﻿Vol. 6 1.] BETWEEN ST. DAVID'S HEAD AND STKTXMBLE HEAD. 587 



untwinned. These appear to be orthoclase. Some of the sections 

 show an outer zone of more acid composition than the interior. 

 Both augite and enstatite are much reduced in quantity, and the 

 chief ferromagnesian mineral is a strongly-pleochroic brownish 

 hornblende. The scheme of pleochroism is : 



a, pale brownish -yellow ; /3, light-brown ; y, dark greenish-brown ; y >/3 > a. 



The extinction-angle is about 15° on clinopinacoid sections. 

 Augite occurs very sparingly, and the enstatite seems to be rather 

 more ferriferous, distinctly pleochroic, and sometimes altered into 

 a fibrous amphibole on the margins. All the dark minerals seem 

 to have preceded the felspars in order of consolidation. The latter 

 apparently grew more acid, until finally the eutectic point of 

 orthoclase and quartz was reached. The iron-ores are represented 

 by ilmenite, rather altered. Apatite is abundant, and there is 

 some secondary epidote. In another specimen, of finer grain, the 

 order of consolidation is apparently different, felspar preceding both 

 enstatite and augite ; with hornblende, reduced in quantity, imme- 

 diately preceding the quartz and micropegmatite. (See PI. XXXIX, 

 fig. 5.) 



At Cam Ffald the rock is very similar, but there is less quartz 

 and micropegmatite ; and at Cam Perfedd the latter structure 

 appears to be absent, but neighbouring interstitial quartz-areas 

 extinguish simultaneously, and are penetrated by a felspar having 

 the appearance of orthoclase. This character is still more pro- 

 nounced at Cam Trelwyd, where orthoclase seems to be more 

 plentiful, both simple and in Carlsbad twins. The earlier felspars 

 are polysynthetic, with extinctions corresponding to andesine. 

 Some augite preceded the felspars, which enclose it in pcecilitic 

 fashion. Augite also occurs in good octagonal sections. Enstatite 

 is about equal in quantity to the augite, and occurs in small fibrous 

 pseudomorphs, slightly pleochroic. Hornblende is rather plentiful, 

 and has an idiomorjDhic habit which becomes more pronounced in 

 the quartz-areas. Ilmenite is scarcer than usual, but apatite is 

 very abundant. Quartz-areas are associated with orthoclase, the 

 latter in idiomorphic crystals, but there is no micropegmatite 

 visible in the specimens examined. 



The rocks in this ridge appear to belong generally to the class of 

 enstatite-diorites. 



The apparent abundance of orthoclase in the Carn-Trelwyd rock 

 almost suggests a monzonite, and I determined the alkalies in 

 this rock, with a view to the estimation of the relative amounts 

 of orthoclase and plagioclase present. The following was the 

 result : — 



Per- Molecular 



centage. proportion. 



Na 2 0=3-726 -060 



K 2 =2073 022 



The orthoclase calculated from these data would thus form 

 12-232 per cent, of the rock, which is scarcely high enough to 



