318 Gerald M. Part — 



specimens the direction of flow is not at all constant even in the area 

 comprised in the field of view of a low-power objective, but a specimen 

 of almost identical rock, slightly coarser in texture, in the Geological 

 Survey Collection (No. E 5706, labelled " Prescelly Mountains, 

 Pembrokeshire ") shows a beautiful and much more regular flow- 

 structure. A second specimen (E 5705, similarly labelled) is com- 

 pletely brecciated. 



A rock from further up the hillside — south-east of Pant-maenog — 

 is poorer in orthoclase. The phenocrysts, nearly all albite, make 

 up a large proportion of the rock and grade down into the ground- 

 mass which approaches the "orthophyric" type in structure, owing 

 to the shorter and stouter habit of many of the felspars, some of 

 which show the " chequer " type of twinning. Quartz occurs, but 

 is not conspicuous except in the occasional vesicles, and the usual 

 interstitial chlorite and granular sphene are present scattered through 

 the ground-mass. In addition there are a number of irregular 

 patches of calcite connected, I believe, with the evident brecciation 

 which this particular rock has suffered, and which is well shown by 

 the bent and broken phenocrysts — sometimes recemented by 

 chlorite. 



The rocks of Cam Afr — the prominent cliff south of the path, 

 leading from Eosebush to Craig-y-cwm, mentioned by Parkinson — ■ 

 are mostly far from fresh, and contain much epidote, chlorite, and 

 secondary silica. As a rule they are highly vesicular, the vesicles 

 being filled with chlorite often with an outer layer of small imperfect 

 spherulites of silica. The phenocrysts are numerous, and, as usual, 

 include both soda-orthoclase and a plagioclase near to albite ; 

 where it is not obscured by secondary products the ground-mass is 

 trachytic and fine in grain. There is a good deal of granular sphene 

 scattered through the body of the rock. 



(c) The " Keratophyre " of Foel Trigarn. 



A trachytic rock of rather different type occurs on Foel Trigarn,. 

 where it fonns the lower and thicker part of the prominent crags on 

 the south and south-west sides of the hill. Most of the hill is made 

 up of soda-rhyolites, ashes, and breccias, which dip about 40° 

 north (almost the same as the northern slope of the hill). The low 

 crags below the stone cairns at the summit on the south side are 

 formed of a sheared dolerite, another band of which is exposed 

 lower down the hill. Immediately below the upper dolerite comes 

 the " keratophyre ". The actual junctions are obscured by heather 

 and scree, but this appears to be a flow intercalated amongst the 

 rhyolites and rhyolitic ashes, which latter are exposed lower down 

 the hill, above the lower dolerite. 



The rock when fresh is grey-green in colour, fine-grained, intensely 

 hard, and breaks with a splintery fracture. It is typically non- 

 porphyritic and non- vesicular, but weathers full of holes, brown and 

 ferruginous, the blocks resembliug lumps of slag — a form of 



