188 



Besides the common association of quartz 

 rock above mentioned, it is frequently found 

 alternating with the primary sandstone. The 

 facility with which two rocks, so nearly allied, 

 may pass into each other, is such as to render it 

 almost superfluous to say that the transition is of 

 frequent occurrence. 



Although quartz rock is, in many of the 

 Scottish isles, found alternating with argillaceous 

 schist, the differences of character are so great, 

 that a transition could scarcely be expected. Yet 

 it takes place with great facility through the in- 

 tervention of the coarser varieties of both rocks. 

 The former first acquires fragments of schist, or 

 else the quartz becomes intermixed with schistose 

 clay, which, gradually predominating, that mine- 

 ral is at length excluded, A minutely laminar 

 intermixture of the two is also occasionally found, 

 of such a nature as to constitute an imperfect 

 transition. 



