OF SOME ROCKS FROM PEMBROKESHIRE. 5a 
compact fragments which are associated with it are quite inert 
between crossed nicols, and present no definite structure. 
No. 78. RochCastle. A dull greyish-white rock. In structure 
almost identical with No. 73. 
No. 79. Roch Castle. A grey compact rock with visible nests 
of quartz. A microcrystalline ground-mass with much felspathic 
dust distributed in patches. Contains very numerous ovoid nests of 
quartz, which in some brecciated portions appear in ordinary light 
to resemble crystals of felspar. Between crossed nicols this 
resemblance disappears. The halleflintas from Roch Castle now 
examined vary but little from those previously described. They are 
brecciated; some contain undoubted angular fragments of quartz 
and of rocks with indefinite structure; one contains fragments of 
what may have been a felspathic rock, or of felspathic débris in 
which the crystals of felspars, if they were such, are now replaced 
by quartz. ‘The whole of these rocks are traversed by numerous 
fissures of secondary quartz, and crystalline nests of the same. 
They are probably indurated and altered bedded ashes. 
No. 80. Plumstone Mountain. This rock shows a very fine- 
grained felsitic ground-mass crowded with fragments of quartz, 
evidently detrital. A quartzo-felspathic ash. 
No. 81. Plumstone Mountain. A _ yellowish-grey compact 
felspathic rock with numerous black spots. A thin section shows 
that it is entirely felspathic; it consists of large felspar crystals 
in a ground-mass of smaller columnar crystals of the same. They 
are both orthoclase and plagioclase ; many of the porphyritic crystals 
are replaced, or partly so, by a dark-yellowish mineral which de- 
polarizes light, and it is minutely dispersed throughout the ground- 
mass. <A porphyritic felsite. 
No. 82, from Goultrop, south side of St. Bride’s Bay, is a rock 
which, macroscopically, appears to consist of felspar and quartz and 
to be distinctly foliated. 
In thin section it presents distinct bands of crystalline quartz 
remarkably clear and free from inclusions, with an occasional large 
erystal of microcline or orthoclase, which are tolerably fresh. These 
alternate with bands of mingled felspars and quartz, the former of 
which sometimes appear as rounded or irregular grains. They 
seem to be much altered, between crossed nicols depolarizing light 
only in minute points and patches. A few small crystals or 
lamine of a green dichroic mica are sparsely scattered, while a 
green structureless mineral with some opacite occupies fissures. 
No. 83, also from Goultrop, is a similar rock, but of a greener 
colour and not so markedly of foliated aspect; a section shows that 
the constituents are not arranged in such distinct bands as in No. 82. 
The rounded and irregular felspar grains are more numerous, and 
some, as in the preceding, show traces of twin striation. A green 
dichroie mineral occurs in wavy bands and is tolerably abundant. 
It is traversed by veins of secondary quartz. 
Oe Ges. Non Lao: QP 
