GEOLOGY OF ANGLESEY. 



225' 



course, frequently appears to pass under the felspathic series. Such 

 facts suggest great caution in trusting to dips in areas where the 

 faulting is not clearly seen. The lithology here, as generally in 

 Anglesey, is the true guide. The Precambrian chiefly consists of 

 felspathic breccia. The fragments are usually of similar composition 

 to the matrix ; but some are of quartzite. The felspathic fragments 

 weather out sharply where the cliffs are washed by the spray. On 

 the whole this rock strongly suggests the breccia near Nun's Well, 

 St. David's. Good exposures are seen at Yynys yfydllyn, Porth yr 

 hwch, and west of Pant yr Eglwys. The thickness, owing to such 

 frequent faulted repetitions, probably is not great. 



The section at Pen bryn'r Eglwys is of much interest. In this 

 great headland, which juts out at the north-west corner of Anglesey, 

 are rocks which are described in the section of the Survey Map as- 

 "gneissic rocks pierced by granite veins ;" and these are represented 

 as passing on the south into " metamorphic foliated rocks." The 

 latter are the felspathic shales and breccias already noticed. The 

 description of the former also requires modification. 



The breccias pass up into the southern slope of the headland, 

 where they are succeeded by a band of quartzite. This is over- 

 lain by a considerable thickness of greenish felspathic beds of ordinary 

 Pebidian type and not much altered. The highest ridge of the 

 promontory consists of this rock. Eollowing these strata across the 

 strike to the north, they are seen gradually to change, the rock 

 putting on a glazed aspect, and mica appearing in small quantities 

 on the lamination-planes. This altered material soon passes into 

 thoroughly foliated gneiss and granitoidite. 



It was rather startling to find true metamorphic rocks associated 

 with comparatively unaltered felspathic beds of the newer series. 

 The granitoidite is very similar to that of the older gneiss ; and my first 

 impulse was to refer it to that group. In such a shattered district 

 as Anglesey, especially in such an area as its north-west corner, a 

 faulting-up of the older series seemed not unlikely. But, after careful 

 examination, I found it impossible to accept this supposition. The 

 transition between the unaltered rock and the schists is gradual and 

 complete. A close comparison furthermore revealed lithological dis- 

 tinctions between the gneiss of the two series, the most important 

 of which is a difference in the colour and lustre of the mica. 



The cause of metamorphism was not apparent. The "granite 

 veins " of the Survey are probably the granitoid bands in the gneiss. 

 Some quartz veins running across the cliffs may have suggested the 

 intrusion of granite. As this metamorphosed mass occurs in the 

 neighbourhood of the Mynydd-Mechell schists, the alteration in both 

 cases may be due to the same general cause. 



YI. Westeen Aeea. 



All the rocks south of the Porth-y-defaid fault and west of the 

 Palaeozoic area belong to the older series. 



1. The Mainland. — This district is chiefly occupied by green 

 chloritic schists (Note 37, p. 234) similar to the rocks described from 

 theMenai anticline, Aberff raw, Craig yr allor, and east of Paris Moun- 



