148 H. HICKS OX SOME PEE-CAMBRIA.F (dIMETIAN 



of the hard bauds frequently found interstratified between the slates, 

 and which, in some cases, shows a semicrystalline texture. 



The dip of the beds all along seemed to be nearly vertical, though 

 the bedding could not be satisfactorily made out in the metamorphic 

 rock. From evidence derived from other sections along the same 

 line, we were inclined to think that the dip of the metamorphic beds 

 was towards the N.W., whilst the dip in the Cambrian slates was to 

 the S.E., and hence that the two series were discordant to one another. 



The crest of the mountain consists of an unaltered conglomerate, 

 in which the pebbles are for the most part made up of masses of 

 quartz, quartz-felsites, porphyritic rocks, and schists, each appearing 

 to resemble the rocks in situ near. The metamorphic rocks crop ou(t 

 immediately under the conglomerate on the mountain-side ; an<<l 

 if our evidence of the dip in these metamorphic rocks is true, then 

 they must be perfectly unconformable to the Cambrians above. 



We did not succeed in touching the conglomerate in the tunnel [ 

 but Prof. Hughes informs me that he found masses of it in the 

 debris at the mouth when the excavation of the central portion of 

 the tunnel was carried on. All we can state in regard to the 

 evidence at this point in the tunnel is, that we noticed a sudden 

 transition from highly metamorphic rocks to unaltered Cambrian 

 beds, the metamorphic rocks being, if possible, in a higher state of 

 alteration near the junction than at a considerable distance deeper 

 in the series, this probably being due to the presence of dykes of an 

 intrusive rock which we found near. 



The evidence at Moel Tryfaen is certainly most favourable to the 

 view that the rocks coloured as altered Cambrian are entirely un- 

 conformable to the Cambrians which rest upon them, and that the 

 metamorphic action does not extend beyond the line of the conglo- 

 merates, which here, as in all other Welsh areas, strongly define the 

 base of the Cambrians. 



Following the direction of strike of the conglomerates to the south- 

 west of Moel Tryfaen, they may be traced to Mynydd y Cilgwyn ; and 

 there again they rest in an unaltered state on highly altered beds, 

 similar in character to the metamorphic rocks on the west of Moel 

 Tryfaen. 



Pen-y-groes to Tal-y-sarn. 



The evidence above given we were partly in expectation of finding 

 from Prof. Hughes's statement, and from the colours on the Geologi- 

 cal-Survey Map indicating an altered series. We also found, how- 

 ever, that the great patch coloured as intrusive porphyry to the 

 west and south-west of these altered beds, and to contact with 

 which it was supposed this change was due, consists also almost en- 

 tirely of similar rocks to those above described, and moreover that 

 the great mass is really made up almost entirely of schistose rocks, 

 breccias, and other altered beds. The general character of the 

 rocks in crossing from Pen-y-groes on the west side to Tal-y-sarn 

 on the east may be described as follows : — About Pen-y-groes fel- 

 stones and quartz-felsites, of either a dark bluish or a pinkish tint 



