OF SOME ROCKS FROM PEMBROKESHIRE. 555 
numerous fragments of felsite and of felspar, some of which is 
plagioclase. A colourless mica is present. 
No. 64. Cambrian sandstone from Ogof-llesugn. This is a much 
finer rock than 63, but with a j-inch objective is seen to consist of 
the same minerals; the felspars, however, with the exception of a 
little plagioclase, seem to have been much broken up. There is 
more of the micaceous mineral than in 63. 
No. 65. Cambrian conglomerate, west of Rhosson Rock. Is 
composed of dirty quartz with fragments of quartzite and quartz- 
schist with mica. 
No. 66. Cambrian conglomerate, west of Rhosson Rock. En- 
closes much quartz, but contains a fragment of a schistose felsite like 
39, also fragments of quartz-schist with a nearly colourless mica. A 
yellowish foliated mineral occupies the interstices between the detrital 
fragments. The remains of felspar are too much altered to allow 
the recognition of its specific character. 
No. 67. Cambrian conglomerate, west of Rhosson Rock. Much 
dirty quartz in a dark cement with fragments of a rock resem- 
bling 33. 
No. 68. This is a pebble from the Cambrian conglomerate, west 
of Rhosson Rock. It is a fairly typical quartz-schist with some 
interstitial felspathic material, and much of a nearly colourless 
mica. 
No. 69. This is a large pebble about 6 inches long by 5 inches 
wide, which has been brought from the Cambrian conglomerate 
of Whitesand Bay. It is a well waterworn pebble, and presents a 
well-marked schistosity. A thin section between crossed nicols 
presents a cryptocrystalline ground-mass enclosing a few crystal 
outlines of felspar. These are now almost entirely replaced by erys- 
talline quartz, which also occupies fissures. An occasional quartz 
erystal such as are characteristic of quartz-felsites is present. It is 
a schistose quartz-felsite and has great resemblance to No. 33. 
No. 70. A dark fragment from the Cambrian conglomerate near 
Castell. Shows in the section a very dark and dirty microcrystalline 
felsitic ground-mass with much quartz. It is a quartz-felsite. 
No. 71. A pebble from the Cambrian conglomerate, Ramsey 
Island. Shows a microcrystalline ground-mass which, with 
ordinary light, presents a brecciated structure; between crossed. 
nicols this is not visible. Quartz and felspar crystals are sparse, 
and the latter much altered. Secondary quartz in fissures. 
The view that the Cambrian conglomerate of St. David’s encloses 
much waterworn débris of the Dimetian is, I think, fully justified 
by the evidence now adduced from the examination of many slides 
of this rock, few of which have failed to afford evidence of the 
presence, not only of pebbles of a rock which under the microscope 
could not be distinguished from it, but also of its individual mineral 
constituents. The slides examined and described here are not 
selected ones, but have been taken as they were cut. The pecu- 
liar quartz of the Dimetian, thronged, as it is, with extremely 
