3-20 ■ Gerald M. Part— 



Tveatherius; wJiich also affects the rhyolites as noted by Parkinson.^ 

 One variety found on the scree-slojDes is full of small white spots, 

 which give it the appearance of a felspathic tuft. This, however, 

 is a form of decomposition similar to that noted by Dr. Elsden m 

 the " porphyrite '"" of CuTu-y-Graig near Mathry.- 



In tliin section (Fig. 6) these rocks are seen to consist mainh 

 of small microlites of felspar about -01 in. in length together "«-it]. 

 much interstitial clilorite, some granular sphene and epidote, and a 

 little interstitial quartz. The sphene in this case has ob^uously 

 arisen from the decomposition of a titaniferous iron-ore miueral. 

 probably ilnienite, of which some irregular remnants remain. One of 

 the pecuKar features of this rock is the abimdance of apatite. In 

 many of the slices the whole rock is riddled "nith needles of this 

 mineral, which must make up at least six per cent of the total bulk. 



The phenocrysts are rare, and include both soda-orthoclase and 

 albite ; vesicles when they occur are filled with quartz and some- 

 times a little limonite. 



The felspar of the microKtes includes both soda-orthoclase and 

 plagioclase. There is a considerable quantity of the former, which 

 •occurs both untwinned and in simple Carlsbad twins, and often has 

 a rather tmdulose extinction under crossed nicols. The plagioclase 

 approximates to albite m. composition, having a lower refractive 

 index than quartz, and giving extinction angles up to 15°. 

 The structure of the rock is " trachytic "' to "" pilotaxitic '". 

 Superficially it resembles very closely the " Kme-bostonite " or 

 " keratophyre '"' of Abercastle, but is richer in oithoclase, chlorite, 

 and iron-ores, and, in addition, is peculiar for its high content 

 of apatite. 



Specimens from the uj)per part of the crags show considerable 

 brecciation, and usually contain more or less calcite. 



There are two specimens (E 5717, 5719) of this tvpe in the 

 Geological Survey Collection, but the rock does not seem to have 

 been previously described. 



(d) Age. 



The evidence for the age of all these lavas is at present very 

 tmsatisfactorv, owing to the cleavage and general absence of 

 recognizable fossils. Parkinson has noted fossils indicating a 

 Bifidus age for the slates in the neighbourhood of Crvmmych .^ms,^ 

 a.nd on this evidence has suggested a " Lower Llandeilo '"'", or even 

 Axenig age for the rhvoKtes of Cam Alw,, which, according to that 

 author, are sUghtlv later than the more basic lavas found on the 

 southern slopes of the mountains (i.e. the rocks which form the main 

 subject of these notes). ^ Owing to the possibilities of strike-faulrlng, 

 this is by no means a certainty. Parkinson also mentions that 

 fossils are occasionaEv foimd in the Rosebush quarries, though he 

 adds that his own search was not successful. I have bke- 



1 Op. cit., p. 465. ° Op. cit,, p. 59S. ^ Ibid., p. 47i. 



