GEOLOGY OF ANGLESEY. 



219 



over and plunge directly towards the quartz at the very junction. 

 A little further to the north, above the farm, the evidence of dis- 

 cordance is, if possible, still clearer. In the quarry is a good section 

 of a greenish slaty rock, with the beds striking directly at the quartz 

 forming the ridge at a distance of about 20 yards. 



It is thus evident that the slaty series is faulted against the quartz 

 group of Bodafon Mountain. 



b. Gneissic Series. 



1. Bodafon Mountain. — This mass cannot be brought into direct 

 stratigraphical relation with the gneiss group, for the reasons just 

 explained ; but the lithological resemblance of this rock to the 

 quartz-schist of Gwalchmai and Mynydd baen is so close as to 

 render the correlation highly probable. In hand specimens some of 

 the varieties are undistinguishable. Prof. Bonney, who was not 

 aware of my views when working on the specimens, groups a typical 

 variety (Xote 29, p. 233) from the summit of the mountain with 

 the Gwalchmai schists. The rock of the western ridge, which is a 

 little more schistose, is also closely represented in the south-western 

 localities. The inlier, surrounded by newer Palaeozoic rocks, which 

 lies to the south-east is precisely similar to the eastern part of the 

 hill. 



2. Section between Liang wyllog and Llanerchymedd. — The suc- 

 cession between these points is similar to that of the coast-section 

 between Ty Croes and Aberffraw ; but the granitoidite is wanting 

 in the latter, and the dip is reversed, so that the two sections repre- 

 sent the two sides of a broken anticline. 



The granitoidite occupies a large area south of Llanerchymedd. 

 Bound Coedana is a grey variety, rather small-grained. To the south, 

 at Bhydgoch, it is much coarser, consisting of fragments of pink 

 felspar in a matrix of smaller bits of quartz, felspar, and chlorite. 

 The older geologists would have called it a porphyritic granite. 



The dark schist is exposed about half a mile further to the south;, 

 at Glanrhyd, being the band of " greenstone " of the Survey. Prof. 

 Bamsay originally suspected the metamorphic origin of this rock ; 

 and, after microscopic examination,- Prof. Bonney is disposed to 

 consider it a hornblendic gneiss, containing felspar, epidote, horn- 

 blende, and some quartz, the felspar being decomposed, and the 

 whole a good deal " messed." Any obscurity arising from such an 

 altered rock is removed if we follow the strike to the north-east. 

 To the south-west of Graig llwyd there is a good exposure of 

 hornblendic gneiss interstratified with granitoidite. The gneiss 

 sometimes passes into hornblende-schist. The hornblende is well 

 crystallized ; and the alteration is very slight. 



The grey gneiss is well seen about two furlongs south of Glanrhyd, 

 in a quarry north of the stream. The dip is high, to the north-west ; 

 and the rock is of the ordinary type. 



rTo rock is seen on the road for two or three furlongs to the 

 south ; but north of Llangwyllog church there is a slight exposure 

 of hatteflinta too small to display dip. There is room for the 



