﻿Vol. 66.] GEOLOGY OF THE DISTRICT AROUND LLANSAWEL. 405 



bedding or fossils. This, together with the scarcity of exposures, 

 has made it impossible for us to produce a complete stratigraphical 

 map based on palseontological evidence, and we have been obliged 

 to leave some of our boundaries conjectural. 

 We classify the succession as follows : — 



fC 3 . Pengelli Shales. 



0. Llansawel Group \ C 2 . Zone of Monograptus communis. 



[C x . Clynmarch or cyphus Grits and Shales. 



f B 2 . Llathige Shales and Mudstones. 



B. Caio Group 1 B T . Pen-y-ddinas Grits and Shon Nicholas 



[ Conglomerates. 



A. Beili Tew Group Beili Tew Grits and Shales. 



The series falls naturally into three groups, sharply marked off 

 one from the other. 



Our lowest or Beili Tew Group (A) is seen in Moelfre and the 

 hills to the south, and in Banc Beili Tew. The most characteristic 

 member is a very tough nodular shale, with no well-marked bedding 

 and containing no fossils : there is also a considerable development 

 of a sandy grit, with a conspicuous red-speckled weathering. This 

 group we believe to be of Hartfell age. 



Our second or Caio Group (B) derives its name from the village 

 of Conwyl Caio, around which it is typically developed. The rocks 

 of this group consist of tough dark-blue mudstones, shales, and at 

 the base a coarse quartzose conglomerate, which makes a strong 

 feature wherever it occurs. South-east of Llansawel there is a. 

 great development of a quartzitic grit, the relations of which to 

 the conglomerate are not easy to determine, for the two are 

 never found together. We believe it to be on much the same, or 

 possibly on a slightly higher, horizon. These Caio Beds cannot be 

 traced as a well-marked band across country, but extend over a 

 distance of 3 miles across the strike, being repeated by a series of 

 parallel folds, along the denuded crests of which the conglomerate 

 occurs. The junction of this rock with the overlying shales and 

 mudstones is often much faulted. The mudstones are characterized 

 by a fauna of Diplograptids and Climacograptids only, and yield an 

 assemblage characteristic of the modestus flags. Prom the con- 

 glomerate were obtained a few fragments of brachiopods and corals. 

 In the grit we have failed to discover any fossils. We therefore 

 believe that in this region the Llandovery period was ushered in by 

 a shallow-water phase, which gave way rapidly to comparatively 

 deep-sea conditions. 



The highest group, which we name the Llansawel Group (C), con- 

 sists of tough, dull-grey, cleaved shales, with softer black partings 

 and interbedded grey banded grits. They have a uniform strike of 

 E. 30° N., and their base forms a well-marked ridge north-west of 

 Llansawel, extending for 5| miles across country. This rise owes 

 its existence to the occurrence among the shales of thick beds of 

 hard sandy grit. These grits and associated shales provide one of 



