CAMBRIAN SERIES IN N.W. CAERNARVONSHIRE. 313 
of irregular outline, as though either imperfectly formed or distorted 
by tension. I am not able to assert positively that any truly glassy 
base still remains, but think this is the case, and feel certain that 1 
Was once present. Subangular grains of quartz, with occasional 
partial inclusion of the base, and small cavities are frequent, also 
felspar crystals rather decomposed, not seldom including microliths : 
these are orthoclase with a little plagioclase. 
(2) From near a Tunnel south of Cwm-y-Glo Station.—Colour a 
dull grey; example of the ordinary type. This rock has a general 
similarity to the last, except that the fluidal structure is less mark~- 
edly parallel and less distinct. Microliths are more numerous and 
larger, and the rock generally has a less glassy aspect, part of the 
slide showing a distinctly cryptocrystalline structure. The rock is 
traversed by cracks, which haye been again cemented; they were 
doubtless caused by the pressure which produced the cleavage of the 
district. 
(3) Ratlway-cutting near the top of the Mass.—A pale pinkish rock 
showing a rude cleavage. A general similarity to the last, but the 
fluidal structure still less distinct, and the rock yet more crushed, 
so that parts of the slide resemble a breccia cemented by a filmy 
pale greenish-yellow mineral. The crystals of quartz* and felspar 
are occasionally crushed in situ and cemented again. This might 
easily be mistaken for a clastic rock, but with crossing Nicols its 
homogeneous character is readily discovered. 
(4) Highest Felsite seen on west side of Llyn*Padarn.—This rock is 
roughly cleaved, and has quite a schistose aspect. Closer examina- 
tion, however, shows that in about ten yards it passes by insensible 
changes into the last. This also accords with its aspect under the 
microscope, where a sort of banded structure and numerous sub- 
angular grains of quartz and felspar suggest a stratified origin. But 
a closer examination with higher powers shows that it is really an 
igneous rock, which has evidently been greatly crushed in situ. The 
fibrous cementing mineral (with crossed Nicols) shows rather brilliant 
golden and blue colours. 
(5) Two specimens from near Brithdirt (one from the cliff over- 
looking the Caernaryon road, the other from a quarry west of the 
farm). These, from the northern massif, are of a purple-red colour, 
like the rock near Rhos Tryfaen. ‘The slides differ but little one 
from another, and only as varieties from (2). ‘There is a little more 
opacite, and the imbedded crystals are rather more numerous. Ono 
or two are probably altered biotite, and some microliths are possibly 
apatite. 
(6) A specimen from the Roadside above the * Antelope’ Inn, near 
the Menai Bridge.—The general character of this is similar to that of 
the last, but the slide is rather more stained by ferrite. The structure 
* The cavities in the quartz are a little larger in this specimen. It also con- 
tains two or three crystals of an altered mica, and there seems to be a little 
calcite in a crack. 
+ A specimen from the same district is described by me, vol. xxxiv. p. 145, 
The description may also serve for these specimens. 
