Mhieralogy of Sky, 73 



the position of these rocks, refer them to the syenite family, although 

 had I met with the same specimens connected with a mountain of 

 granite, and lying under mica slate, I should have referred them to 

 the granites. 



This is far from being the only case in nature where mere 

 mineral distinctions are insufficient to determine the geological 

 situation of a rock. In the stratified classes of rocks, both primary 

 and secondary, these resemblances are frequent, since it is often 

 impossible to distinguish quartz rock from sandstone, the breccias 

 "which it contains from the more recent graywacke, ancient clay 

 s.late from recent, or, as I have shown in this very account of 

 Sky, primitive from secondary limestones. The same rocks seem 

 in some cases to have been repeated at different epochas, while in 

 others they show variations which may perhaps be the results 

 of posterior changes operating on the first deposits rather than 

 the consequences of original diiferences. 



Two other varieties of this rock occurred to me which may be 

 mentioned, although possessing no peculiar interest. In the one 

 chlorite formed a constituent part, and in the other a greenish com- 

 pact steatite was intermixed with the felspar and hornblende, the 

 total compound being not much unlike the porcelain granite of 

 Cornwall. 



Before I conclude these remarks on the trap and syenltic rocks of 

 Sky, it will not be superfluous to enumerate the striking external 

 features in which rocks so nearly associated differ. 



The mountain trap of the CuchuUin is most strongly distinguished 

 from the stratified in the difference of its disposition, in the absence 

 of the columnar forms and decomposing tendency, and in the bar- 

 renness of its surface compared with the deep soil and highly clothed 

 vegetable surface of the latter. In its superior permanence, a per- 



VoL. in. K 



