﻿Vol. 64.] 



ST. DAVID S-HEAD ' EOCK-SERIES. 



285 





1 

 I 



.si 



P? 



.2g 



.9 ?^ 

 1 







1 



-2 



1 



1 





1 







Basic 



— 



— 



— 





— 





- 







Medium... 



— 



— 



— 



— 





- 



— 



— 



- 



Acid 



— 



— 



— 



— 





— 



- 



— 



— 



Aplite 



— 







— 







— 



— 



- 



lY. Peteographical Details. 



I propose now to call attention to some features of petrographical 

 interest shown in these rocks. 



Felspars. — Beginning with the felspars, these are usually 

 rather unsatisfactory for optical study, being generally somewhat 

 turbid, perhaps owing to the fact that outcrop-specimens only can 

 be examined. Where satisfactory extinctions can be measured, 

 these point to the presence of labradorite in the most basic types ; 

 but the proportion of the albite-molecule increases in the more acid 

 varieties, so that all gradations from labradorite through andesine 

 to oligoclase in the zone of assimilation, and nearly pure albite in 

 the aplite-veins, can be traced. The chemical analyses tend to 

 confirm this ; but, as the felspars of the more acid types are 

 sometimes zoned by a species of lower refractive index than quartz, 

 it is clear that even in the same rock they are not always of 

 uniform composition. Orthoclase may be assumed to be a con- 

 stituent of the micropegmatite, which often characteristically frames 

 the felspars in the acid types. The alteration of the felspars presents 

 some noteworthy features. In most cases the alteration-product is 

 granular and opaque. Generally epidote is rare, but occasionally 

 this mineral becomes conspicuous, as along the northern margin 

 of the St, David's-Head mass, where veins of epidosite occur. 

 One of the most interesting changes in the felspar, occasionally 

 exhibited, is the development of a mineral giving rather square- 

 shaped sections and rectangular cleavages, which seems to form in 

 company with quartz from the breaking-down of large felspar- 

 crystals. This mineral has not yet been identified with certainty. 

 In my former paper, where I was relying upon indifferent material, 

 its straight extinction, refractive index, cleavages, and birefringence 

 led me to think that this mineral might be one of the scapolite- 



