﻿612 ME. W. G. EEABFSIDES ON THE GEOLOGY OF [Aug. I905. 



( Amnodd or ShumardiaShules. 

 I Tai-IIerion or Asaphellus-Fla gs. 

 I Nant-ddu or ~Bellerophon-Be&s. 

 Dictyonema-B&rid. 



3 j Niobe- and Psilocephalus-Beds. 

 ; " v P^r«-Beds. lilac ' 



' J Orthis-lenticularis Band. ' 



O , 



Para&o<^a-Beds. 



I Lincfulella-Beds. 

 \ Grits and Flags. 



There are also important intrusions of andesites (almost identical with the 

 Lower Ashes of Arenig), hornblende-porphyrites, and ophitic dolerites. 



The Ffestiniog Beds [22]. 



The oldest rocks of the district occur in the west and south-west, 

 and belong to the shallow -water phase of the Lingula- Flags. They 

 are a monotonous series of grits and flags with hardly any fossils, 

 and determine a belt of most uninteresting, boggy, heather-covered, 

 rolling moorland of only slight relief. Westward they rise to form 

 the steep escarpment which locally determines the main watershed 

 of Wales. Although usually dipping eastward or north-eastward 

 at about 30°, they are so broken up by folds and faults of small 

 amplitude, that, in the absence of any continuous exposure, it is 

 impossible to form any exact estimate of their thickness ; but, as 

 the outcrop is considerably more than a mile wide, that thickness 

 must be considerable. 



The beds are best exposed in the valley of the Lliw, where also 

 may be seen a fine large sill of hornblende-porphyrite, quite like the 

 well-known sills which occur at about the same horizon at Dolgelly. 



The Lingulella-Beds [21]. 



Towards the top these grits and flags become much finer in texture, 

 and in the highest 80 or 40 feet are crowded with specimens of 

 Lingulella Davisii, M'Coy, which, in places, become so abundant as 

 to form the greater part of certain bands (4637 ). x These beds are 

 exceedingly easy to recognize by their mode of weathering, for, 

 from being bluish-grey, they become first quite rusty and brown 

 and then increasingly paler, until finally they are almost white. 

 A similar belt of fossiliferous flags occurs in the same relative 

 position at Penmorfa (Portmadoc) and at Bhobell Fawr ; hence it 

 would seem that the division should be traceable all round the 

 Harlech dome. The beds are well exposed in the two westernmost 

 branches of the Trinant Valley. 



The Parabolina-Beds [20] . 



Passing upward, the Linguletta-Beds become finer-grained and 

 less gritty, giving place gradually to the hard flaggy shales of the 



1 The numerals in parentheses throughout the text of this paper refer to 

 the numbers of the corresponding rock-specimens preserved in the Sedgwick 

 Museum at Cambridge. 



