1854.] PRESTWICH LONDON CLAY. 407 



to the Lower Bagshots ; they are from 5 to 20 feet thick, and are 

 overlaid by 5 to 10 feet of gravel. It is difficult to measure the exact 

 height of these clay cliffs, owing to the continual fall of their upper 

 portion, and to the broken and sloping surface which they conse- 

 quently present. The mean of several observations with an aneroid 

 barometer, and of an approximate measurement with a tape line, gave 

 about 1 30 feet as the height of the cliff at the Coast-Guard station. 

 East End. 



Thence, passing over to Essex, we reach the London Clay cliffs of 

 Southend. A few miles north of this town there are several deep 

 Artesian wells. One at the Union at Rochford traverses the London 

 Clay to a depth of 330 feet, before reaching the Lower Tertiary sands. 

 In another, at Stroud-green, the London Clay is 390 feet deep, whilst 

 at Rayleigh it is 400 feet deep. Allowing for the difference of level, 

 and connecting these several well-sections, the London Clay will be 

 420 feet deep at the point of intersection with our plan. At a short 

 distance from this spot, and nearer Rayleigh, the hills which there 

 attain a height, by aneroid barometer, of about 180 to 200 feet, are 

 capped by 20 to 30 feet of Bagshot sands. Descending into the 

 valley of the Blackwater, at Battle-bridge, an Artesian well has been 

 sunk through 350 feet of London Clay to the Lower Tertiary sands. 

 Connecting together the different points determined or estimated on 

 this line of section, we come to about the same result as in the pre- 

 vious instance, viz. that the London Clay at Sheppey is probably 

 from 470 to 480 feet thick, and that it is about the same on the 

 opposite coast of Essex. 



At Foulness Island, in Essex, there are Artesian wells through the 

 London Clay 400 feet deep. Beyond this place the London Clay gra- 

 dually rises, its base coming apparently within 20 or 30 feet of the 

 surface at the river level at Colchester, and at the sea level at Walton 

 and Harwich ; whilst at Ipswich the Lower Sands come to the surface. 



From the foregoing observations 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 thick. As the 

 regularity of this development does not agree with opinions I have 

 frequently heard expressed on this subject, I may mention in 

 explanation, although probably hardly necessary, that the London 

 Clay was not spread over a denuded land surface, but was a con- 

 tinuation of a series of marine and estuarine deposits, which had 

 previously filled up and smoothed over the irregularity of the old 

 chalk surface. The lenticular shape of this large mass of clay (for it 

 thins off again eastward as it ranges through Belgium) is very 

 striking, and has an interesting theoretical bearing. 



* I am now of course speaking of it apart from the local rliraimition of its thick- 

 ness produced by denudation, which necessarily gives it at present a very variable 

 thickness. 



VOL. X. PART I. 2 F 



