﻿Vol. 66.] TEE3IADOC SLATES OF SOUTH-EAST CARNAEVOSSHIKE. 143 



1000 feet above sea-level, occur in the north and east ; and while, 

 both geologically and topographically, Tremadoc may be said to lie 

 among the Snowdonian foothills, Criccieth stands outside upon the 

 foreland and belongs to the head of the Lleyn peninsula. 



My first introduction to the district was as a member of Prof. 

 Hughes's field-class in the summer of 1901. The field-work upon 

 which the present paper is based was begun in 1902, 1 and each year 

 since that time, sometimes alone, and sometimes with a small party 

 of Cambridge pupils, I have spent a month or more among its rocks. 

 The interpretation of the general structure of the district, which 

 is here put forward, was arrived at in the summer of 1906 ; but, as 

 the result differs so greatly from the views arrived at by earlier 

 writers, I have kept back publication until I could confirm its 

 detail, and by mapping the area of the Moelwyns have proved its 

 extension over a very much wider area. 



II. Previous Literature. 



The district of Ynyscynhaiarn has long attracted the attention 

 of geologists, and its literature affords an interesting insight into 

 the mode of evolution of some of our larger geological terms. 



The first recorded incursion into the district seems to be the 

 visit of Sedgwick in 1831, 2 but, beyond the statement that the 

 rocks are to be included within the newly suggested Cambrian 

 System, little result survives. The first paper devoted to the district 

 is one by J. E. Davis, which appeared in 1846, and is entitled 

 1 On the Geology of the Neighbourhood of Tremadoc' 3 In this the 



(author recognized that the hills of the district are due to differential 

 denudation, and speaks of the Traeth as a recently-filled fjord or 

 sea-loch. He records the unusual general strike of the rocks, and 

 speaks of the major hill-masses as due to intrusive igneous sills. 

 He also announces the discovery of abundant Lingula at various 

 points, along what we now know to be the outcrop of the Lingulella 

 Band. 



In Daniel Sharpe's general memoir on the ' Geology of Xorth 



Wales ' we find the district mentioned, the observations due to Davis 



eing in the main confirmed. 4 In this memoir is found the first 



ccount of the ironstones of the Ynys about Tremadoc, and we 



re told that the intrusive greenstones belong to the Snowdonian 



(mountain) rather than to the Lleyn (peninsular) type. 



In 1847, 5 in Sedgwick's revision of his Cambrian System, we 



1 See Rep. Brit. Assoc. (Belfast) 1902, p. 614. 



2 Introd. to M'Coy's 'Brit. Pal. Foss. Geol. Mus. Univ. Cambridge' 1855, 

 xli ; see also Preface to J. W. Salter's ' Catal. Cambr. Silur. Foss. Geol. 



[us. Univ. Cambridge' 1871, p. xv. 



3 Quart. Journ. Geol. Soc. toI. ii, p. 70. 

 * Ibid. p. 283. 

 3 ' On the Classification of the Fossiliferous Slates of North Wales, &c.' ibid. 



ml. iii, p. 139. 



Q. J. G. S. No, 262. l 



