Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/193

 case that a brook ran along the line of junction, undermining the softer beds, bringing them down into the stream, and then removing them. It was thus that escarpments receded.

Prof. Morris remarked that at an early period the Alps on the south, and the Cumberland mountains on the north, formed the boundaries of a sort of trough, and that this to some extent must have influenced the flow of the rivers both in Britain and on the Continent. He considered that the series of elevations in pre-Permian times had also much to do with the configuration of some parts of the country, and therefore of its river-basins. The evidence of the Oolitic series was that it was deposited in an area of gradual depression, which was subsequently again elevated ; and there was no doubt of the existence of a large amount of land over a great part of Central England during the deposition of some of the later Oolitic beds. Then again came a depression during the period of the White Chalk. With regard to the Severn valley, he recalled the observations of Sir R. Murchison as to its having been an ancient marine channel, connecting the estuary of the Ribble and what is now the Bristol Channel. He cited Prof. Phillips to account for the presence of the Lickey quartz pebbles in the valley of the Thames by the existence of ancient lochs in the Glacial sea.

Mr. Whitaker remarked on the probable extension of the Chalk as far as the Scilly Islands, which was evinced by the flints found on the surface there and in the Land's-End district. He attributed the fact of so many of the streams breaking through the chalk escarpment on the south and so few on the north in the London basin to the difference of the dip in the two cases.

The President could not give in his adhesion to Prof. Ramsay's opinion. To establish so general a view as that propounded, he thought that a more extensive array of facts with regard to the conditions of the river- valleys should have been adduced. He wished for evidence as to the existence of old river-gravels at a greater elevation above the present river Severn, for instance, than that adduced by the author. The elevation of the Alps he regarded as not sufficient to account for the lines of drainage in Britain. It was to be borne in mind that during the Quaternary period the excavatory force of the rivers was much greater than at the present day. He thought there was still much to be learnt as to the causes which led to the direction and extent of the present river-valleys, the original rudiments of which were probably due to other causes than river- action.

Prof. Ramsay, in reply, was inclined to restrict himself to the immediate subject of his paper. With regard to the so-called Straits of Malvern, he accepted the view so far as it assumed that an ancient river-valley had, by submergence, been converted into a strait. He had purposely omitted in his paper all consideration of the Glacial period, for the simple reason that the initial direction of the river- valleys had been given in preglacial times. His object was merely to show the causes of the initial direction of the rivers ; and he could not be expected, in a paper before the Geological