VALLEY AND ITS IIELATION TO THE WESTLETON BEDS, ETC. 181 



affected the Weald and surrounding districts, and referred to Mr. 

 Poulett Hcrope's views that during the elevation of the Weald the 

 beds glided over others, which ^\'ould account for the varying thick- 

 ness of such deposits as the Gaiilt. 



Dr. Evans congratulated the Society and Prof. Prestwich on his 

 having been able to sum the results of the observations of so many 

 years in the series of papers which he had lately read. In taking 

 samples of gravel, and attempting to correlate beds of gravel over a 

 large area, mistakes might arise from local variations ; but he 

 thought that the facts brought forward by the Author went to a 

 large extent to confirm his views. Possibly some further confirma- 

 tion might be found in the country to the west of the axis of eleva- 

 tion that bounded the Westletou Beds. He hoped that Prof. Prest- 

 wich would allow a longer period of time to have elapsed for the 

 production of the i^hysical changes he had described than he 

 had previously allowed for Postglacial changes. As one of the 

 older Fellows of the Society, it would have been impossible for 

 him to refrain from saying a few words in congratulation of one 

 with whom he himself had laboured so long. 



The AuTHOK, in reply, was not surjirised at the difficulty speakers 

 had experienced in following all the details of so intricate a subject. 

 He acknowledged the assistance derived from the admirable Drift- 

 maps of the Geological Survey. He had differed from them in ven- 

 turing upon a chronological classification of the pre-glacial Drifts. 

 He had attempted to describe three kinds of hill-drifts — the AYestle- 

 ton, Brentwood, and Southern Drifts. The Brentwood Drifts certainly 

 originated with the Bagshot Beds, but showed differences which allied 

 them to the other Drifts. He often found a sufficient number of 

 lagstone- and chert-pebbles to show that the Bagshot Beds had 

 been intruded upon and disturbed. In some cases, where infiltration 

 had been suggested to account for the upright position of the 

 pebbles, the pebbles rested upon clay-beds, where infiltration was 

 impossible. He had felt the same difficulty as Mr. Whitaker as to 

 the gravels of Well Hill, and could only suppose they had been 

 derived from a destroyed hill-range. Amongst others Alderbury 

 Hill, three miles from Salisbury, was capped by gravel full of chert 

 and ragstone like the southern drift of the Thames Basin. No doubt, 

 as Mr. Topley had said, the denudation of the Weald commenced 

 in early Tertiary times; but he believed that the great upheaval 

 to which he, the Author, referred took place after the spread of the 

 Leuham Beds. 



