402 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 24, 



in the state in which it was left by Phillips and Conybeare in 1822. 

 The impression likely to be conveyed by the observation on this 

 subject in their valuable work * is one which would suppose its 

 development to be more irregular, and in places considerably greater, 

 than will, I believe, prove to be the case. They state, — " The actual 

 thickness of the London Clay in Sheppey may be estimated, by 

 adding the height of the cliff to the depth of the wells, at 530 feet." 

 They then proceed to say that " it may be supposed to be much 

 thicker in Essex," estimating it at " High Beech to be about 700 

 feet thick ; " and further, they report it to have been pierced at 

 Wimbledon to the depth of 530 feet, without passing through it. 

 They then give a number of lesser measurements, but without stating 

 whether or not the upper beds have been denuded. 



In the following estimates I have deemed it necessary to take only 

 such lines of section as aiford a definite upper horizon — the one 

 obtained by the superposition of the Bagshot sands. The base of 

 the London Clay I have ascertained in all cases practicable by means 

 of well-sections. 



The fine and complete coast sections in the Isle of Wight show 

 that the total thickness of the London Clay at Alum Bay is 193 

 feet, and at White Cliff Bay 363 feet. The Artesian well at 

 Southampton affords another exact measure of 320 feet. Passing 

 over to the London Tertiary area, there exist no definite measure- 

 ments 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 feet, which increases, as it ranges westward 

 towards Basingstoke and Odiham, to 330 feet f . In the neighbour- 

 hood of Reading and Wokingham, judging from a well commenced 

 at Bear Park, the London Clay cannot be less than 370 to 400 feet 

 thick ; whilst at Chobham, where it is overlaid by 265 feet of Bag- 

 shot sands, it was roughly estimated by the well-digger at 400 feet. 

 In the north part of Windsor Forest it has been found about 350 

 feet thick, and if to this we add the height of the ground to where 

 the Bagshot sands set in, it will also give us a thickness of about 

 400 feet. At the foot of Hampstead Hill, in the Tottenham Court 

 Road, and Regent's Park, the I<ondon Clay is from 70 to 140 feet 

 thick ; at the Lower Heath, 285 feet. If to this we add the depth 

 from the top of the hill, and take off 25 feet, as the thickness of the 

 capping of gravel and Bagshot sands, it will give, m round numbers, 

 a total thickness of about 420 to 440 feet to the London Clay at that 

 spot (see fig. 1, p. 404). 



With regard to the exceptional thickness of 530 + and of 700 feet, 

 assigned to the London Clay at Wimbledon and High Beech by 

 Phillips and Conybeare, there are, I believe, errors in both instances. 

 In the first case, the Mottled Clays beneath the London Clay are 

 sometimes so largely developed, that in places not far distant (as at 

 Chertsey and Chobham) there frequently is no sufficient mass of 



* Geology of England and Wales, p. 33. 



t Well-section at Dogmersfield, Jonrn. Geol. See. vol. x. p. 97. 



