512 



PltOF. W. J. PUGH ON THE GEOLOGY OF THE [vol. lxxix, 



111. GrENEBAL SUCCESSION AND StKUCTUKE. 



The rocks of the Corris-Aberllefenni area belong to the Upper 

 Ordovician and Lower Silurian. They consist of inudstones, slaty 

 shales, and slates, with subordinate bands of grit. The arenaceous 

 beds are largely confined to the highest Ordovician and tbe highest 

 Silurian rocks examined. The Ordovician grits pass laterally, in 

 some places, into strong cmartzose conglomerates. 



Synopsis of the Succession. 



v„„,„^„„ a„ . „„ ( Pale mudstones, with numerous lami- 



Ysiwiih Stage •; . , -. , ', 



( nated grit-bauds. 



SI 



P O X T E E TV T r> 



Stage. 



Der w e n 

 Group. 



C w mere 

 Group. 



Bala. 



Llandeilo. 



A b e r c o r r 

 G r o u p. 



Abercwm- 

 c i d d a w 



Group. 



Hengae 

 Group. 



{ Craig ; 

 s L 1 a m 

 (. Group. 



f Pale greyish-blue mudstones, with dark 

 \ graptolitic shale-bands. 



f Blue and dark-blue shales and mud- 

 stones, with thin siliceous seams and 

 a pronounced rusty weathering. 

 -{ Ba>e of the group denned by the 

 zone of G-lyptogr.aptus persculptus 



J or 'Mottled Beds ', consisting of 



^ thinly bedded inudstones. 



3. Garnedd-wen Beds. Mudstones, 

 with bands of grit. Gritty mud- 

 stones and some conglomerates. 



2. Narrow Vein (=Y Faen gnl). Dark- 

 blue slate. 



1. Bed Vein (=Y Faen goch). Dark- 

 blue mudstones, in parts mottled 

 with dark patches. Zone of 

 Dicellograptus anceps. 



Broad Vein (= Y Faen lydan). Bluish- 

 grey mudstones, characteristically 

 mottled, locally greyish-blue slates de- 

 veloped. Trinuclens, Cyclopyge, etc. 



f 2. Nod Glas. Coal-black shales and 

 blocky mudstones, pyritoiis. Di- 

 cranograptus clingani, Dicellu- 

 grajptus morrisi, and Dicellograp- 

 tus forchammeri, etc. 

 1. Ceiswyn Beds. Greyish-blue slaty 

 mudstones, with thin gritty bands. 

 In the lower part are highly 



(_ cleaved dark slates. 



C Rhyolitic lavas and ashes. (The Upper 

 < Acid or Craig v LI am Series of 



A. H. Cox & A. K. Wells.) 



The development of the Silurian rocks is very similar to that 

 described in the Machynlleth 1 area, both in lithology and in 

 fauna. It is proposed, therefore, to adopt the same nomenclature 

 and subdivisions as those given at Machynlleth, in order to avoid 

 so far as possible the confusion of new local names. Further, it is 

 not intended to describe the Valentian rocks in detail, but rather 



0. T. Jones & W. J. Pugh. Q. J. G. S. vol. lxxi (1915-16) p. 343. 



