﻿Yol.6i.] 



ARENIG FAWR AND MOEL LLYFXANT. 



62> 



analyses showed me that, while the lower members of the Ashes 

 of the Upper Series contain only about 63 per cent, of silica, the 

 higher members may contain as much as 72 per cent. 



The Derfel or OrtMs- Limestone [2] . 



Although so thick and so continuous, the Upper Ashes of Arenig 

 end off quite suddenly, and through the massive flaggy ash of the 

 top of the Upper division pass into a few feet of slightly-ashy shale 

 which in turn gives place to a richly-fossiliferous brachiopod, 

 monticuliporoid, and cystoid limestone. Prom the fine exposure in 

 the gorge below the waterfall at Pont Aberderfel, I have called 

 this the Derfel Limestone. It should be considered rather 

 as the basal member of the succeeding belt of black shales than as 

 a member of the volcanic group, for, although intensely pyritous, 

 it contains practically no ashy material. When I first met with it 

 in the valley above Pont Aberderfel I was inclined to think of it as 

 a Drift-carried or faulted-in mass of Bala Limestone, but further 

 work has shown that this is impossible. When traced southward, 

 the Limestone, which in the Derfel gorge is 3 feet thick, seems to 

 die out, and beyond Pistyl Gwyn, where it is 1 foot thick, has 

 not been found. Northward, however, it thickens, and is readily 

 mappable along the Valley of the Gelyn, as also about the Trausnant 

 farm in the Valley of the Serw, and north of Arenig Pach. The 

 fossils obtained by the officers of the Geological Survey at ' Gam, 

 east of Arenig/ also seem to come from the same horizon cropping 

 out east of Gylchedd. In all these places the limestone rests close 

 down upon the highest volcanic beds, and there can be little doubt 

 that it is conformable with them. 



The fauna hitherto obtained is as follows : — 



Trinucleus concentricus, Eaton. 

 Lichees laxatus, M'Coy. 

 Phacops bimucronata ? (Murch.). 

 Cybele verrucosa ? (Dalm.). 



Illeenus. 



Calymene. 



Harpides. 



Eaphistoma cequalis, Salt. 



Clitambonites ascendens, Pander. 



OrtMs Actonics, Sow. 



Orthis (Platystrophia J biforata, Schlot- 



heim. 

 Orthis calllgramma, Dalto. 

 Orthis (Dalmanella) clegantula, Dalm. 

 Orthis plicata, Sow. 

 Orthis sarmentosa, M'Coy. 

 Orthis(Dalmanella)testudinaria ) ~Da\m. 



Orthis (Dalmanella) unguis, Sow. 

 Leptcena rhomboidalis, Welch. 

 Plectambonites quinquecosta, M'Coy. 

 Plectambonites sericea, Sow. 

 Plectambonites transversalis, Dahl. 

 Triplesia insularis, Eichw. 

 Triplesia spiriferoides, M'Coy. 

 Strophomena antiqua, Sow. 

 Strophomena expansa, Sow. 

 Orthotetes pecten, Linn. 

 Grlyptocrimis basalis, M'Coy. 

 Monticulipora lycoptodon, Hall. 

 Monticulipora fibrosa, Goldf. 

 Ischadites microppra, Hall. 

 Ptilodictya dichotoma, Portl. 

 Ptilodictya explanata, M'Coy. 

 Ptilodictya fucoides, M'Coy. 



What the exact horizon of this fauna is I am unable to make out, 

 for all the brachiopods, which are the sole really-common well- 

 preserved fossils, belong to species with very long ranges, and might 



2x2 



