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OF SOME ROCKS FROM PEMBROKESHIRE. 551 
is a purple brecciated rock. Much of the quartz of this rock is 
of a dirty character. The large purple fragments present no defi- 
nable structure and contain no minerals; they correspond very well 
in aspect with the porcellanites. It is a porcellanite breccia. 
No. 28. Quarry north of Board School, St. David’s. This is a 
porcellanite presenting the same structure as No. 19. It is much 
fissured, the fissures being occupied by a colourless foliated mineral 
which depolarizes light. 
No. 24. By Quickwell Bridge, north of St. David’s. A dull 
purple rock. This is a breccia containing a few fragments of a rock 
resembling the porcellanites, clear quartz, and fragments of two 
felspars and altered felsites. The cement is black and opaque. 
The rock consists principally of voleanic débris. 
No. 25, from Maen-dewi. This is a porphyritic quartz-felsite very 
rich in quartz and felspar crystals, both of which frequently present 
a fragmentary or angular aspect. The ground-mass is also very 
confused, and between crossed nicols is microcrystalline and shows. 
traces of an original fluxion structure. Contains chlorite in vermi- 
cular groups of crystals. Looks brecciated, but if so, it is probably 
m situ. 
No. 26, from Rhodiad, is a pebble from a breccia. This some- 
what resembles No. 25. The ground-mass varies in patches be- 
tween a microcrystalline and a cryptocrystalline structure. It is 
pervaded by irregular grains of a mineral which sometimes depola- 
rizes light. This is probably of secondary origin and appears to 
have frequently replaced part of the felspars. | 
No. 27. Another pebble from the breccia of Rhodiad. This is 
another quartz-felsite of a somewhat different character from the 
preceding. It has a clear ground-mass, which between crossed 
nicols is microcrystalline, and contains a smaller number of crystals 
of two felspars and of quartz. 
No. 28, from Treglemais, is a breccia. Much of the rock is 
felsitic ; but there are also fragments which, seen under the micro- 
scope, strongly resemble the hilleflintas, and also a rock with the 
minute indefinite structure of the porcellanite and having no action 
on polarized light. It also contains fragments of felspars and of 
quartz such as might have been derived from a quartz-felsite ; also 
a fragment of a basic rock with columnar felspar. The whole is 
pervaded with a pale-grey mineral which is more abundant around 
the fragments. It depolarizes light strongly. 
No. 29. Same rock as No. 28 and from same place, but contains 
more fragments of felspar and quartz, and also more of the basic 
rock. Contains the same depolarizing mineral, but here it is some- 
what yellowish in colour. 
No. 30. This is a greenish-grey compact rock from Treglemais, 
and is a fragment from the same breccia as No. 29. Ground-mass 
medium-grained crystalline carrying numerous, though isolated, 
spherulitic developments of the same. Felspar crystals are not 
numerous. Some of the secondary mineral characterizing Nos. 28 
and 29 is here also, and some small diverging groups of crystals. 
