320 



of Aberdaron in the Lleyn peninsula is fairly rich in a strongly 

 pleochroic mica. Crystals and grains of zircon are common and when 

 they occur in the mica are surrounded by a strongly pleochroic 

 border. Apatite is also present. The rock from Llanfaelog, Anglesea, 

 exhibits in certain localities most interesting crush-phenomena, as 

 Professor BONNEY has pointed out. These phenomena are similar in 

 all essential respects to those exhibited by the Porthalla " gneiss ; " 

 there is the same white mica occurring partly in ragged scales and 

 partly in the crypto-crystalline condition, there is the same micro- 

 crystalline mosaic of colourless grains, and the same evidence of 

 bending and fracture in the larger constituents. 



It will be seen from what has been stated above that the granitic 

 rocks of Wales, so far as they have been examined, belong to one type 

 and that this type is very distinct from that of the granites of the 

 West of England. It is characterized mainly by the relation of the 

 quartz and the felspar; the felspar occasionally assuming the role of 

 ground-mass and occasionally occurring as a constituent of micro- 

 pegmatite. There is no original white mica and black mica is 

 comparatively rare. In some cases the rock is almost entirely composed 

 of quartz and felspar. In many cases the quartz is rendered turbid by 

 numerous inclusions arranged in planes which often traverse several 

 grains and exhibit a rude parallelism in their mode of arrangement. 

 This is probably a secondary phenomenon due to, or connected with, 

 dynamic metamorphism. In some cases, as at Llanfaelog, the rocks have 

 been foliated and white mica has been developed along wavy planes. 

 This also is in all probability a phenomenon of dynamic metamorphism. 

 The rocks in question resemble granites and granophyres of undoubtedly 

 igneous origin. With regard to their age there is some difference of 

 opinion. Pebbles of the rocks occur, however, in conglomerates which 

 certainly belong to the early part of the Ordovician and probably to the 

 Cambrian period/ 1 ) 



Midland Counties. Rocks of the type just described occur at 

 Ercal Hill and on the W. and S. W. slopes of Primrose Hill.< 2 > The 

 Ercal rock contains only very small traces of any ferro-magnesian 

 constituent. It is essentially composed of quartz and felspar. These 

 two minerals are related to each other as in the St. David's, Twt Hill 

 and Tan-y-grisiau rocks. The quartz is not unfrequently seen to be 

 distinctly idiomorphic with respect to the felspar. Some portions of 

 the mass exhibit the finest possible illustrations of micro-pegmatitic 

 structures. Certain sections of this rock and of a corresponding rock 

 from Twt Hill in Caernarvonshire bear a very close resemblance to 

 specimens of a "granophyre" from Brinzio near Varese. (3) 



k 



(1) Q.J.G.S., Vol. XL. (1884), p. 206. 



(2) See Drs. CALLAWAY and BONNEY, Vol. XXXV. (1879), p. 643. 



(3) HABADA, N.J., Beilage Band II., 1883, p. 1. 



