part 3] LOWER PALAEOZOIC OP ARTHOG-DOLGELLEY. 261 



little by little lighter and more grey-blue as the beds pass quite 

 gradually into the overlying Tremadoc Slates. This transition 

 is even more gradual than that from the Ffestiniog Beds to the 

 Dolgelley Beds. 



The characteristic fossil, Parabolina spinulosa, was obtained 

 near Maes Angharad by Mr. Lake & Prof. Reynolds. 1 The same 

 authors record other fossil localities on the northern flanks of 

 Mynydd-y-Gader, where P. spinulosa Dalmann, Orthis lenticu- 

 lar is Wahl, etc. were obtained at Bryn-rhug. The O.-h-nticularis 

 Bind has also been discovered in the Ceunant stream. 



The whole group is much thinner than the Ffestiniog Group, 

 having a thickness of about 600 feet. 2 Being much softer than 

 the adjacent Ffestiniog and Tremadoc Beds, the Dolgelley Beds 

 usually determine the position of a hollow, hence exposures, parti- 

 cularly of the black beds, are not abundant. The main road 

 makes use of the hollow for some distance, and the black beds are 

 seen in the road-cutting at the top of the steep hill above 

 Arthog. 3 



(2) The Tremadoc Slates. 



The Tremadoc Slates consist of a thick and rather monotonous 

 series of slates and mudstones, usually grey-blue. It is, however, 

 possible to distinguish in this slate-group a sequence which 

 corresponds very closel} r indeed, as regards both lithology and 

 palaeontology, with that established by Prof. Fearnsides in the 

 Arenig 4 and Tremadoc 5 areas. 



1 Q. J. G. S. vol. lii (1896) p. 516. 



2 An igneous rock occurs among the Dolgelley Beds where they are exposed 

 along the northern flanks of Mynydd-y-Gader, and at Bryngwyn near Dolgelley 

 town. Occasional ashy-looking- beds are also developed among the normal 

 dark slates. Mr. Lake & Prof. Reynolds regarded the igneous rock as an 

 andesite-lava separating the Lower and Upper Dolgelley Beds (Q. J. G. S. 

 vol. lii, 1896, p. 514). Followed westwards from the Dolgelley Fault, the rock 

 thins out and disappears near the Derwas Fault. West of this fault no similar 

 rock has been observed. Since, up to the present, no lavas have been proved 

 to occur in the Dolgelley Beds of any other district, we prefer to regard the 

 rock, which is in a very decomposed condition, as probably similar to some of 

 the small intrusions that occur at frequent intervals in the Cambrian rocks 

 north of the Mawddach. The horizon of the Dolgelley and Lower Tremadoc 

 Beds seems to have been a favourite one for small intrusions, and, although 

 no andesitic rocks are found west of the Derwas Fault, small diabase in- 

 trusions are frequent. Small andesitic intrusions have been shown by 

 Prof. Fearnsides to be of frequent occurrence at the corresponding horizon 

 in the Arenig country (Q. J. G. S. vol. lxi, 1905, p. 631). 



3 Near Garth Angharad and Pont Aber-gwynant the hollow is situated on 

 the outcrop of Ffestiniog Beds, not of Dolgelley Beds. This is probably due 

 to the rejuvenation of strike- streams which, originally flowing at a higher 

 level in a hollow on Dolgelley Beds, have now cut down into the harder strata 

 below. The rejuvenation eventually took place in stages, resulting in the 

 production of ' benches,' which form narrow zones of fertile ground. The 

 valley- cutting appears to have been practically completed before the close of 

 the Glacial Period, judging from the manner in which boulder-clay is found 

 ■deep down in the Gwynant Gorge. 



4 Q.J. G. S. vol. lxi (1905) p. 612 ; ibid. vol. lxvi (1910) p. 148. 



5 Ibid. vol. lxvi (1910) p. 157. 



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