18 ME. W. WHITAKEK OX THE [vol. lxxv,. 



shown only on the east, in Sheppey, the one place where Bagshot 

 Sand occurs in the county, and it is the western part with which 

 we are concerned. 



At Shooter's Hill, however, there would seem to be but very 

 little of the London Clay missing, for the sandy or loamy topmost 

 part of the formation, to which the name Claygate Beds has been 

 given, is present. Of late years this has been carefully observed 

 by Mr. A. L. Leach, and his work has led him to go into the ques- 

 tion of the thickness of the London Clay, with the following result : 



' The data at present available indicate 300 ft. as the possible maximum 

 thickness . . . but this estimate is probably too great. It is, however, 

 sufficiently clear that the London Clay of Shooter's Hill is much thinner than 

 that of Hampstead, Essex (530 ft.). Windsor, and Sheppey (480 ft.)- and that 

 this diminution is not due to denudation but is the result of physiographic 

 conditions prevailing during its deposition.' l 



Now, this thinning, to the extent of 200 feet or more, between 

 South Essex and Xorth Kent, if continued southward would bring 

 the Bagshot Beds on to the Blackheath Beds. Whether the 

 decrease in thickness of the London Clay results from original 

 deposition in thinner beds southward, or from erosion of the top 

 beds, does not affect the question under discussion : possibly both 

 methods may have been effective in the south, where the evidence 

 has been destroyed. 



I must own to having been somewhat shocked at first by my 

 friend's depreciation of the London Clay ; but now it is welcome. 



Crossing to the Hampshire Basin, at the far west, in Dorset, we 

 find that the Bao'shot Beds have been described bv Clement Reid 

 as becoming gravelly, like the Reading Beds, but more markedly 

 so. They contain much Greensand chert, quartz-pebbles, and 

 subangular pieces of Palaeozoic rocks, besides Purbeck rocks. 

 Moreover, they cross over the London Clay, so as to rest on 

 Beading Beds, and finally pass over the latter westward and rest 

 on the Chalk. It is inferred that westward ' within a few miles 

 at most the Cretaceous strata must have been completely over- 

 lapped.' 3 These beds, however, are unlike the pebble-beds with 

 which we are concerned and far from any of them : indeed, they 

 are more like Drift-gravel. 



It is between the Test and the Itchen that the Bagshot Pebble- 

 Beds occur in force, and they have been mapped, at or close to the 

 junction of Sheets 299 & 315, at Ganger Common and Knapp 

 Hill, while a large sheet, in which a thickness of over 20 feet, 

 may be reached, stretches from south of Hursley, over Hocombe 

 Plantation and Hiltingburn Common to Cranbury. This is barely 

 divided from an outlier on the east, in Cranbury Park, which again 

 is barely divided from the outlier of Otterbourne Hill, in which the 



1 Proc. Geol. Assoc, vol. xxiii (1912) pp. 115-18 ; 181. 



- ; The Geology of the Isle of Purbeck" Mem. Geol. Sru-v. 1898, pp. 191, 

 194-96 ; and ' The Geology of the Country around Dorchester ' Mem. Geol. 

 Surv. 1899 r pp. 27-32. 



