QUARTZ ROCK. 319 



tity of mica, while their predominant ingredient 

 is quartz in various states of mixture. Such va- 

 rieties holding an intermediate state between two 

 rocks, have an equal claim to be ranked under 

 either ; but as it is a fruitless attempt to define 

 that to which nature has not set bounds, and as 

 no advantages are gained by multiplying terms 

 to express such gradations, it is most proper to 

 class the doubtful specimen in that division which 

 is the predominant one, geologically considered. 

 Thus the occasional presence of mica will no 

 more exclude a specimen or a bed from the series 

 of quartz rock, than the predominance of quartz 

 will remove an occasional bed from the mass of 

 micaceous schist in which it is situated*. 



But the alternations between micaceous schist 

 and quartz rock, are often more decided and on 

 a larger scale ; a sudden and complete change 

 taking place where they meet. Scarba and Jura 

 afford abundant examples of this nature, and it is 

 also to be found in many other parts of Scotland. 

 The space occupied by the quartz rock in these 

 cases is so great, that it constitutes the chief part 

 of the series, and cannot therefore be considered 



