532 Geological Society. 



streams flow between the Lea and Purfleet. There are three rivers, 

 the Crouch, the Blackwater, and the Coin, but they are small, and 

 can carry off only an inconsiderable portion of the water, which falls 

 on about a million of statute acres. 



The London clay in Essex is of great but variable thickness. It 

 is seldom, however, that its actual dimensions can be ascertained, 

 for though the depth of the wells is known, accurate details of that 

 at which the clay commenced and terminated have not often been 

 preserved. Dr. Mitchell gives the following list of the total depth 

 of wells, selected from a very large number : — 



Stratford 247 feet. 



Ilford 301 — 



Dagnam Hall 404^ — - 



Brook-street 340 — 



Upminster 192 — 



Parsonage, Warley 390 — 



Grange Hill, near Fairlop 398 — 



Dunton 344 — 



Battle Bridge 350 — 



Ferry-house on the Crouch 360 — 



Rochford Union workhouse 330 — 



Wakering Marshes 400 — 



Foulness Island < , '\kk'L ■> nr,\ 460 — 

 l^clay, 100 to 160 J 



Clay- street, Walthamstow 190 — 



Lough ton, in Epphig Forest 324 — 



Epping 270 — 



Horsley Park, near Ongar 340 — ■ 



Booking 370 — 



Braintree 420 — 



This variation the author conceives, is partly due to the uneven- 



ness in the surface of the chalk ; but in some instances to the un- 



dulatory nature of the country, the difference in the depth of the 



wells agreeing with the increase in the rise of the ground. When 



this is the case, the bed in which the water is found is the same in 



the adjacent wells, and consequently the variation in the outline of 



the surface is due to denudation, and not to unequal elevation. 



Thus, in the two wells close to the turnpike at Romford, water was 



found at the depth of 100 feet, but half-way up the hill between 



Hare-street and Havering Ate Bottler at the depth of 250 feet ; at 



Bocking it was obtained at 370, but at higher ground, at Braintree, 



close adjoining, at 420. Again, at the union workhouse in Rochford, 



the well is 330 feet deep, and at Stroud Green, on the road to Rug- 



leigh, where the surface is higher, it was necessary to sink 390 feet. 



At North Fambridge is a well 388 feet deep, the water rising to 



within 10 feet of the top : but at another well in the same parish, 



dug in lower ground, there is a constantly flowing stream. 



In the New River Company's well at the end of Tottenham Court 



Road, chalk was found at the depth of 150 feet ; but in that near 



Pond-street, at Hampstead, the main spring in the bed of sand 



