316 Gerald M. Part— 



chilled crust has not always been preserved. Slate}' material has. 

 been washed into all the irregularities in the igneous rock, and in 

 some cases there is a certain amount of debris of the latter in the 

 slate in immediate contact with it. 



The actual junction is not exposed along the southern side of the- 

 hill, but a specimen of the igneous rock taken from within a few 

 yards of it> shows signs of brecciation and contains patches of calcite, 

 particularly in the felspar phenocrysts. This might be taken tO' 

 indicate the albitization of a previously more calcic felspar, but 

 I have only observed the presence of calcite in this area in those 

 rocks, both rhyolites and trachytes, which have been brecciated or 

 Aveathered, and on the whole I am inclined to regard its introduction 

 as due to these causes, and not connected with any secondary 

 albitization. 



Two hundred yards north-west of the farm Mynydd-du (f mile 

 north of Rosebush) a grey cleaved rock, very like a tufi in appearance, 

 crops out above the j)ath a few feet below blue-black slates similar 

 lithologically to those of Banc-du and the Rosebush quarries. It 

 contains abundant phenocrysts of soda-orthoclase and albite, 

 together with a few irregular patches of chlorite set in a ground- 

 mass which varies in texture from micro- to crypto-crystalline.. 

 This, however, was oiiginally glassy and shows beautiful peiiitic 

 structure picked out in chlorite (Fig. 3). The patches of varying 

 texture sometimes show a certain angularity and there are parts 

 which are distinctly " ashy " in appearance, but on the whole this 

 rock appears to be a fluxion-breccia rather than a taff. 



(c) Craig-y-cwm type : Interstratified with the slates in Craig-y- 

 cwm (north-east side of " Prescelly Top " — marked Foel-cwm- 

 cerwyn on the Ordnance Map), a few yards south-east of the old 

 slate quarries, is a flow of quite a different character. This is a 

 pale grey rock with white mottling, and of rough, almost gritty, 

 texture. There are numerous elongated vesicles filled with quartz 

 and limonite, and small patches of finer grain and of somewhat 

 darker colour. These resemble the ma,rginal portion of the rock, 

 Avhich is, in addition, more vesicular. The whole rock weathers 

 a rusty brown. 



Under the microscope (Fig. 4) there are scattered phenocrysts of 

 soda-orthoclase and albite about -04 in. in length. The ground- 

 mass has a poecilitic structure, consisting of grains of quartz usually 

 about -02 in. in diameter, with interstitial chlorite and granular 

 sphene and enclosing large numbers of felspar microlites. These, 

 which lie at all angles in the matrix, are often rather ragged and 

 tend to occur in radiating sheaves. Their refractive index is 

 distinctly lower than that of quartz, and the extinction angles (up 

 to about 16°) indicate that the majority are a plagioclase of 

 approximately the composition of albite. Some which are untwinned 

 or simple twins, and give almost straight extinction, are a soda- 

 orthoclase. Needles of apatite are very abundant, and there is 



