CAERNARVONSHIRE AND ANGLESEY. 305 
APPENDIX. 
Notes on the Microscoric Srructursu of some Rocks from CAERNARVON- 
SHIRE and ANGLESEY. By Prof. T, G. Bonney, M.A., F.R.S., 
Sec. G.S. 
I. Road-side near Port Dinorwig, Caernarvonshire (p. 296). 
Tuts rock has an ill-defined granular structure, not unlike that of 
a microcrystalline quartz-felsite ; but after careful study I am of 
opinion that it is an altered rock, probably once a fine felspathic 
mud. A good many minute grains of quartz can be distinguished. 
There is a fair quantity of opacite in parts, rather earthy-looking, 
and a pale yellowish mineral in filmy microliths, perhaps allied to 
vermiculite. 
Il. From the Eifl Range (p. 299). 
(a) From the Hifl range, south-east side.—Structure cryptocrys- 
talline, the felspathic component being rather decomposed and con- 
taining many microliths, probably of secondary origin. Some opacite, 
a few larger grains of quartz and (probably) of decomposed felspar. 
I believe the rock to be a quartz-felsite. 
(6) Eifl range, east side.—Structure cryptocrystalline ; possibly 
a little glass even yet remains ; indications of flow-structure ; several 
erains of quartz and one or two of felspar. General character similar 
to the quartz-felsites described on p. 317, and like them an old rhyo- 
lite. As happens sometimes with them, the rock has been consider- 
ably crushed ; the fractures are filled in by a minutely crystalline 
mineral, showing rather bright colours with crossed Nicols. 
IIL. On coast north-west of Hifl range (p. 299). 
Structure crystalline, but rather finely so, sometimes inclining to 
porphyritic. Chiefly consists of felspar and quartz, much of the 
latter interstitial with the former rather than in separate grains ; 
also mica, iron peroxide (possibly some ilmenite), and a few micro- 
liths, possibly apatite. The felspar is rather decomposed; but a 
plagioclase as well as orthoclase can be recognized. Part of the 
mica exhibits the characteristics of biotite; but much of it is replaced 
by a green dichroic mineral not unlike chlorite, which, indeed, it pos- 
sibly is; but I think most is only one of the green magnesia-iron 
silicates which often replace biotite. I have no doubt the rock is 
igneous, but in its present condition, and without chemical analysis, 
hesitate whether to class it with the quartziferous mica-syenites or 
mica-diorites, or the fine-grained granites (the granite-porphyry 
of many petrologists). 
IV. Boduan mountain, south side (p. 299). 
Structure rather microcrystalline than crystalline; but as the 
