292 PROF. A. H. COX AXD ME. A. K. WELLS ON THE [voL lxxvL, 



The felspars of the ground-mass include both plagioclase and 

 orthoclase. The former is distinctly idiomorphic, whereas the 

 latter is irregular in outline ; sometimes a partly - idiomorphic 

 plagioclase is surrounded by allotriomorphic orthoclase. The 

 orthoclase occasionally presents the patch}' or streaky appearance 

 indicative of a soda-orthoclase. All gradations between perfectly 

 untwinned crystals and markedly patchy ones may be observed. 

 There is evidently a fair amount of soda in the molecule. 



Fig. 5. — Sections of felspar-twins, Crogenen (jranopliyre, 

 crossed nicols. X 50. 



C 224 



a.k.'W. del. 



Chlorite occurs rather abundantly as specks, granules, threads, 

 and larger aggregates, especially round the felspars, or separating 

 the felspars from quartz, and even enclosed in quartz. Pieochroism 

 is very marked in the chlorite of the larger patches, whereas the 

 chlorite of the smaller granules and threads is pale with hardly 

 observable pieochroism, though it may sometimes be birefringent. 

 It is not easv to see from Avhat original mineral this chlorite has 



