388 ME. H. H. THOMAS AXD PEOF. 0. T. .TO>^ES ON THE [Sept. I912, 



of albite and perthite, with possibly some anorthoclase. The 

 St. David's granite, as shown by the analysis made by Persifor 

 Frazer and quoted by Hicks (14, p. 528), is, like the one under 

 description, particularly rich in soda ; in fact, the analysis of the 

 St. David's rock shows 45 per cent, of albite, 4 per cent, of orfcho- 

 clase, and not more than 2*7 per cent, of anorthite. 



In the St. David's Dimetian, specimens taken from Bryn-y-garn 

 show albite to be the only plagioclase-felspar present, and they 

 ■contain perthite like the specimens from Silver Hill. 



The mica of the St. David's Dimetian is slightly more decomposed 

 than that of the Brawdy and Brimaston masses, for it seldom shows 

 the pleochroic halos characteristic of this mineral in its fresh state ; 

 also it appears to have been more highly titaniferous, for its decom- 

 position has resulted in the separation of a considerable quantity 

 of secondary sphene. 



The St. David's mass, like those of Brawdy and Brimaston, has 

 pegmatitic modifications. 



The quartz -porphyry. — The macroscopic characters of this 

 rock have already been mentioned (p. 383). Microscopically it 

 •exhibits corroded quartz-phenocrysts, alkaline felspars, and a 

 little decomposed biotite, set in a beautifully microgranophyric 

 ground-mass (E 9022). It is obviously related petrographically 

 to the granite, and is almost certainly a product of the same 

 magma. 



The felspar of the phenocrysts is chiefly albite ; the crystals are 

 ragged in outline, much twinned, and of smaller dimensions than 

 the quartz-crystals. They are for the most part rendered turbid 

 by micaceous products of decomposition. 



Around the quartz-crystals is a fairly wide resorption-halo of 

 microgranophyric material, in which the quartz extinguishes simul- 

 taneously with that of the phenocryst. Similar halos surround the 

 albite -crystals ; but, in this case, the felspars of the granophyre and 

 crystal are in optical continuity. In the remaining portion of the 

 ground-mass quartz is predominant, and extinguishes in irregular 

 patches after the manner of the ground-mass of many devitrified 

 rhyolites ; but the structure of each patch is minutely granophyric. 



The rock is evidently rich in silica, and the soda-content must 

 be high, on account of the prevalence of albite. It would compare 

 closely with the elvans of Cornwall, but would probably show a 

 difference in having an excess of soda over potash. In the elvans 

 there is a greater percentage of potash, and generally an absence 

 of granophyric structure in the ground-mass. 



The Knaveston diorit e. — The diorite is a dark-grey medium- 

 grained speckled rock, in which hornblende-crystals are obvious 

 to the unaided eye. Under the microscope it is seen to consist 

 chiefly of pale bluish-green hornblende, albite-oligoelase felspar, a 

 little augite, and much sphene. The hornblende sometimes behaves 

 ophitically towards the felspars, and contains small patches of 

 augite. 



