part 4] DISTRICT AROUND COHEIS AND AUERLLEPENNI. 513 



to give some account of the Upper Ordovician rocks, concerning 

 which, in this area, we have so far comparatively little information. 



Most of the important slate that is quarried in this area occurs 

 in the Ordovician : accordingly, to various parts of the Upper 

 Ordovician local names have been applied, and it is proposed in 

 this paper to retain those names so far as possible. 



The area may be regarded as constituting part of the south- 

 eastern flank of the Harlech Dome. The beds strike approximately 

 from south-west to north-east, and dip steeply south-eastwards. 



The area is traversed by a series of anticlines and synclines, the 

 axes of which are transverse to the general direction of strike, and 

 trend approximately north-north-east and south-south-west, in 

 some places, indeed, practically north and south. A conspicuous 

 southward pitch is noted throughout the region described. 



There is comparatively little faulting, except in the neighbour- 

 hood of the great Tal-y-llyn or Bala Fault. Here the mapping of 

 the upper margin of the volcanic rocks has revealed several faults 

 ranging approximately parallel to the Tal-y-llyn Fault. The 

 northern part of the area is affected by transverse faults of com- 

 paratively small effect, which trend north-west and south-east, and 

 apparently die out south-eastwards. Southwards the only impor- 

 tant fault is that which runs parallel to the strike at Aberllefenni. 



The rocks of the area are affected by a powerful cleavage, which 

 is approximately parallel to the strike of the beds. Generally, the 

 cleavage is vertical or very steeply inclined south-eastwards ; but 

 the inclination of the cleavage-planes diminishes somewhat as the 

 margin of the volcanic rocks is approached, though it is still as 

 much as 60° to the south-east. The diminution in the inclination 

 of the cleavage-planes is generally accompanied by a diminution in 

 the dip of the beds. 



IV. Detailed Description or the Beds. 

 Llandeilo. 



Craig 3^ Llam Group. — The highest volcanic rocks of the 

 Ordovician in this area are particularly well exposed in the great 

 bluff on the south side of the Tal-y-llyn A^alk^y called Craig y 

 Llam, and also on the north-western slopes of Mynydd y Waun 

 and Mynydd Ceiswyn. They have been called by Prof. A. H. Cox 

 A Mr. A. K. Wells 'the Upper Acid Craig y Llam Series.' 

 They consist of rhyolitic ashes and lavas, which weather to a 

 greyish -white. 



For the purpose of this paper, I have only examined the volcanic 

 rocks immediately underlying the Ceiswyn Beds; but it is stated 

 that the group as a whole is about 900 to 1000 feet thick. 1 



The boundary between the volcanic rocks and the overlying 

 sedimentary rocks is sharp and abrupt, and can be mapped accu- 

 rately. The junction is well exposed in several localities : as, for 



1 A. H. Cox & A. K. WelJs, Ker, Brit. Assoc. (Manchester) 1915, p. 424. 



