120 Geological Society : — 



the Weald and of the Thames Yalley."— Part III. On a Southern 

 Drift in the Valley of the Thames, with Observations on the 

 Pinal Elevation and Initial Subaerial Denudation of the Weald, 

 and on the Genesis of the Thames." By Prof. Joseph Prestwich, 

 D.C.L., F.B.S., &c. 



In this third part of his paper the author gave a description of 

 the characters of the Southern Drift, showing how it differs from the 

 Westleton Beds in the nature of its included pebbles, which consist of 

 flints from the Chalk with a large proportion of chert and ragstone 

 from the Lower Greensand, while there is a total absence of the 

 Triassic pebbles and Jurassic debris characterizing the Northern 

 Drift. He traced the drift through Kent, Surrey, Berkshire, and 

 Hampshire, and described its mode of occurrence. 



Another preglacial gravel was then discussed under the title of the 

 Brentwood group, and its age was admitted to be doubtful. 



The author then entered into an inquiry as to the early physio- 

 graphical conditions of the Wealden area, and gave reasons for 

 supposing that a hill-range of some importance was formed in the 

 Pliocene period after the deposition of the Diestian beds. From the 

 denudation of this ridge, he supposes that the material was furnished 

 for the formation of the Southern Drift, which may have been de- 

 posited partly as detrital fans at the northern base of the range. 



The relation of the Southern Drift to the Westleton Shingle and 

 other preglacial gravels was considered, and the Westleton Beds were 

 referred to a period subsequent to that of the formation of the 

 Southern Drift. 



The influence of the meeting of the earlier Wealden axis with 

 that of the folding which produced the escarpments of central 

 England was discussed, and it was suggested that the result would 

 be the genesis of the Thames valley and river. 



The following summary gives the results of the author's inquiry 

 as developed in the other parts of the paper. He holds : — 



1. That the Westleton shingle ranges from Suffolk to Oxfordshire 

 and Berkshire, rising gradually from sea-level to 600 feet. 



2. That the lower Tertiary strata were coextensive with this 

 shingle. 



3. That the upraising of the Westleton sea-floor, with its shingle, 

 preceded the advance of the glacial deposits, and that the latter be- 

 come discordant to the former when traced westward, occupying 

 valleys formed after the rise of the Westleton Beds. 



4. That the Tertiary strata and Westleton Beds on the north 

 border of the Chalk-basin were continuous until the insetting of the 

 Glacial period, when they were broken through by denuding 

 agencies. 



5. That none of the present valleys on the north of the Thames 

 Tertiary basin date back beyond the Preglacial period. 



6. That the same date may be assigned to the Chalk, and pro- 

 bably to the Oolite escarpments. 



7. That in the Thames basin, besides the Northern Drift, there is 



