120 ON THE FLEXURES ANI> 



and remarkable in gneiss, as well as in the hornblende 

 schist by which it is often acompanied. Bythe alternation 

 of these rocks, which are generally so strongly marked 

 by their differences of colour, they are indeed often 

 rendered visible at a distance where they would other- 

 wise escape notice. They abound also in micaceous 

 schist, where they offer examples of intricacy far more 

 minute than even in gneiss. The flexures of quartz 

 rock are, on the contrary, either on so large a scale as 

 to be insensible, except in a considerable space, or, 

 when they occur on a scale somewhat smaller, the 

 appearances are exceedingly partial. Through the 

 whole of a most extensive examination of the quartz 

 rock of Scotland, I have found but two examples of 

 this kind, and even those were barely sensible in an 

 extent of a few inches. The value of this fact will 

 hereafter be apparent ; although it appears in some 

 measure at variance with those already stated respect- 

 ing the tortuous nature of quartz veins. 



It may perhaps be con sidered remarkable, th at although 

 argillaceous schist is subject to flexures, it rarely, if 

 ever, exhibits those intricate contortions found in 

 micaceous schist or gneiss. Yet, in primary lime- 

 stone,^ the curvatures are often as great as they are in 

 the last of these rocks ; though never exhibiting that 

 extremely tortuous character so often found in mica- 

 ceous schist. 



With respect to the whole of the later, or secondary 

 strata, consisting of limestone, shale, and sandstone, 

 the same general remark will suffice. They are all 

 subject to curvatures ; but in no case have these been 

 observed to be sudden or considerable, much less, 

 intricate. 



Before proceeding to inquire respecting the cause 

 of these appearances, it remains to see whether any ol 



