MINERAL CHARACTEES OF STRATA UNCERTAIN. 32] 



different ages are often of similar composition ; and though 

 Sir. R. Murchison shows us, as in the case just cited, that 

 he has himself in past times been misled by trusting to lith- 

 ological evidence ; yet his reasoning, all through Siluria, 

 shows that he still thinks it natural to expect formations of 

 the same age to be chemically similar, even in remote re- 

 gions. For example, in treating of the Silurian rocks of 

 South Scotland, he says: — "When traversing the tract be- 

 tween Dumfries and Moffat in 1850, it occurred to me that 

 the dull reddish or purple sandstone and schist to the north 

 of the former town, which so resembled the bottom rocks 

 of the Longmynd, Llanberis, and St. David's, would 

 prove to be of the same age ; " and further on he again 

 insists upon the fact that these strata " are absolutely of 

 the same composition as the bottom rocks of the Silurian 

 region." 



On this unity of mineral character it is, that this Scot- 

 tish formation is concluded to be contemporaneous with 

 the lowest formations in Wales ; for the scanty paloeontolo- 

 gical evidence suffices neither for proof nor disproof. Now, 

 had there been a continuity of like strata in like order be- 

 tween Wales and Scotland, there might have been little to 

 criticise in this conclusion. But since Sir R. Murchison 

 himself admits, that in Westmoreland and Cumberland, 

 some members of the system ^* assume a lithological aspect 

 different from what they maintain in the Silurian and 

 Welsh region," there seems no reason to expect miueralogical 

 continuity in Scotland. Obviously therefore, the assump- 

 tion that these Scottish formations are of the same age 

 with the Longmynd of Shropshire, implies the latent be- 

 lief that certain mineral characters indicate certain eras. 



Far more striking instances, however, of the influence 

 of this latent belief remain to be given. Not in such com- 

 paratively near districts as the Scottish lowlands only, does 

 Sir R. Murchison expect a repetition of the Longmynd 



