304 PROP. T. Gr. BONNE! ON THE [May ICJOOy 



Discussion. 



Mr. Strahan said that he was glad to see the Author once more 

 engaged upon the interesting problem of the origin of the Bunter 

 sediments. He thought that a comparison of them with the breccias 

 at the base of the Keuper Marls was instructive. The latter con- 

 sisted of material derived from the cliffs against which they rested ;. 

 they seemed to have travelled a few yards only, and resembled scree 

 that had fallen into water. The Bunter pebbles, on the other hand, 

 bore evidence of a long journey, and generally were so perfectly 

 rounded as to suggest that they must have been derived from some 

 pre-existing conglomerate. 



As to the direction of their transport, he recalled the fact that 

 the pebbles decreased rapidly in size from south to north. The 

 shingle-beds of Shropshire passed into thick sandstones with a few 

 insignificant pebbly seams in Cheshire and Lancashire. This increase 

 in the thickness of the sediments pointed to the area of maximum 

 subsidence having lain to the north, and both this and the distri- 

 bution of the pebbles suggested transport from the south. This was 

 an old objection to the Author's theory, and presumably would be 

 met by him. His precise observations on the composition of the 

 pebbles could not fail to be of the greatest value in the investigation 

 of this problem. 



Prof. Sollas remarked that the Author's theory of the fluviatile 

 nature of the Bunter Beds was becoming very generally accepted 

 both in this country and abroad. The case presented in favour of a 

 northern origin for the bulk of the material forming the Pebble-beds 

 was no doubt a strong one : but it was not without its difficulties, 

 and the objection urged by Mr. Strahan required to be met. A river- 

 system might include tributaries from very different sources, and the 

 speaker was disposed to think that some of the Bunter pebbles of 

 the Midlands had been brought from the south. The remarkable 

 similarity between the material of the pebbles of Budleigh Salterton 

 and that of the Gres de Mai could not fail to impress the observer : 

 it was not merely lithological, but paheontological ; and few would 

 dispute that the Budleigh-Salterton pebbles had a southern origin. 

 But it was admitted that pebbles as similar, lithologically and 

 palasontologically, to the pebbles of Budleigh Salterton as these 

 are to the Gres de Mai occurred in small numbers in the Midland 

 district. Again, while the Bunter thickened from the Midlands 

 towards the north-west, in Devon it thickened towards the south ; 

 and thus precisely the same kind of evidence which was adduced 

 in favour of a northern origin also existed in favour of an origin 

 from the south. During the deposition of the Bunter, South 

 Wales and Somerset formed a tract of dry land, which would have 

 intercepted the current of a river flowing northward from Budleigh 

 Salterton ; but this barrier might have been easily turned on the 

 east, and the stream of Gres-de-Mai pebbles might possibly have 

 passed along a line between the Mendips and the Wiltshire Downs. 

 The difference in form of the Budleigh-Salterton pebbles and those 



