PROCEEDINGS 
OF 
THE GEOLOGICAL SOCIETY OF LONDON. 
Vot. III. 1839. No. 64. 
May 22.—William Fane De Salis, Esq., M.A., Carlton Gardens, 
and George Fleming Richardson, Esq., of the British Museum, were 
elected Fellows of this Society, and Professor Ehrenberg of Berlin 
was elected an Honorary Member. 
A paper was first read, ‘‘On the Wells found by digging and 
boring in the gravel and London clay in Essex, and on the geolo- 
gical phenomena disclosed by them,” by Dr. Mitchell, F.G.S. 
Essex consists chiefly of London clay, but that portion of the 
county which lies to the north-west of a line drawn from Harlow 
to Ballingdon Hill, near Sudbury, and the long ridge extending 
from Purfleet to East Tilbury, are composed of chalk. Extensive 
districts, however, are covered by thick deposits of gravel, sand, 
and other detritus, varying in depth from 10 to 300 feet. In 
Wakering Marshes and Foulness Island, there are 300 feet of 
sand between the vegetable soil and the London clay. 
The wells formed in the gravel are supplied by land springs, the 
water, when enough, being collected in a reservoir excavated in the 
London clay. They are often not more than 12 feet in depth; 
but it is impossible to estimate the number of feet to which they 
must be sunk in any district, on account of the great inequalities 
of the outline of the chalk. At Stanway, near Colchester, the clay 
was found to be 45 feet from the surface; but at the Union work- 
house, less than a quarter of a mile distant, and on the same level, 
it was necessary to sink 60 feet before it was reached. "When the 
London clay forms the surface there are no land-springs, as the 
clay is generally impervious to water; but in some places it is sandy, 
and permits the percolation of water. That much of the rain which 
falls in Essex penetrates downwards, is evident from the smallness 
of the number and size of the brooks and rivers. Very little water 
enters the Lea on the west side; and into the Thames only four 
streams flow between the Lea and Purfleet. There are three rivers, 
the Crouch, the Blackwater, and the Coln, 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 
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