part 3] lower paleozoic of abthog-dolgelley. 259 



The correlation of the strata with those in other districts is 

 discussed in a later section (§ V, p. 315). It may, however, be 

 helpful to the reader to indicate at this stage that the Moelyn 

 and Crogenen Slates together correspond to the main mass of 

 the Bifidus Slates of other areas, also that the Cefn-Hir Ashes 

 correspond to the Stapeley Ashes of Shropshire and to the Lower 

 Andesitic Ashes of Arenig Mountain. 



The various beds dip steadily southwards or south-eastwards at 

 angles of 30° to 70°, so that a north-to-south traverse gives an 

 ascending sequence. But the regular succession is somewhat 

 disturbed by a few strike-faults which cut out certain groups 

 of strata. It is further complicated by the presence of a number of 

 intrusive masses, which, although generally sill-like in habit, yet 

 transgress and accordingly cover up certain of the stratified rocks. 



Detailed Description of the Succession. 



(A) Upper Cambrian. 

 (1) The Lingula Flags. 



It was in the Dolgelley district that the tripartite division of the 

 Lingula Flags was first established by T. Belt. 1 The Maentwrog 

 Beds — the lowest of these three divisions — occup} r the ground 

 on the north side of the Mawddach Estuary, but are not found on 

 the south side.~ The strata belonging to the other two divisions 

 are fully represented. They appear, however, so similar to the 

 corresponding beds elsewhere round the Harlech Dome, as (for 

 example) in the Moel Llyfnant and Criccieth districts, where they 

 have been fully described by Prof. W. G. Fearnsides, 3 that a 

 detailed description here seems unnecessary, especially in view of 

 the already existing accounts by Belt, and by Mr. P. Lake & Prof. S. 

 H. Reynolds 4 of the beds in the immediate vicinity of Dolgelley. 



The main points with regard to the lithology may, however, be 

 briefly summarized as follows. 



(i) The Ffestiniog Beds. — These consist here, as almost 

 everywdiere in North Wales, of a great series of greyish-blue 

 flaggy slates interbanded with white laminated quartzite-bands 

 and with fine-grained siliceous grits or ' ringers,' which often 

 display the peculiar contorted or ' curled ' bedding described by 



1 ' The Lingula Flags of Dolgelley ' Geol. Mag. 1867, pp. 493, 536 ; and 

 ibid. 1868, p. 5. 



2 For a recent description of these beds, see A. R. Andrew, ' The Dolgelley 

 Gold-Belt' Geol. Mag. 1910, p. 165. 



3 ' The Geology of Arenig Fawr & Moel Llyfnant ' Q. J. G. S. vol. lxi (1905) 

 p. 612 ; and ' The Tremadoc Slates of South-East Carnarvonshire ' ibid. 

 vol. lxvi (1910) p. 142. 



4 ' The Lingula Flags & Igneous Rocks of the Neighbourhood of Dolgelley ' 

 Q.J. G. S. vol. lii (1896) p. 511. 



Q. J. G. S. No. 303. t 



