324 QUAT2: ROCK. 



tion/of the micaceous laminae into large .flags ap-? 

 plicable to the purposes of architecture, o.-: c 

 ml Tracing die progress of the granular kind, we 

 find them at length consisting; of grains so large 

 as to appear mere agglutinjatjons of gravel, with 

 occasional vacuities among the grains ; nor is it 

 uncommon to find.spec4iens^containing 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 dissimilar 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 gfay%ack fg easy, and, in these 

 islands, abundantly obvious. 



Quartz rock never appears to ; con tain ; any un- 

 essential minerals capable of modifying its gene- 

 ral characters; and it is equally remarkable that 

 it is very rarely the repository of any indepen- 

 dent mineral, except garnet and pyrites. , ^ ^ 



The colours are extremely limited. In the 

 purer varieties, white is the predominant hue ; 

 but they arc ^Qccasiohally to be found tinged with 



