﻿144 me. w. a. feaknsides on the [May 1910, 



find an accurate description of a section across the heart of the 

 district, and meet with the term 'Tremadoc Group' applied as a 

 name for the slates between and below the greenstone intrusions, 

 that is, as a term embracing all rocks from the Upper Llandeilo 

 (Zone of Climacograptus peltifer) downwards to the Maentwrog Beds 

 or Lower Lingula Flags. We are told that such a group can be 

 recognized by its mineral character, and great stress is laid upon 

 the diagnostic value of the occurrence of pisolitic iron. We are 

 informed that the slaty members of this Tremadoc Group have 

 yielded two graptolites, an Asaphics and a Lingula. We gather 

 further from that paper that the bed containing the Lingula 

 (Lingulella Band) has been traced a great way around the Harlech 

 dome, and that it affords an excellent datum-line for the determi- 

 nation of the geological horizon of other beds, both above and below. 1 

 A little farther on in the paper the beds below and about the Lingula 

 datum-line in the Tremadoc district are compared with similar 

 rocks to the south of Ffestiniog, and the term ' Ffestiniog Group ' 

 is hinted at as a name for these. 2 In the final summary, however, 

 the terms Tremadoc and F'festiniog are used synonymously, 

 and in the figure of the general vertical section the Tremadoc Group 

 is shown resting upon ' Crystalline Slates' [op. cit. p. 156). 



By 1851 it would seem that the progress of the official geological 

 survey of Wales under Ramsay and Selwyn, and in this district 

 by Salter especially, had shown the necessity for an amplification 

 of the 1847 scbeme : hence, in the introduction to M'Coy's 

 ' Systematic Description of the British Palaeozoic Fossils in the 

 Geological Museum of the University of Cambridge' 1855, p. xx, 

 >we find the nomenclature entirely recast, and the older Ffestiniog 

 « or Tremadoc Group subdivided by Sedgwick thus : — 



'' I Bala Group. { ^ er * a } a - 



A L [ Lower Bala. 



P 



,3 f Arenig Slates and Porphyries. 



•■•Cambrian. ■{ "i% Ffestiniog Group. I Tremadoc Slates. 

 ^ [ Lingula Flags. 



<P -d n f Harlech Grits. 



I Bangor Group. { L i anberis Slates. 



In this scheme the term Ffestiniog is retained as a collective 

 name for the three middle subdivisions, while Tremadoc is com- 

 pletely specialized and applied to a definite slate series forming the 

 middle member of the Ffestiniog Group. 



In Sedgwick's classic paper on the ' Classification & Nomen- 

 clature of the Lower Palaeozoic Rocks of England & Wales ' 3 the 



1 Quart. Journ. Geol. Soc. vol. iii (1847) p. 143. 



2 Ibid. p. 144. 



3 Ibid. vol. viii (1852) p. 136. 



