﻿Vol. 64.] BALA AND LLAIfDOVERY EOCKS OP GLYN CEIRIOG. 571 



nodules and appear to pass gradually into the limestone at the base 

 of the Glyn Grit. 



The fauna of the Dolhir Beds is by far the richest in the 

 district, both in the number of species and in the number of indi- 

 viduals. So abundant indeed are the fossils, that scarcely a blow 

 of the hammer fails to reveal some trace of organic life. A few of 

 the species are found in the beds below, and some pass up into the 

 Glyn Grit ; but the greater number are confined to the Dolhir Beds 

 themselves. 



On examining the list of species from the district (pp. 572-73), it 

 will be observed that the corals, crinoids, and cystoids are almost 

 entirely confined to this horizon. A few of these pass upwards into 

 the Glyn Grit, and one or two have been found in the Pron-Frys 

 Slates; but none occur in the beds below the Dolhir Group (excepting 

 in Nant Tyn-y-twmpath, where Echinosphoerites arachnoidea has 

 ocen found). Polyzba also are very much more abundant in these 

 beds than in any others, and the genera Ptilodictya, Phyllojoora, and 

 Ramipo7^a are practically confined to this series. A considerable 

 number of brachiopods have been found only in the Dolhir Group ; 

 while, among the trilobites, Cybele, Cheiruriis, Hemopleurides, and 

 Trinucleus seticornis have not been found in the district, excepting 

 in these beds. 



(8) Glyn Grit. — The bed or series of beds that has hitherto 

 been supposed to represent the Corwen Grit is not so uniform in 

 character as the Corwen Grit of Corwen, and towards its base it is 

 sometimes very calcareous, so much so indeed as to form a definite 

 limestone. This limestone, however, is not constant, and west of 

 the Ceiriog it seems to be represented by a soft, brown or choco- 

 late-coloured sandstone, which is very much decayed and extremely 

 fossiliferous. The limestone itself is best seen in the two small 

 quarries on the top of MynyddPron Frys. It has also been worked 

 on the side of the hill south of Hafod-y-gynfawr, but the quarry 

 is now overgrown. 



At the quarries on the top of Mynydd Fron Frys the limestone 

 is overlain by a platy sandstone, and near the junction the two are 

 interbedded with each other. Elsewhere the grit generally consists 

 of an irregular, platy, and fairly compact sandstone. But, towards 

 Tomen-y-meirw,^ it is sometimes very coarse and contains frag- 

 ments of slate. 



Commencing at Tomen-y-meirw, the sandstone strikes nearly 

 due eastwards to a fault which throws it to the south. It is again 

 exposed in a small quarry by the roadside. Another fault, which 

 may be seen in the quarry itself, throws it again to the south into 

 the wood of Cefn-coch. It traverses this wood from west to eastl 

 and runs thence to Pant-y-graig, where it forms a line of sraal, 

 cliffs in which many fossils may be found. The Aber-chwil Fault 

 cuts it off at the eastern end of this clift*, and farther east it forms 



^ Tomen-y-meirw is a turauhis Ijing just on the northern border of our 

 map (PI. LIII), about 400 yards west of Nant Tyn-y-twmpath. 



Q. J. G. S. No. 256. 2 q 



