AKD PEBIDIAN) EOCKS IK CAEENAEVONSHIEE. 149 



predominate; a little further east some highly siliceous-looking 

 rocks, in which are disseminated throughout minute grains of quartz, 

 with spots also of highly vitreous quartz. We found in these 

 last rocks tolerably good indications of lamination and bedding; 

 and they are undoubtedly made up of ashy materials subsequently 

 altered, now a kind of quartz-felsite breccia*. 



In another road-side quarry, a little further on, the rocks assumed 

 a more schistose nature, but were otherwise much like the former. 

 The bedding here was clearly seen ; and the strike of the beds was 

 apparently from N.E. to S.W., dipping to the N.W. at an angle 

 of from 75° to 80°. 



This quarry would seem to be as near as possible about the centre 

 of the great mass coloured as intrusive felspar trap and quartz-por- 

 phyry. The same general characters seemed to prevail all along to 

 the fault on the east of the village of Tal-y-sarn. In the village 

 itself we came on to a dyke of truly intrusive porphyry, of a cha- 

 racter very unlike any we had previously met with in crossing the 

 great felsitic series. 



This, according to Mr. Hudleston, is an undoubted greenstone 

 porphyry, with large crystals of felspar in a dark-green subcrystal- 

 line base, containing also some hornblende and felspar disseminated, 

 and a good deal of epidotic (?) matter. This appeared to have been 

 intruded nearly at the junction of the Cambrians with the metamor- 

 phic series. 



The dyke itself is probably altogether in the Cambrian rocks ; and 

 some slight alterations visible in the Cambrian rocks near here and 

 at some other points must be attributed to contact with or proximity 

 to one or more of these dykes. 



On the above evidence we feel justified in placing the so-called 

 altered Cambrian of Moel Tryfaen and neighbourhood, and the whole 

 of the rocks in the large area coloured as intrusive felstone and 

 porphyry to the west and south-west of Moel Tryfaen and Tal-y-sarn 

 (excluding only some more recently intruded dykes), with the Pre- 

 Cambrian rocks ; and we shall moreover endeavour to show by 

 lithological evidence to which group in the Pre-Cambrian rocks they 

 appear to belong. Before doing so, however, it will be necessary to 

 see whether the characters made out to be typical of this district are 

 continuous in the area similarly coloured to the north-east, towards 

 and beyond Llyn Padarn, west of Llanberis. 



Llyn Padarn. 



As anticipated, we found that the rocks similarly coloured with 

 those to the west of Tal-y-sarn and Moel Tryfaen, along the north and 

 south shores of Llyn Padarn, were for the most part identical in 

 character with them, though, on the whole, rather more highly por- 

 phyritic. As numerous dykes of greenstone and melaphyre seemed 

 to have been intruded into the series here, it is possible that 



* See Mr. Davies's Note appended. 



