330 PBOF. O. T. JOXES ON THE [Sept. I912, 



In the above-cited paper, p. 155, he asserts 



' that all the groups .... are superior to the slates and porphyries of the- 

 whole Cader Idris range, and I think also superior to the Bala limestone ; . . . . 

 The groups occupy .... a kind of trough between the older rocks of Pem- 

 brokeshire and the chain of Cader Idris, and are thrown into astonishing 

 undulations.' 



The groups that he established were not mutually exclusive, and 

 are not arranged (as he believed) in a general ascending sequence 

 from west to east. It is pleasant to find, however, that as the- 

 result of so brief a visit he had discovered the true stratigraphical 

 position of the group of rocks as a whole, and also their general 

 structure. Two of his sub-groups were subsequently adopted by 

 Walter Keeping and myself, namely, the Aberystwyth Group and 

 the Plynlimon Group. Eeference will be made to other results 

 below. 



About the year 1845 appeared Sheets 4, 5, and 6 of the Horizontal 

 Sections of the Geological Survey, which crossed in various places 

 the district lying between the Old Eed Sandstone and Cardigan 

 Bay. These indicate in a beautiful manner the nature of the 

 small-scale folding of the area ; further, it is clear from these 

 sections that the more important structural lines were made out. 

 From the remarks accompanying the sections it is easy to construct 

 a table of the various groups of rocks along these sections which 

 the surveyors regarded as contemporaneous. The order of the strata 

 in the various districts is everywhere correct, but the correlation of 

 the rocks along the Yale of Towy with those lying between the 

 Yale of Teifi and the sea is at fault. Considering, however, the 

 highly folded nature of the country and the absence of any guiding 

 fossils, it is remarkable how truthful these sections are, although 

 made nearly 70 years ago. 



Sir Andrew Eamsay^ in the * Geology of ^orth Wales,' records 

 fossils from several localities within the area, as, for instance, 

 Pishguard (graptolites), Abereiddy (various fossils), Cardigan- 

 (graptolites), Newcastle Emlyn (various fossils), etc. 



Another attempt to unravel the rock-groups and internal 

 structure of Central Wales was made by Walter Keeping.^ The- 

 most important result of his investigation was the discovery of 

 graptolites in several localities, thus proving that the rocks were 

 not everywhere so unfossiliferous as had been supposed by earlier 

 investigators. By this time the value of graptolites as an aid in 

 identifying rocks had been demonstrated by the researches of Prof. C. 

 Lapworth, Mr. J. Hopkinson, and others. By some means, however, 

 the fossils obtained by Keeping at several points represent, as is now 

 known, various horizons, as if they had been obtained mainlj' from 

 loose material. He was thereby prevented from recognizing the- 

 true succession of the rocks, and was led to believe that only one^ 

 great group existed over the whole area ; most of the graptolites 

 were assigned to the Upj^er Birkhill Group of the South of Scotland. - 



1 Mem. Geol. Surv. vol. iii, 2nd ed. (1881) p. 8. 



2 Q. J. G.S. vol. xxxvii (1881) p. 141. 



