Geological Society of London. 421 



silica and alumina as did the limestone ; and Mr. Maw concluded 

 that they were left behind in the cavities after the calcareous matter 

 had been removed by watery dissolutions. The average of a 

 number of analyses of the white Tertiary clays of Dorset, Hants, 

 and Devon, showed a similar resemblance to the average compositions 

 of a number of examples of Chalk and Chalk-marl, after deducting 

 the carbonate of lime and other matters soluble in carbonated water, 

 implying a derivation from the watery dissolution of the Chalk. 

 The geographical distributions of the Tertiary white clays seemed to 

 favor such an origin, and the improbability of their derivations from 

 the felspathic rocks was supported not only on geographical but on 

 chemical grounds. The average of the analyses of felspar showed 

 that the proportions of silica to alumina was about as three to one, 

 and in the average of a number of analyses of clays as two to one, 

 indicating that felspathic rocks could not provide the proportion of 

 alumina found in the white clays ; but the proportion of silica to 

 alumina in Chalk corresponded as nearly as possible with their 

 composition. 



14. " On the Post-glacial Structure of the South-east of England." 

 By Searles V. Wood, Jun., Esq., F.G.S. 



This paper was an outline of the principal points deduced by the 

 author from his Geological survey of the country included in the 

 Ordnance sheets Nos. 1 and 2, where the glacial clay approaches 

 nearest to the Thames Valley beds ; and from a survey on a smaller 

 scale of the glacial beds over a much larger area. Both of his maps, 

 with a manuscript memoir upon the subject, have been placed by 

 him in the Library of the Society. 



The author took up the structure at the southerly and westerly 

 edges of the principal tracts of glacial beds, and in the parts where 

 these are divided by great troughs of denudation ; he showed the 

 manner in which the denudation, commencing at the first upheaval 

 of the glacial sea-bed, has descended through the Lower Tertiary 

 and Secondary deposits, accompanied by the formation of successive 

 gravel beds during its progress. His conclusion was, that the re- 

 moval of the Lower Tertiary strata over much of the South of Eng- 

 land, and the excavation of the Weald Valley, as well as the great 

 denudation which the Liassic, Oolitic, and Cretaceous beds have 

 imdergone in the west of England, are to bo traced principally to the 

 progress of the post-glacial denudation. This denudation had its 

 inception in the upheaval of a portion of the glacial sea-bed, and was 

 accompanied by a long succession of subterraneous disturbances, 

 which have brought up the Secondary and Tertiary rocks of the 

 south and west to the elevations they now occupy, and left the early 

 emerged portion of this sea bed (represented by the detached tracts 

 of the Upper and Middle Glacial formations) at lower levels than 

 much of the denuded area of the south and west. 



The following specimens were exliibited : — 



A series of Elephants' teeth from the Norwich Mammaliferous 

 Crag and the Forest-bed of Norfolk ; exhibited by the Eev. John 

 Gunn, M.A., F.G.S., showing (a) the transition of the {Loxodon) 



