86 ROCK-METAMORPHISM. 



materials precisely such as would result from the mechanical 

 degradation of the gneisses, syenites, and other silacid members 

 of the Archaeans. The abundance of argillytes, sandstones, and 

 other mechanically produced deposits, the rare accompaniment 

 therewith of limestones, and the contemporaneous presence of 

 non- calcareous fossils admit of no difficult explanation ; but 

 when viewed in connexion with the occurrence of 6000 feet of 

 crystalline limestones in the preceding Archseans, the problem 

 becomes inexplicable. 



It must strike every geologist that, if such an enormous thick- 

 ness of calcareous rocks was available during the Lower 

 Cambrian period (that is, when these sandstones and argillytes 

 were in course of derivation, through disintegration and denuda- 

 tion, from the Archaean gneisses &c.), we ought to expect that 

 the great lime-bearing series referred to yielded, through 

 organic intervention, a considerable amount of limestones. But 

 where are they ? Certainly not in the diffused quantities, or the 

 " belts" and "bands," that are known, particularly as it is 

 questionable that the lime in these cases is much in excess of the 

 amount which could have been generated by the contemporary 

 action of the carbonic acid, then pervading the atmosphere and 

 different waters, on the calcium silicate in the labradoritic, 

 hornblendic, and augitic debris produced by the disintegration 

 and denudation of the Archseans during the Cambrian periods. 



With respect to the period when the Archsean crystalline 

 limestones were completely elaborated (for our hypothesis in- 

 volves slow processes requiring immensity of time), we can offer 

 no decided opinion. It would appear improbable that their 

 denudation, operating throughout a vast chronological term that 

 embraced the two Cambrian periods, and producing miles in 

 thickness of debris, would give rise to no more than the small 

 quantity of limestones that were deposited during these periods. 

 On the other hand, however, there seems to be great probability 

 that the crystalline limestones constituted the great factors which 

 so vastly increased the number of organisms with calcareous 

 skeletons, and the consequent calcareous deposits, during the 

 Silurian periods. 



To account for the paucity of early post- Archaean limestones, 

 we offer the suggestion that, during the Lower Cambrian period 

 there was no great series of crystalline limestones included 



