OVERLYING ROCKS. 509 



A. Claystone, common, or indurated, with im- 

 bedded scales of mica, sometimes regularly crystallized. 



This rock occurs both in veins and in masses 

 in the Western Isles of Scotland, but appears to be 

 rare. 



B. Grains, or imperfect crystals of quartz imbed- 

 ded in a simple base. 



a. Base of claystone. 



b. Base of indurated claystone. 



c. Base of clinkstone. 



d. Base of compact felspar. 



These occur together with the simple rocks 

 that form their bases, and are common in Arran. 

 Varieties c, d, form rocks of a sufficiently remark- 

 able appearance to attract the notice of a collector. 

 The Colours necessarily vary according to that of 

 the bases. 



C. Crystals of felspar imbedded in a simple base ; 

 or in a base apparently simple. Porphyry. 



a. Base of claystone. 



b. Base of indurated claystone. 



c. Base of clinkstone. 



d. Base of compact felspar. 



These form the most common of the simple 

 porphyries. The imbedded crystals may consist 

 of common felspar, of glassy felspar, or of both 



