268 PROr. C. LLOYD MORGAN ON THE PEBIDIAN 



result of great thrusts from the north-west, and that they had been 

 folded over a crystalline axis (Dimetian) in Pre-Cambrian times. In 

 Post-Cambrian times there had been a great fold of Cambrian and 

 Ordovician rocks over the Pre-Cambrian axis, and immense dislo- 

 cations of the strata had taken place, especially in the arch-limb of 

 this fold. On the north-west side of the axis the beds have been 

 intensely cleaved and sheared, and newer rocks have been thrust 

 for great distances over older rocks ; whilst on the south-east side 

 they have been inverted and faulted so as to cause the older beds to 

 overlie the newer. On this side the rocks are much less cleaved 

 and sheared, and the overthrusts are of less magnitude. Paults were 

 referred to in the fossiliferous strata showing displacements of from 

 500 to 12,000 feet. A great anticlinal arch bent over a core of 

 Archaean rocks, and broken into a succession of sheared surfaces, 

 could alone furnish any satisfactory explanation of these conditions. 



I'he Pebidians themselves, both the volcanic and sedimentary 

 portions, had also been sheared. Whatever might be the relations 

 in other areas, here there was an immense unconformity. Where 

 could all these pebbles have come from ? He referred to the existence 

 of large pebbles of various rocks in the conglomerates in different 

 areas, and maintained that the Pebidian had been folded, altered, 

 and cleaved before a stone of the Cambrian had been deposited. 



As regards the Dimetian, he was not surprised at the difficulty 

 the Author found in mapping it. He himself now admitted that 

 the rock was originally, like the massive Archaean gneisses, of 

 igneous origin. It was of Pre-Cambrian age ; but bethought it had 

 been too much crushed to assume a gneissic character. The pebbles 

 at Chanter's Seat were undoubtedly fragments of Dimetian. 



Prof. Blake thought the views of the two disputants were not so 

 far removed in matters of fact ; both agreed that there was a well- 

 marked overlap or unconformity. In so far as details were concerned, 

 he could verify the sections given ■ by Prof. Morgan. The main 

 question is the importance to be assigned to the unconformity. He 

 formerly thought it very great ; but later work in Caernarvonshire, 

 and still more recently in Shropshire, had shown that there was a 

 volcanic series more satisfactorily classed with the Cambrian than 

 with the underlying series, and of this the St. Davids rocks might 

 possibly be the equivalents. 



Prof. Hughes regretted the absence of the Director-General, as 

 the paper seemed to be chiefly a re-opening of the old controversy in 

 Avhich he had ably expounded the same view as that now put 

 forward. The paper secondly suggested corrections of details in the . 

 interpretation of the structure of the district, which, though valuable, 

 were not of great importance in the general question. 



He would have been glad if the six-inch maps on which the 

 Author had drawn his lines had been produced, if only to be sure 

 that they were speaking of exactly the same sections. 



He gave diagrammatic illustrations of his own view that the 

 Cambrian was unconformably trangressive across the Archsean, and 

 that the constituents of the conglomerate varied according to the 



