188 DE. WILLS AXD ME. SMITH OS THE LOWER [vol. Ixxviii, 



Groom & Lake, In addition to the forms recorded by them, we 

 may note Sarpes dorani^ Portloch and Splicerocoryplie tJiom- 

 soni (.'') Reed. 



(8) G-lyn-Corwen G-rit. — The Glyn Grit at Glyn is a platy 

 sandstone passing down into a limestone, the two rocks being 

 interbedded at their junction. The peculiarities and fauna of 

 these rocks were described b}' Groom & Lake at some length. 

 When traced westwards an upper massive grit is found to come in 

 about a mile west of Glyn. and it is this upper grit which has 

 been proved by us to be the continuation of the CorAven Grit. The 

 disappearance of the platy grit westwards has just been noted. It 

 is interesting, however, to find near the Ty"n-y-twmpath stream 

 that the upper massive grit is succeeded upwards by thin beds of 

 OTit interbedded with the Lower Valentian Slates, in the same 

 manner that farther east the platy grit passes upwards into the 

 slates, as described by Groom & Lake.- Xear Corwen shaly bands 

 in the grit resemble lithologically the Dolhir Beds. 



Thus it appears that we must treat the Corwen and Glyn Grits 

 as parts of a single formation, conformable with the rocks above 

 and below ; but, if Ave judge from the fossil as Avell as fi'om the 

 litholosrical evidence, the formation is more closelv related to the 

 Ordovician than to the Silurian. Further reference to this is 

 made on pp. 190 &. 191 in describing their equivalent farther north. 



Relation of the Ashgilliaii to tlie Caradocian. 

 (See Table II., p. 1S9,. & fig. 3, p. 185.) 



The relation of the Ashgillian to the Caradocian in the Xorth 

 Berwyns may now be reviewed in the light of the facts set out in 

 th.e preceding pages. 



The hiatus shown by Dr. Groom & Mr. Lake to exist between 

 the Dolhir Beds proper and the Caradocian Bryn Beds in the 

 Glyn-Ceiriog region appears to be filled farther west by tlie 

 Blaen-y-cym and Ty'n-y-twmpath Beds, the former yielding 

 graptolites probably indicative of an Tapper Caradocian age, and 

 the latter containing a tyj^ical assemblage of Ashgillian trilobites. 



The complete sequence appears to be dcA^eloped at most places 

 west of Gelli ; but, east of that localit\', the rapid diminution in 

 thickness of the Dolhii- Beds proper, that come to outcrop, and the 

 absence of the Ty"n-y-tAvmpath ^ and the Blaen-y-cwm Beds, are 

 best explained by faulting along a gently-dipping plane, a view 

 put forward by Groom & Lake (see fig. 3, p. 185). This fault was 

 described in detail as the Dolhir Fault, and was shown on 

 their map as separating the Bryn and Dolhir Beds nearly as far 

 west as Ty'n-y-twmpath. Our discovery of the Pen-y-Gi-aig Ash 

 west of G-elli and our present knowledge of the Blaen-y-CAvm and 



1 Q. J. G. S. vol. Ixiv (1908) pp. 572-73. - Ibid. p. 57G. 



^ The available palffiontological evidence is insufficient to prove or disprove 

 the identity in age of tlie Dolliir Limestone and the Tv'n-y-twmpatli Beds. 



