part 3} iowee paleozoic of akthog-dolgellet. 265 



a large area. 1 This stands in strong contrast with the relations 

 obtaining farther north-west in the Lleyn peninsula, where there 

 is a rapid transgression of the Arenig Grit across all the members 

 of the Cambrian System, and the discordance is so great that the 

 unconformity can be clearly seen in cliff-sections. 3 



Through the region farther east and nearer Dolgelle} r the 

 relationship between the Ordovician and the Cambrian Systems 

 must remain uncertain for the present, owing to the great pre- 

 valence of faults and of intrusions. Throughout this part of the 

 district the Cambrian and Ordovician rocks are invariably separated 

 one from the other by diabase-intrusions. Mr. Lake & Prof. 

 Reynolds 3 have shown that eastwards along Mynydd-y-Gader there 

 is a progressive increase in the amount of the Cambrian strata 

 that are cut out by the intrusion, so that, first the Tremadoc, and 

 later the Dolgelley Beds, disappear ; while we find that still farther 

 east, in the Aran Valley, the basal Ordovician beds rest in close 

 proximity to beds well down in the Ffestiniog Group. The absence 

 of the higher Cambrian beds is undoubtedly due in part to the 

 presence of a strike-fault — the Mynydd-y-Gader North Fault (see 

 p. 312) ; we find that all the strike-faults in this district increase 

 in throw to the eastward (p. 313), so that absence of a greater 

 thickness of Cambrian rocks to the east may possibly be due 

 entirely to faulting. At the same time, one of us (A. K. W.), 

 working in the country north-east of Dolgelley, finds some evidence 

 pointing to the presence of a considerable unconformity in that 

 direction. Thus the absence of Tremadoc Beds in the country 

 south-east of Dolgelley may be due in part to an unconformity, 

 and only in part to faulting. 



(B) Ordovician. 



(1) Arenig: The Basement Series. 



The Ordovician strata begin with a series of arenaceous beds, 

 grits and sandy flags, resting on the underlying Tremadoc Slates 

 without any evidence of transition, the junction between the two 

 groups of strata being quite sharp. 



The Basement Series is perhaps best seen on the slopes south of 

 Llyn Wylfa, while the junction with the Tremadoc Slates is clearl} r 

 displayed south-east of the eastern end of the lake. The lowest 

 members of the group here consist of well-stratified grits in beds 

 2 to 4 feet thick. The dip of the grits is about 40° south-south- 

 eastwards ; but, as the dip of the Tremadoc Slates cannot be 

 made out owing to cleavage, it is impossible to say whether there 

 is any actual discordance. 



The lowest grits are generally dark grey, highly felspathic, and 

 hence rather ashy-looking, though in places they become bluer,. 



1 W. G. Fearnsi'des, Q. J.G.S. vol. lxi (1905) p. 618. 



2 T. C. Nicholas, ibid. vol. lxxi (1915) p. 139. 



3 Ibid. vol. lii (1896) p. 511. 



