666 DR. C. CALLAWAY ON THE [Nov. 1902, 
quoted in Geikie’s ‘ Text-book of Geology, ' and an analysis of an 
American obsidian furnished to me by Mr. Holland. 
GranNuLAR Fetsite, Ruyouitic OBsIDIAN, RuYOLITE, 
Y Graig, Gaerwen, Medicine Lake. Euganean Hills. 
Anglesey. (U.S.A, Geol. Surv.) (Vom Rath.) 
Per cent. Per cent. Per cent. 
DMIGH. Datmatarece mere 73°48 73°51 76:03 
Titanic oxide...... 0:29 ae ue 
PANUIAITIA boc cls/eiee 14-79 14-42 13°32 
Ferric oxide ...... 0:03 0:46 poe 
Ferrous oxide ... 1:04 1:49 1:74 
Manganous oxide. trace trace ate 
Piimnes saceenee-cace ns 0:53 1:26 0°85 
Magnesia ......... 0°43 0°33 0°30 
IP otasiiat.c ect abeos 4°24 4:29 3°83 
Oda ge teascneseee nen 4°40 4:03 5°29 
Sulpbur-trioxide. 0-03 
Phosphorus-pent- 
ORIG paces 0-02 0:04 
Combined water. O81 0°40 uae 
Loss... 0:32 
100°09 100°23 101°68 
It will be seen from these analyses that the Anglesey felsite 
does not differ materially from Vom Rath’s rhyolite, and is almost 
identical with the American obsidian. 
The felsite, in the first stage of crushing, is converted into the rock 
originally known in Anglesey as ‘hiilleflinta.’? It then passes 
progressively into quartzose and micaceous schists, the phenomena 
being similar to those so well seen at Y Graig,* in the eastern district. 
The least modified form of the felsite as it occurs in Central 
Anglesey is the ‘ hiilleflinta’ of Hicks. The following slide (1) is from 
his typical locality (Llanfaelog): it has undergone greater change 
than the type-specimen from Y Graig. In ordinary light, it is 
colourless and nearly transparent, irregular clear patches being 
surrounded by slightly dingy material, and some opacite in small 
patches is present. The clear spaces prove to be mainly quartz. 
The dingy patches are largely composed of minute microliths of 
white mica lying confusedly intermingled. Some felspar also 
appears to be present, but I cannot clearly detect crystalline forms 
amid the tangle of mica. Prof. Bonney writes of it, ‘I think it 
is a devitrified rhyolite (or dacite), with sericite forming from the 
felspathic constituents.’ 
A specimen of the ‘ hilleflinta’ (389) from Porth Nobla shows 
more differentiation. The groundmass consists of water-clear 
granules of various sizes and definite outline. Some of them are 
angular, and are evidently the result of fracture, but most of them 
Ist ed. (1882) p. 137. . 
2 I made some comments on this rock ina previous paper, Quart. Journ. 
Geol. Soe. vol. liii (1897) p. 350. 
3 Ibid. p. 351. 
