Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/192

 same lines as that of the modern easterly streams. He pointed out that the great W.N.W. disturbances, many, if not all, of which were in part post-Tertiary, must be taken into account in this inquiry : e. g. the synclinal of central Devon running into the English Channel near the Isle of Wight — the anticlinal of the British Channel and the Weald, which we know was a barrier in pre-Carboniferous times, from the different character of the Coal-measures of Wales and the Culm-measures of Devon — Mr. Godwin- Austen's ridge bringing up the old rocks under London — the barriers which caused the Lower and Upper Silurian of S. Wales to differ so much from those of N. Wales and the Lake-district, and were the indirect cause of the bosses of Silurian rocks which project through the newer rocks in Central England — the barriers that divided the northern coal-fields — the Craven Faults and the great valley which runs along them, — these and many others have obviously affected recent denudation. A slight tilt to the west would send the drainage again to the west along some of them ; and the question involved the consideration of all traces of changes of level.

Prof. Duncan observed that one important point in the paper was the hypothetical dip of the Chalk, on which the existence of the Severn was made to depend; and commented on the denudations which must have taken place during the Glacial and Pliocene periods. He differed from the author in his view of the character of the Oolitic period, which he regarded as one of great oscillation. As to the amount of Palaeozoic land-surface in Cretaceous times, he maintained that the purity of the Chalk deposits and their freedom from any terrestrial waste bore evidence of the distance of the land at that time. The depth of the sea in which they were formed was immense ; and in the Upper Cretaceous period the oscillations were also great. He disputed the fact of the Miocene period of Europe having been continental in character, especially as regards the upper and middle parts of the deposits, in which Corals abundantly occurred. The elevation of the Alps was, he maintained, of a slow progressive character, which could hardly have affected so great an area as supposed by Prof. Ramsay.

Mr. Evans called attention to the relation of the present flow of many rivers to the last elevation of the land at the close of the Glacial period. The deposits of the Severn valley, he thought, proved its preglacial origin, and consequently supported Prof. Ramsay's argument ; but the condition of the land at the close of the Glacial period was also to be fully taken into consideration, as the previously existing channels had in many instances been obliterated during that period. To a great extent Mr. Evans agreed with Prof. Ramsay, but he would wish to see the explanation carried down to a later date.

Mr. Green remarked, in illustration of the retrogression of escarpments, that he had had some opportunity of observing the process while still in progress. In the Carboniferous rocks of the north of England, where the dip of some hard rock was in a certain direction and it was overlain by softer strata, it was constantly the