part 3] 



LOWER PALEOZOIC OP ARTHOU-DOLCiELLEr. 



2.57 



1912. P. Lake & S. H. Reynolds.— ' The Geology of Mynydd-y-Gader,. 

 Dolgelley ' Q. J. G. S. vol. lxviii, p. 345. 

 This paper deals with the area immediately south of that previously 

 described by the same authors. The rhyolitic rocks which succeed the 

 Tremadoc Slates were found to be followed eventually by beds of Llanvirnian 

 age. These latter were styled the ' Ashy Series ', and were regarded as 

 forming the upper portion of the Mynydd-y-Gader Volcanic Series. They 

 consist of ashes and slates yielding Didymograptus bifidus (Hall) and D. mur- 

 chisoni (Beck). Hence it was supposed that both Lower and Upper Llan- 

 virnian Beds were there represented. The exact relation of the volcanic 

 rocks to the Tremadoc Beds was however still obscure, since no Tremadoc 

 fossils indicative of any higher horizon than the Dictyonema Zone had been 

 found, nor were Ordovician fossils indicating any lower Zone than that of the 

 D.-biftdus Beds obtained. The rhyolitic rocks were, however, found to rest 

 transgressively upon or against the various divisions of the Upper Cambrian, 

 and the apparent transgression was ascribed to faulting. 



In 1912 one of us began an examination of the higher beds of 

 the Cacler-Idris range. During the progress of the work, it was 

 found advisable to extend the investigation into the ground south 

 of the Grwernan Faiilt and west of the area described by Mr. Lake 

 & Prof. Reynolds. A preliminary report l on the results obtained 

 from the examination both of this ground and of the main 

 Cader-Idris area was presented at the Manchester meeting of the 

 British Association in 1915. 



The present paper contains a detailed description of the lower 

 portion of the sequence outlined in that report. 



II. The Stratioraphical Succession. 

 The general stratigraphical succession is as follows : — 

 ? Llandeilo. Lower Basic Volcanic Series. 



f Cefn Hir Ashes.' 



Crogenen Slates. 

 -{ Bryn Brith Beds. 



Moelyn Slates. 

 [__ ' China-stone ' Ashes. ~"| 



Lower 

 Llanvien. 



Lower Llanvirn 

 and Arenig. 



Arenig. 



Upper 

 Cambrian. 



Lower Acid or 

 r" Pont Kings Slates. \- Mynydd Gader 

 r Volcanic Series. 



J Lower Ashes. -J 



[_ Basement Series. , 



? jt ,. ., f Upper Pencil-Slates. 



| Upper Dictyonema Band 



rn , i oi i. I Asaph ell us Beds. 



f IremadocSlates. < T * -r, ■-. 01 , 



| Lower Pencil- Slates. 



Dnlo- 11 "R rl i Lower Dictyonema Band, 

 ge ey e s. ^iobe Beds. 



Ffestiniog Beds. 



_ [Maentwrog Beds] (not seen south of the Mawddaeh 

 Estuary). 



1 A. H. Cox & A. K. Wells, ' The Ordovician Sequence on Cader Idris ' 

 Rep. Brit. Assoc. (Manchester, 1915) 1916, p. 424. 



