OF ARVONIAN ROCKS FROM PEMBROKESHIRE. 293 
with more numerous crystals and fragments of orthoclase felspar. 
In places the felsitic matter has assumed a spherular structure, in- 
distinctly radial, and without a peripheral definition, but passing into 
the microcrystalline ground-mass; this is best observed between 
crossed Nicols. 
This absence of definition of the spherular limits of the felsitic radii 
observable here, and also in some other Arvonian rocks*, though not 
unknown in some trachytic examples, I am disposed to regard as 
evidence of a subsequent rearrangement of part of the felsitic con- 
stituent. 
6. Near Church Schools, St. David’s (p. 289).—Consists of an 
intimate microcrystalline association of felspathic substance with 
quartz-grains. The felspathic ingredient constitutes by far the larger 
proportion of the ground-mass, and has a great tendency to develop 
into small columnar crystals, which, though possessing an indefinite 
outline, are yet distinctly discernible. Here and there appear a 
crystal or group of crystals of orthoclase (as evidenced by the 
Carlsbad twins) distributed porphyritically. A similar radial-sphe- 
rular arrangement of part of the felsitic substance to that in No. 5 
is shown, but, though more distinctly developed, is much less fre- 
quent. Viridite in patches pervades the whole rock. 
7. Road east of Trepewet, St. David’s (p. 289).—A compact rock 
with a very splintery fracture, and resembling hornstone. 
Under an inch objective the ground-mass of this rock is seen to 
be exceedingly fine-grained, much of it having no action on polarized 
light, but it is variegated by patches having a microcrystalline struc- 
ture and a strongly depolarizing action. Numerous angular and 
subangular grains of quartz are dispersed throughout. 
This rock has a considerable resemblance to the hilleflintas; and 
Tam inclined to refer it to that type, but I have not yet met with a 
Swedish example presenting so decided an illustration of detrital 
origin. 
8. Llanhowel Quarry, in field north of Church (p. 290).—A light 
yellowish-grey rock, very compact, with some disseminated mica. 
Microscopical examination reveals that it consists almost exclu- 
sively of felspar and quartz. ‘The felspar constituent of the ground- 
mass is largely in the form of minute slender prisms, its orthoclase 
affinity being here and there evidenced by the Carlsbad twinning ; 
whilst that of the larger porphyritic crystals is a plagioclase, as 
indicated by the faintly coloured strive observable between crossed 
Nicols ; this felspar appears to be much altered. The remainder of 
the felspathic constituent is more indefinite, and with the slender 
prisms and the quartz-grains forms the fine-grained ground-mass, 
Some of the quartz appears as large, more or less rounded crystalline 
grains, similar to those familiar to us in the quartz-felsites. Numer- 
ous crevices are occupied by a colourless, diverging, fibrous mineral, 
which is probably an epidote. 
* Quart. Journ. Geol. Soe, vol. xxxiy. p. 164. These are Nos. 1 & 2 described 
under and then supposed to belong to the Dimetian, but now associated with 
the Aryonian on the grounds mentioned in the present paper.—H. H, 
J.G.8. No, 138, x 
