466 MR. 0. T. JONES ON THE HAKTFELL-VALENTIAN [Nov. 1909, 



and Cwm Ystwyth as the same, but placed the Aberystwyth Grits 

 far below them. His most important achievement was the discovery 

 of graptolites in several places, from which he was able to infer 

 that the beds were on the whole of Upper Birkhill age : — 



' From the fossil evidence, therefore, there can be no hesitation in referring 

 our Mid-Wales rocks to the same age as the Upper Birkhill of Scotland ; and 

 beyond the occurrence of the three species just mentioned in the rocks of the 

 Devil's Bridge, there is nothing that conveys the slightest hint that any of our 

 strata are newer than Upper Birkhill.' (P. 166.) 



In regard to the general structure, he recognized 



' four different lines of north-and-south foldings between Aberystwyth and 

 Plynlimmon — namely : (1) the principal [anticlinal] axis of Aberystwyth ; (2) 

 the principal [synclinal] axis of Plynlimmon ; and (3) the two minor axes 

 running (a) from Ystrad Meurig through Llantrisant and Cwm Symlog, and 

 (b) along the gorge of the Rheidol about Pont Erwyd and the Devil's Bridge.' 

 (Pp. 154-55.) 



I shall not attempt to criticize Keeping's work in detail. His 

 paper contains much that is good ; but, from my investigations in 

 the Pont Erwyd district, I have been forced to an entirely different 

 conclusion in regard to the stratigraphical succession. As a fact, 

 Keeping's paper carried a very strong hint that something was 

 wrong, in the form of a palaeontological appendix by Prof. Charles 

 Lapworth, to whom a few of the graptolites had been sent for 

 identification. From the evidence before him Prof. Lapworth 

 arranged the beds in the following descending order : — 



' D. The M. turriculatus beds of the Devil's Bridge. 

 C. Next below (or perhaps identical with D) the Cefn Hendre grits [part 



of the Aberystwyth Grit Group]. 

 B. Then comes the rich graptolitic zone of Morben and Cwm Symlog. 



A. Lowest of all lie the Diplograpsus beds of Steddfa Gurig.' 



(P. 167.) 



According to this arrangement, the Metalliferous Slates (A & B) 

 are older than the Aberystwyth Grit Group, and the beds A, 

 which are on the supposed synclinal axis of Plynlimon, ought to be, 

 according to Keeping, almost the highest beds in the district. 



It is a striking tribute to the value of the graptolite as an index 

 of geological age, and to Prof. Lapworth who first made that value 

 clear, that he was able to establish the order of succession of the 

 strata from an inspection of a few graptolites, and to maintain it 

 in the face of supposed stratigraphical evidence leading to a 

 diametrically opposite conclusion. 



Since the appearance of Keeping's paper no one had worked on 

 the district, until I began there in the summer of 1903 ; but when, 

 in 1900^ Dr. Herbert Lapworth established his Khayader suc- 

 cession (6), showing that there at any rate Keeping's interpretation 

 would not hold, doubt was thereby cast on the other districts with 

 which he dealt ; and in 1905, when the Geologists' Association 

 visited Plynlimon, Dr. Lapworth found Gwastaden or Lower 

 Llandovery graptolites near Eisteddfa Gurig (7). 



