JURA. QUARTZ ROCK. 



that, in these cases, it often admits of being split in the 

 direction of the micaceous laminse, into large flags appli- 

 cable to the purposes of architecture. 



Tracing the progress of the granular kinds, we find them 

 at length consisting of grains so large as to appear mere 

 agglutinations of gravel, with occasional vacuities among 

 the grains ; nor is it uncommon to find specimens contain- 

 ing rounded pebbles of large size. In these varieties the 

 marks of mechanical formation are obvious ; and they are 

 rendered still more so by the occasional intrusion of dissi- 

 milar fragments, such as jasper, clay slate, and micaceous 

 schist, examples of which occur so as to form considerable 

 beds both in Scarba and in Jura. From these, the passage 

 into common graywacke is easy, and, in these islands, 

 abundantly obvious. 



Such is a general sketch of the prevailing mineral fea- 

 tures of quartz rock. Respecting its accidental characters 

 it may be added, that it is a metalliferous rock, although 

 not hitherto included in that division ; since the mine of 

 Tyndrum is situated in strata of this substance that alter- 

 nate with micaceous schist. Occasionally also it contains 

 those prolonged concretions and other well known forms 

 which are common in the secondary sandstones, and which 

 have been supposed in these to originate in organic remains 

 or impressions. Finally, I may remark that it is rarely 

 contorted, when compared to those rocks with which -it is 

 associated ; although instances of such irregularities may 

 be witnessed in some places, and very conspicuously at the 

 Whiten head in Sutherland. 



