Vol. 56.] GEOLOGY OF NOKTHERN ANGLESEY. 2-33 



Acid Intrusions. — The acid dykes are rarer than the basic, 

 but have a greater average width. They are granophyres, quartz- 

 porphyries, and microgranites ; for field-description they may be 

 spoken of generally as felsites. A group of them is shown, on 

 the Geological Survey map, south of Llanfechell, coursing in the same 

 direction as the basic dykes, and, like the latter, terminating at the 

 boundary-thrust. I have observed one on the coast at Trwyu 

 Cemlyn, and others near Cemaes Pier and at a few localities inland. 

 In no instance, however, have I found them to be injected into any 

 other rocks than those of the Green Series. Even at Cemaes Pier, 

 though they strike at the neighbouring crush-conglomerate and 

 limestone-masses of Penrhyn, they fail to penetrate them. 



From evidence which will be stated below, I infer that these acid 

 dykes were injected before the basalts, but their difference of age 

 may be slight ; and seeing that, so far as I have examined them, 

 they yield no evidence of crushing, I regard them as 'post-movement' 

 intrusions. These rocks weather with a white crust, and have a 

 splintery fracture. They are pale grey, bluish, brown, or flesh- 

 coloured, with needles of a green hornblendic mineral, and some- 

 times contain corroded crystals of quartz. Some of the dykes are 

 beautifully spherulitic, as described by Prof. Blake. 1 



The following notes have also been supplied by Prof. Watts : — 



[N.A. 23 ; Bwchanan, south of Llanfechell.] A beautiful spheru- 

 litic felsite. Groundmass minutely granular or cryptocrystalline. 

 Fibrous chlorite occurs between the spherulites, and long needles, 

 once probably actinolite, run through the spherulites. Felspar- and 

 also hornblende-phenocrysts. Some epidote is present, and a pink, 

 pleochroic mineral occurs in some of the spherulites. 



[N. A. 127; ridge north of Coeden.] Granophyre. Large pheno- 

 crysts of quartz and felspar. Groundmass noncrystalline, made 

 up of moderately large grains of quartz, felspar, and micro- 

 pegmatite. 



[N.A. 109 ; Hafod-onen.] Granophyre. Orthoclase-phenocrysts, 

 often twinned on the Carlsbad plan. Many of these phenocrysts 

 show an internal structure resembling that of micropegmatite, and 

 most of them are encrusted and completed by a granophyric growth. 

 The structure of this rock is very beautiful, and it has been fully 

 described by Prof. Blake Qoc.cit.). See also p. 251. 



Composite Dykes. — The injection of a fissure with both acid 

 and basic material is well known from Prof. Judd's descriptions of 

 the composite dykes of Arran,' 2 and similar dykes are known to 

 occur in Ireland (County Down). 3 Hitherto, so far as I am aware, 

 they have not been recognized in Southern Britain, and their occur- 

 rence in another portion of the Irish Sea area should therefore 

 prove of some interest. 



My attention was first attracted to the association of acid and 



1 Rep. Brit. Assoc. 1888 (Bath) p. 410. 



2 Quart. Journ. Geol. Soc. vol. xlix (1893) p. 536. 



3 G. A. J. Cole, Trans. Roy. Dublin Soc. ser. 2, vol. v (1894) p. 239. 

 Q. J. G. S. No. 222. t 



