Edward Greenly — Sillimanite Gneisses in Anglesey. 495 



of gneissose rocks is associated with the granite. Some of these, as 

 about Craig yr Allor, are banded hornblendic gneisses and foliated 

 diorites, whose strong resemblance to much of the Archaean gneiss 

 of the North-west Highlands of Scotland has been pointed out by 

 Sir A. Geikie. 



But in the neighbourhood of the Holyhead Road the coarse 

 granites can be seen in contact with quartzose and micaceous 

 schists. The junctions are very clearly intrusive, the granite often 

 passing quite abruptly across the foliation planes of the schists, 

 and containing many included fragments of them. More frequently, 

 however, the relations are those of sills, the granite lying between 

 parallel folia, or passing across them at very gentle angles. 

 Moreover, the schists contain, close to their junction with the 

 granite, thin seams of quartz, which are full of fine needles of 

 sillimanite, forming thoroughly characteristic " Faserkiesel." The 

 granite shows no sign of chilled edges, being, so far as I have 

 yet observed, quite as coarse at the junction as elsewhere. The 

 phenomena are, in fact, very closely allied to those described by 

 Mr. Barrow in the S.E. Highlauds of Scotland, and also in a paper 

 by Mr. J. Home and myself, read before the Geological Society of 

 London, on June 10 last, on " Foliated Granites and their Relation 

 to Crystalline Schists in Eastern Sutherland." Fine granulitic 

 quartzose schists, containing idiomorphic magnetite, also occur. 



Although sillimanite occurs in certain seams in the schists close 

 to the Holyhead Road, it does not appear to be abundant; and the 

 crystalline texture of the schists is, on the whole, rather fine. But 

 a little distance along the strike, to the north-east, the whole series 

 of phenomena become much nioi*e pronounced. Coarse, wavy 

 gneisses appear in which sillimanite is abundantly developed, 

 knots and irregular seams of hard, pale sea-green " Faserkiesel," 

 full of fine, lustrous needles, being conspicuous on their rugged, 

 weathered surfaces. Innumerable granite bands and len tides are 

 interfelted with these gneisses, exactly as in Eastern Sutherland, 1 

 and synthetic gneisses of the same type occur all along the strike 

 to the neighbourhood of Llanerchymedd. The beautiful biotite 

 gneisses of the inlier at Tafarn-y-botel (whose resemblance to rocks 

 of the Scottish Highlands has been noticed by Mr. Blake) are 

 almost identical in character with those of Kinbrace, in Sutherland, 

 a region of excessive metamorphism and intimate granitic injection. 

 Sillimanite is of frequent occurrence, being sometimes included 

 in the micas, which are large and well developed. The thin 

 granitic seams contain large and beautifully striated oligoclase. 



The whole series of phenomena closely correspond with those 

 of Eastern Sutherland, and I have little doubt that they are of 

 essentially the same nature. Certain rock types occur, however, 

 which have not yet been observed in the latter country. 



1 In the recognition of these peculiar modes of granitic injection -will, 

 I think, be found a reconciliation of the conflicting views that have been 

 held as to the intrusive or " interbedded " character of the granite. I should like 

 also to draw attention to the very vivid and faithful description of these phenomena 

 by Sir A. C. Ramsay (" Geol. North Wales," ed. 2, p. 243). 



