584 PROF. T. G. BONNEY ON SOME ROCK- 
from the railway-cutting near Llanfaelog *, consisting of orthoclase, 
microcline, and oligoclase or albite, with quartz in rather irregular 
granules. ‘The last, as is usual in these rocks, contains numerous 
very minute cavities, generally empty, but now and then showing a 
tiny moving bubble, together with other enclosures, apparently 
filmy microliths. The felspar also contains many of the latter. 
There are a few groups of vermiculites of a chloritic mineral. 
11i (“ School,’ west of Cwaen-hen).—A green schist, con- 
stituents very minute. Microscopically examined, this is seen 
to consist of extremely minute granules of quartz, giving to parts 
of the slide a chalcedonic aspect, and minute films of a very pale- 
green micaceous mineral, with a darkish dust, which may be provi- 
sionally called ferrite. The two principal minerals are arranged in 
bands in which the one or the other predominates, and these exhibit 
beautiful corrugations. I should conjecture, from their appearance, 
that they had been produced subsequently to the segregation of 
the minerals. The micaceous constituent may be of more than one 
species, some scales seeming to be more like a chlorite, others more 
resembling a true mica. With crossed nicols, and the flakes placed 
at an angle of about 45° with the vibration-planes, fairly bright 
colours are given. It is, in short, the mineral or mineral group so 
frequently found in these grey-green or olive-green schists of 
Anglesey and elsewhere; and though this rock is rather more 
minutely constituted than most of my own specimens, I should 
group it with the schists of Holyhead Island and the adjoining part 
of Anglesey, and with those of the Menai district. 
112 (Brwynog, p. 571).—A greyish-green schistose rock, whether 
a satiny slate or a true schist it is not easy to say. The difficulty 
is not wholly removed by microscopic examination. The consti- 
tuents are extremely minute, but appear, on examination with a 
high power, to be fairly well defined, and to differ from those last 
described only in size; though one or two parts of the slide do not 
look so highly altered. I therefore feel doubtful, as the rock has eyi- 
dently been much compressed, whether to regard it as a member of 
the last group, which I think we may safely consider Pre-Cambrian, 
or as one of the older Palzozoic rocks exceptionally altered. 
113 (N.W. of Byttia, Amiwch, p. 568).—A pale-green slaty or 
schistose fragment, in a gritty matrix. Under the microscope the 
rock appears to be less highly altered than the last: it consists of 
quartz, some probably in minute fragments, a brown argillaceous 
material, and a fair amount of a greenish chloritic mineral in films 
and also in streaks. I think, from the general aspect, that the frag- 
ment was nearly in its present condition when broken off from the 
parent rock, and that it is rather a schistose slate than a true schist. 
114 (Tywyn ridge, p. 578).—A rounded pebble, about 2 inch in 
longer diameter, in a greyish-green matrix, which exhibits a distinct 
cleavage. Under the microscope the pebble is seen to be one of the 
coarse granitoid gneisses, upon which, as I have so often described 
* Quart. Journ. Geol. Soe. vol. xl. p. 201. 
