TJie Carboniferous Succession below the Coal- Measures. 449 



point of view tins P. giganteiis bed is succeeded by the pebbly- 

 conglomerate — a bed which can be traced in the sequence from 

 Gwernymynydd to Llangollen. It is interesting to note that the 

 upper beds of limestone at Castleton, Derbyshire, contain occasional 

 quartz pebbles. 



(xxxii) The outlier of Carboniferous Limestone at Corwen was 

 also visited by one of us. The quarry exposes the upper beds of 

 the limestone, and is altogether in the zone of Zonsdaleia, the upper 

 Dibunophylbim zone, but it is impossible to ascertain the thickness 

 of the beds below those that are quarried. At the top there are 

 thick, well-bedded limestones, beneath which are shales with nodules 

 in layers and thin limestones. The beds worked in the quarries are 

 estimated by Morton at 80 feet. The upper limestones contain — 



Lithostrotion Portlocki (Bronn). Martinia glabra (Mart.). 



Syringopora sp. Productus gigauteus (Mart.). 



Rhynchonella pleurodon (Phill.). P. punctaius (Mart.). 



The 10 feet of black shaly beds below the limestone are full of 

 corals, and contain — 



Alveolites septosa (¥\em..) . L. PortlocJci (Bronn). 



Cyathophyllum cf. regiuin, Phill. Zonsdaleia rugosa, M'Coy. (Very 



Dibunophyllum sp. abundant.) 



Lithostrotion irregulare (Phill.). Phillipsastrmaradiata,'E.&,'B.. (Common.) 



L.jiinceimi (Flem.). Seminula ambigua (Sow.). 



(xxxiii) South of the Dee the limestone outcrops almost con- 

 tinuously for 12 miles from Fron-y-cysyllte to Llanymynech, the 

 outcrop varying from a half to one mile in width. It is of interest 

 that at Fron, according to Morton, only the upper moiety of the 

 series, as exposed in the Eglwyseg ridge, is present, demonstrating 

 the fact that the Wenlock beds in this position were not submerged 

 till later on in Carboniferous time. This also seems to be borne out 

 by the various sections of limestone at Bron-y-garth, Craignant, 

 Llawnt, Porth-y-waen, and Llanymynech. At the latter place 

 Morton estimated the whole limestone series to be 450 feet thick, 

 as against 1,200 feet at Ty-nant, near Llangollen. 



Morton ^ shows the following variation in thickness in a distance 

 of 4 miles from west to east : — 



Tv-nant. Tan-y-castell. Trevor Rocks. Bronheulog. Fron. 



1,200 feet. 1,025 feet. 607 feet. 295 feet. 115 feet. 



And as the upper part of the series is similar in each localitj', it 

 follows that the floor on which the limestone was deposited was 

 very uneven, and a considerable overlap of the upper beds occurs 

 locally and in a definite manner from south to north. 



(xxxiv) A traverse from Sweeney Mountain, through Treflach 

 Wood Quarries, gives a fairly complete view of the sequence in this 

 area. The old tramway which afforded the section of Cefn-y-Fedw 

 Sandstone published by D. C. Davies^ and by G. H. Morton^ has 

 now unfortunately largely disappeared, but many of the beds can 



' G. H. Morton: op. sup. cit., p. 39. 



•^ Geol. Mag., Vol. XII (1870), p. 69. 



^ G. H. Morton: op. sup. cit., p. 93, pi. vi. 



DECADE V. — TOL. III. — NO. X. 29 



