OF SOME ROCKS FROM PEMBROKESHIRE, 549 
Dimetian and of a felsite respectively. The ground-mass of the 
latter is micro-crystalline, and shows small fragments of quartz torn 
from the Dimetian, which latter is scarcely, if at all, changed. 
There is no evidence of a passage from one rock to the other. Macro- 
scopically the junction is well marked, though the edges are welded 
together. 
No. 9 is a bluish-grey compact rock, with circular or ovoid 
patches resembling spherules. In thin section it resembles No. 7, 
but is still finer grained; the minute black mineral is present here 
also in abundance. The patches consist of a micro-crystalline 
eround-mass similar to the rest of the rock, and contain some 
diffused chlorite ; they are surrounded by a belt of a slightly coarser 
texture than the remainder of the rock. This is also from Porth-— 
clais farmyard. 
No. 10, from a field north of Porth-melyn, is a compact grey 
rock, with numerous crystals of altered felspars and some quartz. 
Under the microscope the ground-mass is highly crystalline, with a 
tendency to a spherulitic structure in places, and encloses numerous 
erystals of orthoclase and plagioclase ; the quartz is not so abundant. 
There is much of the green minerals distributed throughout, some- 
times replacing the felspar crystals wholly or in part. This is a 
porphyritic quartz-felsite. 
No. 11. Hast of Dowrog Pool. A dull dark-green rock, showing 
some enclosed small quartz crystals. Under the microscope it is 
seen to consist principally of fragments of quartz, much of which is 
of the dirty character of the Dimetian; some, however, are clear, 
and such as characterize the quartz-felsites. ‘The small amount of 
felsitic ground-mass is micro-crystalline, and is pervaded by the 
bright-green mineral, which also fills fissures and cracks in the 
quartz crystals, and appears to have replaced most of the felspar. 
There is much iron-pyrites in small crystals, also magnetite and its 
decomposition-products. A few felspar crystals or fragments show 
traces of Carlsbad twinning. The whole appears to have been much 
crushed, for the rock, as well as the quartz fragments, is fissured 
and fractured in all directions, and the quartz much displaced. 
There is much secondary quartz. 
No. 12. Carn, Ramsey Island. Macroscopically this is an ex- 
cessively fine-grained rock, of a greenish-grey colour and hornstone- 
like fracture. Weathers white, the weathered surfaces presenting 
strong indications of bedding. It resembles an.old rhyolite, but on 
examination of a thin section it appears as though it would group 
best with the hilleflintas, though it differs from the Roch-Castle- 
rock type in the larger size and character of the individual grains 
and in the absence of quartz nests. This is a very puzzling rock, 
which for the present I am inclined to regard as a quartzo-felspathic 
ash. : 
No. 13. Carn, Ramsey Island. This is a dark-grey rock, much 
marbled with yellowish white. In thin section it presents a 
micro-crystalline ground-mass, which is pervaded by a yellowish 
substance having no action on polarized light. Quartz and felspar 
