550 T. DAVIES ON THE MICROSCOPIC STRUCTURE 
crystals are present, the former being much fissured. This is a 
quartz-felsite. 
No. 14, from Carn, Ramsey Island, is much like No. 12, but con- 
tains distinct quartz fragments. It may be regarded for the present, 
like No. 12, as a quartzo-felspathic ash. 
No. 15. Aber Mawr, Ramsey Island. A greyish rock, with 
quartz and felspar crystals. The ground-mass is medium crystal- 
line, with numerous small quartz and felspar crystals ; this encloses 
larger crystals of the same minerals, though the two felspars are 
much altered or perhaps immature. Around some of the felspar 
erystals the dendrito-graphic structure is slightly developed: this 
structure also appears in parts of the ground-mass. Contains a 
little calcite in spaces, secondary quartz, and much epidote with 
chlorite, which appears in some cases to replace the felspar. This 
is a porphyritic quartz-felsite. 
No. 16, from Well on the road to Caerfai. A bluish-grey rock, 
with many quartz and felspar crystals, and disseminated iron-pyrites. 
The micro-crystalline ground-mass of this rock varies very much in 
texture, and encloses abundance of quartz, which is frequently much 
fissured and fragmentary. This appears to me to be a quartz-felsite 
breccia. 
Nos. 17 and 18. West Creek, Nun’s Well, are fragments from a 
breccia. No. 17 contains two felspars, in a very fine-grained ground- 
mass; one crystal appears to be partly replaced or honeycombed 
by quartz. In No. 18 a dust-like substance, haying no action on 
polarized light, pervades the whole ground-mass, though thicker in 
somewhat irregularly distributed patches. It is much fissured, and 
traversed by veins of secondary quartz. There are evidences of 
felspar crystals, around some of which appear traces of a former 
fluxion-structure. A mineral in slender prisms has been replaced 
by a black opaque substance. Epidotic and chloritic minerals are 
present, but much obscured by the decomposition-products of ferri- 
ferous minerals. These appear to be altered felsites. 
No. 19. Caerbwdy Valley. A grey or greenish-grey compact 
banded rock, with hornstone-like fracture. Under the microscope, 
in ordinary light, it presents innumerable exceedingly minute grains 
of a very pale yellowish transparent mineral, in a clear ground-mass. 
Much of a greyish opaque substance is disseminated throughout. 
This is a porcellanite. - 
No. 20 is an arkose-like rock from Porth-lisky. It consists of 
fragments of felspar and quartz in a confused micro-crystalline 
ground-mass, which also contains much of a grey dust-like substance 
diffused throughout. This is probably a quartz-felsite breccia. 
No. 21. West side of East Harbour, Porth-lisky. This is a fine 
schistose greyish rock with a white silky mineral. Under the 
microscope it exhibits fragments of quartz much resembling that of 
the Dimetian, separated by a filmy micaceous mineral which feebly 
depolarizes light. There are also occasional fragments of a rock 
resembling the porcellanites. 
No. 22 is from the east side of West Harbour, Porth-lisky. It 
