part 4.] district around corris and aberllefennt. 537 



They trend north-west and south-east, and faults with a similar 

 direction have been located by Prof. A. H. Cox & Mr. A. K. Wells 

 in the Arthog-Dolgelly district, and by Miss G. L. Elles in the 

 Bala country. 



Both fault-planes are inclined south-westwards, the more 

 northerly one at about 60 '. Both possess a small downthrow to 

 the north. 



The cleavage. — All the rocks in the Corris-Aberllefenni area 

 are affected by a very pronounced cleavage, although the degree to 

 which they have been affected has depended to some extent upon 

 the physical characters of the rocks themselves. The coarser- 

 grained and less homogeneous rocks have not yielded so much 

 to the compressional movements. 



The cleavage-planes trend almost exactly parallel to the strike 

 of the beds, but are not so gi'eatly affected by the minor folding. 

 They are steeply inclined or practically vertical, as a general rule. 

 The average inclination over much of the area is about «S0° to 8~> 

 south-eastwards, but the inclination diminishes somewhat with the 

 diminution in dip as the margin of the volcanic rocks is approached. 

 In the dark slaty group at the base of the Ceiswyn Beds, 

 immediately above the volcanic rocks : as, for example, at the head 

 of the Llefenni Valley, the cleavage-planes are inclined south- 

 eastwards at 00°. 



YI. Comparison avith other Areas. 



It is proposed to give some general comparisons with other areas 

 in the case of the Ordovician rocks alone. The Valentian rocks 

 are closely similar to those described farther south in the Machyn- 

 lleth area, where the succession lias been compared in detail with 

 that obtaining in other districts. 



Central Wales. — The Ty'n-y-maen Group of the Plynlimon 

 Stage, described at Machynlleth, corresponds with the upper part 

 only of the Garnedd-wen Mudstones at Corris. The lithological 

 characters ars closely similar in both areas. 



There is no doubt that the Abercorris Group may be correlated 

 with the Plynlimon Stage at Plynlimon. 1 From the basal group 

 of the Plynlimon Stage, namely the Nant-y-Moch Flags, Prof. 0. 

 T. Jones records the following graptolites : — 



Dicellograptus anceps. Climacograptus scalaris var. 



Orth ograptus trv/nca tus. miserabilis. 



Orthoijraptus cf. mvtabilis. 



The fauna is the same as that of the lied Vein at Corris. The 

 Red Vein, and probably the Narrow Vein, may, therefore, be 

 correlated with the Nant-y-Moch Flags. The Garnedd-wen Beds 

 are equivalent to the Drosgol and Brynglas Group at Plynlimon, 



1 O. T. Jones, Q. J. G. S. vol. lxv (1909) p. 468. 



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