464 ME. 0. T. JONES ON THE HAETFELL-VALENTIAN [Nov. I909, 



and heather, gives to the country a decidedly monotonous aspect, 

 which is but little liable to seasonal variation. 



At first sight, this monotony seems to extend to the rocks, for in 

 a hurried traverse, especially across the higher ground, one might 

 readily believe that the country for mile after mile contained nothing 

 but an interminable series of pale greenish shales and mudstones of 

 so forbidding an appearance that it was out of the question to look 

 for fossils. It is only by working carefully along the numerous 

 sections that the great variety of the rocks and their exceptionally 

 fossiliferous nature is appreciated ; and it is only then that the 

 geological structure becomes revealed in the disposition of the sur- 

 face-features, which seem at first to defy any attempt at a simple 

 explanation. 



II. Histoeical Review. 



The general features of the region of Central "Wales were ably 

 described by Dr. Herbert Lapworth (6) 1 in a paper read before the 

 Geological Society in 1900, wherein he showed how a certain 

 amount of order had been gradually evolved out of the chaotic 

 mass of slates, grits, and conglomerates which covers an area of 

 something like 1800 square miles. It is unnecessary for me to go 

 over the ground there dealt with, and I shall therefore confine 

 myself to the literature which bears closely on the district de- 

 scribed in this paper. The list on p. 467 includes all the references 

 that I have been able to find. 



I am not aware that Murchison ever visited this part of Wales, 

 which was not coloured on the map accompanying the ' Silurian 

 System '; the rocks were believed by him to belong to the Cambrian 

 country of Sedgwick. 



We owe to Sedgwick (1) the first attempt at a classification of 

 the rocks. He made ' hasty traverses ' across the district on two 

 occasions — 1832 and 1846, — and as a result he inferred the rocks 

 to be below the ' Upper Silurian,' for he says : — 



' I believe the true demarcation of the Upper Silurian rocks [base of Wen- 

 lock] passes in an irregular line from the neighbourhood of Mallwyd to the 

 hills near Llanidloes, leaving the Plynlimmon ridges far to the west.' (P. 151.) 



With regard to their inferior limit, he remarks : — 



'these groups .... are superior to the slates and porphyries of the whole 

 Cader Idris range and I think also superior to the Bala Limestone.' (P. 155.) 



He divides the rocks within these limits into four principal 

 groups [p. 151], namely : (1) the Aberystwyth Group ; (2) the 

 Plynlimmon Group ; (3) the Upper South Welsh Slate Group 2 • 

 and (4) the Cambro-Silurian Group. He apparently regarded them 



1 These numerals in parentheses refer to the List of the Literature at the 

 end of this section, p. 467. 



2 On p. 153 of the paper Sedgwick denotes Group (3) 'The Ehayader Slates/ 



