part 4] DISTRICT AROllXJJ CORIitS A^D ATJEHLLEFE^M. 511 



There is little doubt that the succession given by Davies 

 should be reversed, and that the ' Narrow Vein ' overlies the 

 ' Broad Vein ', which in its turn overlies the ' Black Rock '. This 

 reading is corroborated by the fossils which have now been obtained 

 from the different groups. 



In addition, Davies gives a list of quarries in the Corris area, 

 stating the particular slate-bed in which each quarry is located, 

 and this, in many cases, requires modification. 



In 1881, Sir Andrew Ramsay l described the ' Bala Beds, south 

 of Cader Idris ' in some detail. He stated that no fossils had been 

 discovered between Aberdovey and the Bala Limestone beyond 

 Dinas Mawddwy. He considered that the sandy beds of Garnedd- 

 wen might be ' on the general parallel of the Bala Limestone, or 

 of the fossiliferous strata not far below' (op. cit. p. 10b'). This 

 latter statement is improbable, for the sandy beds of Garnedd- 

 wen are part of the highest Ordovician group in the Corris area. 



In the same 3 r ear, Walter Keeping 3 described the occurrence of 

 graptolites in this area, and, so far as I am aware, that is the first 

 and only record of fossils. Up to that time the area had been 

 considered by all investigators as entirely devoid of fossils. Keep- 

 ing referred the Corris rocks to his ' Metalliferous Slate Croup ', 

 and recorded from Corris and Taren y Gesail Monograptus 

 sedgwicJcii (/) Portlock, M. tenuis (7) Portlock, Glimacograptus 

 scaiaris Hisinger, and Ortkoceras sp. The graptolites were, in 

 all probability, obtained from the now disused slate-quarries in the 

 Birkhill rocks at Corris and Taren y Gesail. 



After Keeping's paper, the only other reference to the area is 

 that by Prof. W. G. Fearnsides in 1910. 3 He states that a mono- 

 tonous series of dark banded mudstones overlies the Llandeilian 

 volcanoes along the southern flanks of Cader Idris and the Arans, 

 and that they probably, therefore, represent some part of the 

 Caradocian. 



Prof. A. H. Cox and Mr. A. K. Wells have been working for 

 some years in the adjoining area around Dolgelly and the main 

 Cader Idris range. The details of their investigations, concerning 

 the lower part of the Ordovician succession, have recently been 

 published, 4 and a summary of the general Ordovician succession up 

 to the highest volcanic rocks is contained in a Report to the 

 British Association. 5 



The present paper describes the succession of the Upper 

 Ordovician and Lower Silurian rocks lying south-east of the Cader 

 Idris area, and is intended, in some measure, to bridge over the 

 gap in our knowledge concerning the rocks and structure of the 

 country lying between Cader Idris and Machynlleth, the geology 

 of the latter place having been described in 1915. G 



1 ' The Geology of North Wales ' Mem. Geol. Surv. 2nd ed. (1881) p. 105. 



- Q. J. G. S. vol. xxxvii (1881) p. 162. 



:i Jubilee Vol. Geol. Assoc. 1910, pp. 799, 804, 816. 



1 A. H. Cox & A. K. Wells, Q. J. G. S. vol. lxxvi (1920-21) p. 254. 



5 A. H. Cox & A. K. Wells, Rep. Brit. Assoc. (Manchester) 1915, p. 424. 



' ; O. T. Jones & W. J. Pu-jh, Q. J. G. S. vol. Ixxi (1915-16) p. 343. 



