CONTORTIONS OF ROCKS. 119 



The last species of contortion calling for remark, is 

 that found in rock veins, and it presents difficulties 

 peculiar to itself; at present, apparently unsurmount- 

 able. Micaceous and argillaceous schists are, as is 

 well known, very commonly traversed by veins, often 

 numerous, and generally slender. In the former rock, 

 they invariably, I believe, consist of quartz ; and, in 

 the latter, sometimes of this mineral, and, at others, of 

 calcareous spar. When the rocks that contain them 

 are contorted, the veins are at the same time bent ; 

 following their sinuosities, however intricate, without 

 any fracture or breach of continuity. However in- 

 flexible these minerals may appear, it is plain that they 

 must, in these cases, have undergone flexion. It is 

 impossible, either that open fissures occupying these 

 intricate directions could have been formed in the rock 

 after it was bent, or that, if previously existing and 

 empty, they could have been so bent without being 

 obliterated ; waiting to be filled, at a future time, by 

 means of those infiltrations of carbonat of lime or 

 silica which produce them. A case even more diffi- 

 cult sometimes occurs in gneiss ; where veins of quartz 

 are found most intricately contorted, although the laminae 

 of the including rock show no corresponding indica- 

 tions of flexure. These are difficulties which, among 

 a thousand others, must remain for future explanation ; 

 and how far the solution of these cases may modify 

 any of our theories, it is impossible at present to 

 foresee. 



Of the 'Rocks subject to Flexures and Contortions. 



It was already observed, that contortions were chiefly 

 abundant and conspicuous among; the more antient 

 strata; and, of these, they are exceedingly common 



