DR. HICKS IN ANGLESEY AND N.W. CAERNARVONSHIRE. 201 
eneisses as I have examined. The quartzes are rather full of minute 
cavities, often irregular in form; some of these appear to be empty, 
and look as if coated by a dusky pigment; others, however, contain 
small moving bubbles. The felspars are rather decomposed, and 
contain minute “flecks” of a pale filmy mineral; but probably 
orthoclase and a plagioclase with rather small extinction-angles are 
present. In these and in all other respects the rock agrees with 
specimens already described. 
(2) Railway-cutting near Llanfaelog, to show Change of Colour from 
Decomposition.—The less altered parts of the specimen show this to be 
another variety of granitoidite, having a little more of a dark mica- 
ceous constituent and being a shade more definitely crystalline. The 
rock has evidently been greatly crushed 2m situ, the felspar being in 
places absolutely pulverized, and the quartz grains broken through 
again and again. Along these fissures, and in certain cases laterally 
also, water has evidently percolated, converting the crushed felspar 
into kaolin, and depositing stains of iron peroxide. 
(3) Railway-cutiong near Llanfaelog—tThis rock is identical with 
one which I have already described (Quart. Journ. Geol. Soe. vol. 
xxxv. p. 305, &c.). Were no controversy involved, it would, I 
think, be accepted without hesitation as a gneiss, so closely does it 
resemble specimens from the oldest Archzan gneisses in various 
parts of the world. ‘The quartzes contain, as in the preceding 
slides, the same minute enclosures, among which are small fluid 
cavities with tiny moving bubbles. There is, as usual, a closely 
twinned plagioclastic felspar, and, I think, a little microcline. Mica 
is more abundant than in the other two specimens. Most of it has 
heen a black mica, but it is now much altered, having been replaced 
by a viridite or a minute chloritic mineral, by flecks of a colourless 
mica like paragonite, and by others that are rendered opaque by a 
black staining. This rock gives indications of crushing, but not to 
the extent of the others. 
(4) and (5). Sections cut from the Pebbles in a specimen of Conglo- 
merate from the north end of Liyn-faelog.—The one pebble is 1Z inch, 
the other about 27 inch in longest diameter, both being well 
rolled. (4) is afragment of rock macroscopically and microscopically 
in all important respects identical with (3), while (5) is more nearly 
allied to (1) and (2). Even the quartzes, with their numerous 
minute inclusions and fluid-cavities, correspond ; and there are the 
indications of local crushing already described. The slide cut from 
(4) also includes a portion of the matrix. This is composed of 
angular and subangular fragments, mingled with, and sometimes 
imbedded in, a black dust, which probably to a large extent consists 
of an iron oxide. These fragments are mostly quartz and felspar ; 
the former exactly resemble the quartzes in the three preceding 
specimens ; the latter are probably derived from similar rocks. <A 
micaceous mineral, exactly like that described in (8), is also present. 
With these occur some fragments of a different nature; one is a 
slightly felspathic quartz-grit, with a little of a minute chloritic 
