158 ON THE CHARACTERS AND DISPOSITION 



soft claystone, with a schistose tendency on exposure, 

 in no respect differing from those of the Trap islands 

 of the western coast. And the same facts, precisely, 

 occur in Guernsey ; sx> that a mineralogist, or a 

 superficial geologist, would, unhesitatingly, call that 

 a basalt which is but a graduating portion of the 

 general mass of granite. 



In each case, all these rocks form a common gra- 

 duating mass, and are therefore of the same date; 

 but, in Shetland, there occurs another instance which 

 presents, with similar features, an interesting variety 

 of origin. In that region, there is an extensive mass 

 of red granite of an ordinary structure, accompanied 

 by a dark one composed of the quadruple compound, 

 hornblende, mica, felspar, and quartz. In the latter, 

 the same gradation into basalt may be traced, in a 

 manner equally perfect ; and that this mass is granite, 

 is evinced by its inferiority to gneiss under all the 

 usual well-known appearances. But it is also of a 

 prior date to the red granite ; as the latter every where 

 penetrates it by its veins, just as it does the approxi- 

 mate strata. Hence its antiquity is no less unques- 

 tionable than its character. 



Thus it is proved that granite, or at least a 

 rock originating in the same causes, may possess 

 the characters of some of the most common varieties 

 of the most recent traps. It remains to reverse the 

 proposition, and to adduce instances of the gra- 

 nitic character among these recent unstratified 

 rocks. 



If the rock of Arran be considered a granite, which 

 it is esteemed to be, the same latitude must be ex- 

 tended to that of St. Kilda. Mica is often absent 

 from both ; and both contain, in certain parts, cavities 

 in which felspar and brown quartz are crystallized. 



