548 T. DAVIES ON THE MICROSCOPIC STRUCTURE 
AppEnDIx.—Notes on the Microscopic Srructure of some Rocks 
from PrempBroxEsHIRE. By Tuomas Daviss, Esq., F.G.S. (of the 
Mineralogical Department of the British Museum). 
Nos. 1-5. These are textural varieties of the Dimetian, Nos. 1 
and 2 being from Porth-lisky, Nos. 3 and 4 from Porth-clais, and 
No. 5 from a quarry south of the Camp. It is difficult to add to 
the descriptions already given of this rock. The quartz is of a 
peculiarly dirty aspect, being filled with enclosures, by far the 
larger portion of which are not fluid, but of a fine dusty nature, 
resembling much the dusty débris of felspars. It is often of a 
rounded character, like that of a typical quartz-felsite, and fre- 
quently in isolated crystals, surrounded by a mass of what may be 
immature or undeveloped rather than altered felspar. There is 
orthoclase and sometimes plagioclase and microcline. No mica is 
present, but in No. 4 there are evidences of the former presence — 
either of hornblende or of biotite, probably the latter. The green 
chloritic mineral occupies interstices or fissures, or partly replaces 
the felspars, and it presents no indication of its having resulted 
from the decomposition of a micaceous constituent. A rhombohedral 
carbonate is present in No. 1, which is probably a dolomite. The 
peculiar dendrito-graphic structure existing between the quartz and 
the felspar is very striking in Nos. 2 and 3—a rock which, probably 
from crushing, presents a remarkably brecciated aspect. No. 51s 
banded, the darker bands containing much of a fine-grained trans- 
lucent grey mineral, which depolarizes light. 
I may remark here that on a former occasion I thought this rock 
might be grouped with the quartz-schists (Q.J.G.S. vol. xxxiii. 
p. 232). At that time, however, but little microscopical work had 
been done upon the massive schists, and it was the unfamiliar 
crystalline relations of the constituents which led me to that view. 
No. 6 is another specimen of Dimetian from the Brimaston 
Mountain. It is coarse-grained and, macroscopically, shows slight 
signs of foliation. Under the microscope the quartz exhibits the 
same dirty aspect as in the preceding, and also, as in those rocks, 
frequently appears in individual crystals resembling those of a 
quartz-felsite. There are traces of a mica, but it is much altered 
and obscured by opacite. The felspars are in large immature indi- 
viduals, and, with the quartz, present sometimes a dendrito-graphie 
character. The rock seems to have been much crushed. Though 
from a different area, this belongs to the Dimetian type. 
No. 7 is a purplish-grey compact rock, intrusive in Dimetian at 
Ogof-llesugn. It exhibits a fine micro-crystalline ground-mass, with 
a few crystals of orthoclase and plagioclase. The whole is pervaded 
by minute grains and crystals with square outline, of a black and 
opaque mineral. It is much fissured, the fissures being lined with 
secondary quartz. This is a felsite, slightly porphyritic. 
No. 8 is from Porth-clais farmyard, and exhibits the junction of 
two rocks, which under the microscope present the characters of the 
