PLATEATJ-GEAVELS OF EAST BEKKS AND WEST SURREY. 557 



33. IS'oTE on the Plate aij-geavels o/East Berks and West Surrey; 

 their Age, Composition, and Structure. By the Eev. A. Irving, 

 D.Sc. (Lond.), B.A., F.G.S, (Eead May 21, 1890.) 



(Abridged.) 



In the year 1883 my first paper on Tertiary geology * was published 

 by the Geologists' Association. In dealing with the gravels of the 

 Bagshot district I pointed out that my observations up to that date 

 had led me to regard the Plateau-gi^avels as Pre-glacial and con- 

 temporaneous with the Crag. This view has been worked out in 

 fuller detail by Prof. Prestwich in Part III. of his recent paper 

 " On the Mimdesley and "Westleton Beds and their extension in- 

 land." t It was a gratification to find myself in accord with him 

 as to their pre-glacial age. Por several years past it has been in 

 my mind to give the results of further studies in this subject to the 

 Society ; he has happily forestalled me, and brought to the con- 

 sideration of the subject a much wider range of data than I could 

 command. Still it seems that the argument by which he arrives at 

 the conclusion that these Plateau-gravels are of later Pliocene age is 

 not a very strong one ; for, when the Diestian age of the Lenham 

 deposits is admitted (and Mr. Clement Eeid's conclusion t that they 

 both are of older Pliocene age is also admitted), the assumption (which 

 forms the cardinal point of the wiiole argument) that certain outlying 

 sands high up on the slope of the North Downs are contempo- 

 raneous with the Lenham deposits has no other support than 

 their being at approximately the same height ; while the litho- 

 logical evideiice points to their Eocene age §. This might very 

 well be, if — as seems probable from various considerations — the 

 north-western portions of the Weald partook in later Eocene time 

 of the movement which is recorded in the upheaval of the axis of 

 Kingsclere and Inkpen \\. Again, there is the absence of all marine 

 deposits of Miocene age in the South of England, indicating (in 

 addition to many other collateral considerations) a high degree of 

 probability that the Miocene was the period of maximum elevation 

 of this part of Europe, rather than one of depression beneath the 

 sea. To this long period of Miocene (and Pliocene) elevation of the 

 Weald we must look, I think, mainly for the supply of the vast 

 amount of flinty subangular detritus under notice. Erom long 

 exposure on the surface of the Chalk, as chemical erosion proceeded, 



* See Proe. Geol. Assoc, vol. viii., " On the Bagshot Beds of the London Basin, 

 and their associated Gravels," 



t Quart. Journ. Geol. Soc. vol. xlvi. pp. 155 et scq. 



J See Naiure (1886), vol. xxxiv. pp. 341, 342. I have discussed this matter 

 with Mr. Eeicl, and have examined the Lenham fossils in the Museum of 

 Practical Geology. 



§ See some remarks by the author in the Geol. Mag. 1888, pp. 123, 124 : and 

 1890. p. 406. 



||_ In Topley's memoir " On the Geology of the Weald " there are many facts 

 which point to this conclusion. 



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