﻿The Tarannon Series of Tarannon. 167 



Ab. Dolgadfan Group. 



Zone of M. convolutus. 

 Zone of M. fimbriates. 



Aa. Fachdre Group. 



Zone of Dimorphograptus Swanstoni. 



The Tarannon Series iii this district has a maximum thickness of 

 3500 feet, but thins somewhat as it is traced north-westward. It 

 rests conformably on Llandovery rocks below, and passes up without 

 a break into Wenlock beds above. This rock-series is strati- 

 graphically continuous from base to summit, and includes the four 

 divisions of the Brynmair, Gelli, Talerddig, and Dolgau Groups, 

 which, while they possess distinctive features of their own, are 

 bound together by common palaeontological characters. The lowest 

 two, namely the Brynmair and Gelli Groups, consist mainly of 

 grey shales and mudstones with beds of thin flags, which increase 

 in number and thickness as one ascends the sequence. The 

 Talerddig Group is distinctly an arenaceous one, and contains 

 numerous bands of thick grit which are generally massed together 

 at four or five distinct horizons. The highest member of the 

 Series, the Dolgau Group, answering to the local ' Tarannon Shales ' 

 of the Geological Survey, consists of pale-grey and purple mudstones, 

 the latter being inconstant in number and thickness in different 

 parts of the district. 



The strata of the overlying Wenlock Series present all the 

 characters of the Denbigh Grits and Flags of North Wales. Some 

 2000 feet are developed in this district, the upper beds consisting 

 of grits and flags, while the lower are mainly shales and mudstones. 



The Llandovery Series, which underlies the Tarannon Scries, 

 has, at present, been recognized only in the western part of the 

 district, namely in the valley of the Twymyn, and its rocks are 

 brought to the surface by an anticlinal fold. Representatives 

 of nearly the whole of the Llandovery beds have been met with at 

 different localities, and live distinct graptolitic zones have been 

 recognized. The rocks, which consist almost entirely of soft shales 

 and mudstones, are probably not more than 400 feet thick. 



A comparison of the graptolitic lists shows that the Tarannon 

 Series, as here defined, corresponds almost exactly with the Gala 

 or Queensberry Group of the South of Scotland, includes all the 

 palseontological zones hitherto assigned to the Tarannon, and fills 

 up the whole period intervening between the Llandovery below and 

 the Wenlock above. It includes the extreme beds which have been 

 mapped as Tarannon by the Geological Survey in Wales ; and in 

 the Tarannon District, at all events, the thickness of the Series is 

 equivalent to its maximum development elsewhere. 



