270 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [Apr. 16, 



eastward it seems to be thin ; and if we class the upper sands near 

 Heme Bay, &g., with the underlying "Woolwich and Beading Beds, 

 " the basement-bed itself might be considered in this area to merge 

 into the thin seam of sandy clay just at the base of the great mass 

 of the London Clay*." "Westward of Eeading it is from 2 to 5 feet 

 thick, and in Marlborough Forest it has been shown to consist merely 

 of a line of pebbles (see p. 262). 



London Clay. — Of the London Clay itself Mr. Prestwich has 

 observed the westerly thinning, as before stated. To quote his words, 

 " It would appear that the London Clay gradually expands as it 

 ranges from west to east, at first rather rapidly until it attains a 

 thickness of from 300 to 400 feet, and then very gradually until, in 

 the neighbourhood of London, it averages from 400 to 440 feet thick. 

 In the Isle of Sheppey, and on the opposite Essex coast, however, it 

 reaches its greatest development, being there apparently as much as 

 470 to 480 feet thickf." The thinning is, however, much sharper 

 on the west of Eeading than Mr. Prestwich has supposed. He shows 

 that a few miles to the south-east of that town the London Clay can- 

 not be less than 370 feet thick ; and says, '' there exist no definite 

 measurements in the neighbourhood of Hungerford or Newbury; 

 taking, however, into consideration the dip of the beds and the 

 height of the hills, I do not think that the entire thickness of 

 the London Clay there exceeds 200 to 250 feetj." During the 

 progress of the Geological Survey the data wanted for the measure- 

 ment of the thickness of the London Clay were found, and my friend 

 and colleague Mr. Bristow tells me that its thickness on the south 

 of Newbury is not more than 50 or 60 feet, and that westward 

 towards Hungerford it is, if anything, less. I have shovni that at 

 Oare, on the north of Newbury, it is less than 20 feet, the Bagshot 

 Sand being there within that vertical distance of the Eeading Beds §. 

 On the west of Great Bedwin it has been proved to be not more than 

 15 feet thick ; and in Marlborough Forest the London Clay proper 

 seems to have whoUy thinned out ||, aU that there remains of the for- 

 mation being a pebble-bed forming part of its " basement-bed." 



Of the Bagshot Beds, which belong to the Middle, and not to the 

 Lower Eocene Series, I do not now treat. Enough to say that Mr. 

 Bristow teUs me that south of Newbury the Lower Bagshot Sands 

 are at least 100 feet thick ; but that, as they are not capped by any 

 of the Middle Bagshot Beds within some miles distance, their full 

 thickness cannot be given. 



Effect of the Westerly Thinning of the Lower Eocenes. — The result 

 of the westward thinning of the Lower Eocene strata is, that in 

 that direction the Bagshot Beds gradually get nearer to the Chalk. 

 In Marlborough Forest we have seen (p. 262) that there is but 15 feet 



* Quart. Journ. Geol. Soc. vol. x. p. 130, foot-note. 



t Ibid. vol. X. p. 407. % Ibid. vol. x. p. 402. 



§ Memoir illustrating Sheet 13 of the Map of the Geological Survey, p. 54. 



[| This thinning-out does not necessarily indicate the original edge of the 

 basin, but may be for the most part due to denudation before the deposition of 

 the Bagshot Beds. 



