Geological Society. 533 



between the London clay and the chalk, was 330 feet from the 

 surface. 



The London clay in Essex varies greatly in colour, being in some 

 places yellow or red in the lower part, but in many localities it is 

 blue to the bottom. It is sometimes uniform in composition through- 

 out, but more frequently, even when only 100 feet in depth, divided 

 into two or three portions by beds of sand. In the well at the site 

 of Fairlop Fair it was 398 feet thick, and uniform throughout. In 

 the Dengey and Rochford hundreds, where the clay is from 300 to 

 400 feet in thickness, it is divided by beds of sand into three or 

 four parts. A bed of sand also usually occurs between the clay and 

 the chalk. These alternations Dr. Mitchell is of opinion, indicate 

 successive periods of turbulence and tranquillity. 



A sufficient supply of water is sometimes obtained in the first bed 

 of sand, but it is more often necessary to sink to that resting imme- 

 diately on the chalk, on reaching which a vast volume of water 

 rushes up, and compels the well-digger to ascend precipitately to 

 the surface. Cement-stones are sources of great impediment, par- 

 ticularly to well-borers, a week or fortnight being occasionally spent 

 in punching through a single mass. At the bottom of the clay a 

 layer frequently occurs, and is technically called the water-rock, 

 because, being penetrated, a powerful spring rushes up. 



The water is sometimes, but not very often, combined with a saline 

 substance, probably sulphate of magnesia, as that salt is abundant in 

 the waters of the London clay in Surrey, and solid magnesia occurs 

 at Stamford Hill, near London. Foul air is not unknown in the 

 wells, though it has done little harm in Essex. Its nature has not 

 been ascertained, but Dr. Mitchell conceives, that it is probably 

 sulphuretted hydrogen, as in Middlesex and Hertfordshire that gas 

 has been most destructive. In the chalk of Surrey carbonic acid gas 

 is very troublesome, and has sometimes produced fatal effects. 



There is, perhaps, no part of the world where artesian wells are 

 more general, or are more useful than in Essex. In the vale of the 

 Lea they have been bored with the greatest facility and at a small 

 expense. In Waltham Abbey the cost is usually about 16^. In 

 the district of Bulpham Fen, seven miles south from Brentwood, they 

 yield a large supply of water. In the marshes, as well as along the 

 coast, and in the islands of Essex, they have proved of the greatest 

 utility. Formerly, in some seasons, when the ditches became dry, 

 the cattle suffered, the fishes died, and the farmer lost severely on 

 his stock ; but by the aid of artesian wells the ditches are now kept 

 full all the year, and the farmer and landlord are accordingly bene- 

 fited. In Foulness Island there are no natural springs, and until 

 lately no water, except atmospheric, collected in the ditches. In hot 

 seasons this water became putrid, but the inhabitants and the cattle 

 continued to partake of it as long as it lasted ; and supplies were 

 then obtained, at the distance of seven miles, from the east end of 

 the island. Artesian wells now keep the ditches full of fresh and 

 sweet water, labourers are obtained at reduced wages, and farmers 



