492 ME. W. B. E. KIKG OlS" THE [vol. lxxix, 



small building called Pwll-y-wrach-isaf, in the Marchnant valley, 

 has yielded many excellent specimens, particularly of Pterygo- 

 metopus juJcesi, which Miss Elles found to be characteristic of the 

 calcareous-ash horizon at Bala. 



Pen -y - garnedd black -shale group. — At Pen-y-garnedd 

 at the present time the calcareous group is seen to be followed by 

 some 45 feet of jet-black soft shales, with an 18-inch bed of 

 phosphatic material at the base. This section (which will be 

 described in more detail in a Geological Survey Memoir) may be 

 noted here, since it is the best exposure of the phosphate-bed ; 

 also the shales above the phosphate-bed have yielded moderately 

 well-preserved graptolites at several horizons. 



The change from limestone to shale is not very sudden, for 

 several thin beds of dark shale are intercalated in the upper part 

 of the ashy calcareous beds, marking a fluctuation of conditions of 

 deposit before the black-shale type definitely set in. The same con- 

 ditions are described hj D. C. Davies 1 at the Cwm-gwynen Mine, 

 which he visited when it was being worked ; here, however, he 

 notes calcareous shales with echinoderms and brachiopods resting 

 upon a phosphate-bed 10 to 15 inches thick. No mention is made 

 of the thick bed of black shale, and I also failed to identify it in 

 the adit when I visited the mine in 1919. It should, however, be 

 home in mind that strike-faulting frequently cuts out the black- 

 shale group. 



Miss Elles has examined some of the material collected at 

 Pen-y-garnedd, and identifies the graptolites as belonging to species 

 of Orthograptus. The occurrence of graptolite-bearing shales 

 at this horizon is of interest, for the forms prove to belong to 

 the Diplograptus-pristis Zone of Sweden, which usually is not 

 represented by graptolite-bearing beds in this country. 3 Strati- 

 graphically, therefore, this horizon is the highest part of the 

 Pleurograptus-linearis Zone, and as such belongs to the highest 

 Caradocian rocks of Britain. The following species of graptolites 

 have been identified by Miss Elles : — 



Orthograptus pristis (Hisinger). 

 Climacograptus minimus Carruthers, 

 C. styloid ei is Lapworth. 



Orthograptus truncatus var. pau- 

 peratus Elles & Wood. 



0. calcaratus var. basilicus Lap- 

 worth. C. scalaris var. miserabilis Elles & 



0. aff. quadrimucronatus (Hall). Wood. 



Besides containing graptolites, these shales yield numerous small 

 horny brachiopods and dwarf hinged brachiopods, among which the 

 following appear to be commonest: — 



Plectambonites albida Reed. I Lingula obtusiformis Wade. 



Lingula cf. brevis Portlock. ] Siphonotreta cf. micula M'Coy. 



On tracing this zone south-westwards to near Aber Marchnant 



1 Q. J. G. S. vol. xxxi (1875) p. 358. 



2 ' Summary of Progress for 1919 ' Mem. Geol. Surv. 1920, pp. 4 & 5 ; also 

 L. J. Wills &B. Smith, Q. J. G. S. vol. Ixxviii (1922) p. 186. 



