1854.] PRESTWICH LONDON CLAY. 403 



interstratified sands to afford a water supply, so that wells have been 

 carried through the London Clay, and then passed almost at once 

 into the Mottled Clay, in consequence of which works have been 

 given up and abandoned. This may have occurred at Wimbledon ; 

 and possibly no line of demarcation having been drawn between the 

 two clays, they were put down together as one mass of London Clay *. 

 We know the thickness of the London Clay to be 230 feet deep at 

 the Asylum at Garrett, and 210 feet at Mortlake ; if to these we add 

 the height of the Wimbledon Hill above those places, it will hardly 

 give a thickness of 400 feet to the London Clay at Wimbledon. The 

 line of section (fig. 1, p. 404) between Hampstead and Wimbledon, 

 which takes the evidence afforded by wells at intermediate stations, 

 shows a structural result in conformity with the foregoing obser- 

 vations f. 



The other case, at High Beech, arises from an error, originating 

 apparently with the Trigonometrical Survey %, in which that spot is 

 stated to be 750 feet high. Phillips and Conybeare, knowing the 

 height of the ground at a well at Epping to be 340 feet above high 

 water mark, and the thickness of the clay to be 392 feet, added to 

 the latter the additional height of the ground at High Beech, or of 

 300 feet more, — giving an estimate, therefore, of about 700 feet. 

 I find, however, by observations with the aneroid barometer, that 

 the height of High Beech above the sea level can hardly exceed 440 

 feet ; and further, I find that the hill is capped by a thin covering of 

 gravel and Bagshot sands, amounting together probably to about 20 

 or 30 feet. In the valley of the Lea, to the west of High Beech, 

 the depth to the base of the London Clay is about 50 feet § at 

 Waltham Cross, 40 feet at Waltham Abbey, 122 feet at Sewardstone, 

 and 80 feet at Turkey-street, Enfield, whilst at Northaw the chalk is 

 at the surface in the valley to the east of the village. Again, at 

 Loughton, a few miles east of High Beech, a well has traversed the 

 London Clay, and reached the Chalk at a depth of 320 feet. Now, if 

 we run a line of section (fig. 2, p. 404) from Northaw, across the valley 

 of the Lea, through High Beech, to Loughton, and take a calculated 

 thickness afforded by the several above-mentioned well-sections 

 (though our line does not exactly pass through any of them), we 

 may approximate very closely to the thickness of the London Clay 

 at these places, and conclude that at High Beech it cannot exceed 

 430 to 440 feet. At Hunter's Hall, near Epping, and at a height, 

 according to Conybeare and PhiUips, of 4 1 feet above the Thames, 

 the Lower Tertiary sands were reached at a depth of 350 feet. If 



* Allowing for this, the reported thickness still seems 100 feet too much. 



f Mr. Mylne informs me that the Chalk has recently been reached at Mr. 

 Beaumont's, on the east side of Wimbledon Common, at a depth of 465 feet. 

 —J. P., Jun., Sept. 1854. 



t Vol. iii. p. 306. Langdon Hill, Essex, is given at 620 feet ; but this hill surely 

 cannot exceed about 400 feet above the sea level. 



§ I am not quite satisfied as to the exact correctness of these measurements, since 

 in some of the wells the Mottled Clay has, I suspect, been included in the thick- 

 ness of the London Clay. Making, however, these corrections where they seem 

 necessary, these numbers cannot be far from the truth. 



