION, 



whole of the rocks, riot unlike that which the lime- 

 stones do to the siliceous and argillaceous strata. This 

 should be expected from the presumed rarity of these 

 animals in the antient ocean. 



It has been supposed by some geologists, that all 

 the calcareous strata, of whatever age, were the ex- 

 clusive produce of animals. That possibility is coun- 

 tenanced by the phenomena of the Coral islands ; 

 though the accessary causes, arising from the decom- 

 position of previous limestones, must be admitted as 

 far as regards the secondary strata. But the mere 

 existence of primary limestones thus operating by their 

 destruction to assist in producing new ones, is not 

 itself a proof that these are original, and independent 

 of animal sources. The existence of animal remains 

 in primary schists has just been mentioned ; and I 

 have elsewhere described one instance in which these 

 occur in a calcareous quartz rock situated beneath 

 gneiss. Thus far, they might have contributed to 

 the production of even the primary limestones ; and if 

 they are not more frequently found among them, 

 causes for that are not wanting. 



In the first place, primary limestones are, not only 

 comparatively rare, but geologists having adopted the 

 hypothetical opinion that they ought not to contain 

 animal remains, make it a rule invariably to rank such 

 instances among their transition series ; without 

 thinking it necessary to investigate the subject by the 

 rigid rules of pure geological analysis : by position, 

 and relation towards the neighbouring strata. It is 

 further obvious, that the primary rocks have undergone 

 great disturbance, and, in many instances, serious 

 changes ; and, even among the secondary strata, in 

 such cases, the animal remains are often obliterated. 

 The fusibility of limestone has already been demon- 



