180 i'kof. j. prestwich on a southekn drii'x in the thames 



Discussion. 



The Chaieman remarked on the difficulties connected with the 

 identification of gravels over large areas, and with the problems of 

 Tertiary phj'siography. 



Mr. WhitakePv noticed the wide range of the paper, and believed 

 that there was no problem more difficult in the geology of the South of 

 England than the classification of various deposits of gravel. Prof. 

 Prestwich, he understood, put in the same class the gravels of Nor- 

 wood, Hampstead, Eayleigh, and of the hills between Faversham 

 and Canterbury. The geological surveyors had not felt that they 

 could successfully correlate these various gravels. At IS'orwood, 

 Mr. Spurrell considered the gravel was an old Wandle gravel, while 

 he himself rather inclined to sujDpose it an old Thames gravel. The 

 geological surveyors had coloured certain gravels as plateau-gravels, 

 without any attempt to assign them to any definite age. He (the 

 speaker) had noticed pebbles on end in many gravels. When Prof. 

 Prestwich supposed that some gravel was synchronous with Crag- 

 deposits, he could not follow his line of reasoning. As regards the 

 supposed Wealden ice-field, he felt that the beds were not strong 

 enough to carry much wear, and that the hills would soon be denuded. 

 The pebbles in the Ped Crag showed that the sea of that age extended 

 to the Lower Greensand, pebbles of Ammonites hiplex- which had 

 been phosphatized in Lower-Greensaud times occurring in the 

 phosphate-de^Dosits of Ped-Crag age. Between Southampton and 

 Winchester he had seen hardly any high gravel which he was inclined 

 to classify as Drift. He agreed with the Author as to the import- 

 ance of insisting on the occurrence of a southern drift ; but he 

 pointed out the difficulties in identifying gravels by their included 

 fragments. Whether the breaching of the escarpment of the Chalk 

 was so late as the Author had considered it seemed to him doubtful. 



Dr. Ievixg said that he was in accord with the Author as to the age 

 of the plateau-gravels of Berkshire. He had published his views as 

 to the fluviatile character of these gravels, and now brought for- 

 ward evidence of glaciation at lower levels, proving their pre- 

 glacial age. 



Mr. Toi'LEY was interested in the distinction between Southern 

 Drift and Westleton Shingle. He had not altogether appreciated the 

 distinction Avhich Prof. Prestwich had endeavoured to draw, and 

 thought that some of the materials of a river would be eliminated as it 

 flowed further from its source. As regards thedenudation of the Wcahl, 

 the Author had shown that the denudation of the Chalk had ex- 

 tended so far that Tertiary deposits rested directly upon the Lower 

 Greensand. He did not follow the Author's mode of estimating the 

 lieight of the Wealden hill-range, for much denudation must have 

 'succeeded the deposition of the Tertiary beds previous to the forma- 

 tion of the Southern Drift. He thought changes of drainage in the 

 Weald were frequently due to the cutting back of escarpments, and 

 he gave an illustration of the method in which this might be done. 

 He called attention to the important earth-movements which had 



