510 JWtOF. W. J. PL'trH ON THE OEOLOUV uF THE [vol. lxxix, 



district : namely, the Upper Ordovician lying above the volcanic 

 rocks and the Valentian, especially the Upper Valentian or Tarannon 

 Series, which covers so large an area between Corris and Machyn- 

 lleth, and the rocks of which are ' violently contorted '. 



Sedgwick included ' a part of the slate series on the south 

 flank of Cader Idris, descending towards the country nearMachyn- 

 lledd ' in his ' Upper Cambrian slate group, or the Trilobite 

 group, or (geographically) the Bala group' (ojj. cit. pp. 157, 158 J. 



In 1852, Sedgwick 1 again referred to this area. In giving a 

 classification of the Lower Palaeozoic rocks, he stated that the 

 ' Bala Group ' Avhich overlies 'the Arenig slates and porphyries ' is 

 of great thickness, and that the lower part of the group is finel} r 

 developed on the south-eastern flanks of Cader Idris. 



In the same year appeared the Geological Survey Horizontal 

 Section No. 26, which indicated the general arrangement of the 

 rocks along a line through Cader Idris south-eastwards, and 

 passing between Corris and Aberllefenni. 



In the following year, Sir Andrew Ramsay 3 stated that Bala 

 and Llandeilo rocks overlie the volcanic rocks of Cader Idris, and 

 occupy the country on the south-east. He comments upon their 

 unfossiliferous nature. 



In 1855, the Geological Survey Map 3 of the Cader Idris country 

 was published. The rocks of the Corris-Aberllefenni area are indi- 

 cated as being of Lower Silurian age. Along the south-eastern 

 margin of the volcanic rocks the symbol ' b 2 ' may be observed, 

 indicating 'Llandeilo and Arenig' beds, while all the country on 

 the south and south-east is labelled 'b 3 ', indicating ' Caradoc 

 or Bala ' rocks. 



In 1872, the Rev. W. S. Symonds 4 referred briefly to this area, 

 and, in describing the unfossiliferous nature of the rocks, stated 

 that they appeared ' to resemble, rather the nearly unfossiliferous 

 grits of the Lower Llandovery series, than the fossiliferous Caradoc 

 strata of Bala and other districts.' 



Six years later appeared Mr. I). C. Davies's ' Treatise on Slate 

 & Slate-Quarrying' (1878, pp. 61-61), and, so far as I am aware, 

 this contains the most detailed description of the rocks of this 

 area that has been published. Davies classified the rocks of the 

 Corris area as belonging to the Llandeilo Series, and considered it 

 erroneous to correlate them with the Bala Group. He describes 

 the slate-beds in some detail, and gives a horizontal section of 

 the strata. He recognizes the slate-bed locally known as the 

 ' Narrow Vein ', and states that it is overlain by the ' Hard Rock '. 

 This latter group is, according to Davies, overlain by the ' Broad 

 Vein ' or ' Broad Slate-Bed ', and he cites Abercwmeiddaw as a 

 quarry in the ' Broad Vein '. The ' Broad Vein ', according to his 

 section, is overlain by the ' Black Rock '. 



1 Q. J. G.S. vol. viii (1852) p. 148. 



2 Ibid. vol. ix (1853) p. 163. 



:s Geological Survey of Great Britain, 1-inch Sheet, Old Series, 59, N.E. 

 * ' Records of the Rocks ' 1872, p, 104. 



