102 Fisher—Natural Pit at Lexden. 
a mile distant to the east. It so happens that several Artesian wells 
have been sunk not far off, so that we know the sequence of beds 
in the neighbourhood. The late Mr. J. Brown, of Stanway, sup- 
plied me with the following section of the well at the Colchester 
Water-works, which is situated on the edge of the same valley a 
mile to the east. 
Section of the Artesian Well at the Colchester Water-works.* 
Vegetable soil and low-level gravel . . . 12 feet. 
London Clay, with much green sand in the 
lowerbeds tale a ue eyes. epoca nieie ro ghte il Oommen 
Woolwich and Reading beds. . . .. . 20 4, 
Chalk-borederr suas op. piel en ve 
Plenty of good' water at . - - 9. . . . 294 5, 
At another well, which was commenced, but abandoned, at the 
Cavalry-barracks, about two miles tc the south-east of the subsi- 
dence, the thickness of the London Clay and Lower Tertiaries was 
found to be 211 feet, and the Chalk was reached at 237 feet. 
Noticing that at the water-works the Chalk was reached at 142 feet, 
this gives the difference of depth for the top of the Chalk, at the 
two localities, as 95 feet, which must be very nearly the difference 
of surface-level of the two places. This shows that the surface of 
the Chalk is, in all probability, nearly horizontal, and consequently 
deeply covered by clays at Lexden. We are precluded, therefore, 
from attributing the subsidence to an ordinary pipe in the Chalk,f 
for such could not have been formed beneath so great a thickness of 
clays. Had Chalk been the immediate subsoil, such a subsidence 
might have occurred; for it is probable that the pipes formed by 
the percolation of rain-water through Chalk are not always filled up 
by the falling-in of the superficial beds keeping pace with the forma- 
tion of the cavity. The workmen at Horstead Chalk-pit in Norfolk 
(where the Chalk is wrought beneath a thick covering of Norwich 
Crag) assured me that they occasionally came upon hollow pipes, 
roofed over by the ‘uncallow,’ i.e. the Crag deposits. This is quite 
probable, on account of the firmly compacted bed of great flints 
which immediately cover the Chalk. But such an explanation will 
not apply to our present problem. 
That there must be a vacuity somewhere beneath the subsidence 
is clear. That it should be in the gravel is impossible, because the 
stratification, as exposed in the sides of the hole for about 10 feet, is 
perfectly regular; that it exists in the London Clay is also im- 
possible ; but that such a cavity should exist in the Chalk is pro- 
bable. I conceive it may be connected with the chasm from which 
the supply of water to the Colchester Water-works is obtained. 
* IT am indebted to Mr. Prestwich for a correction to Mr. Brown’s note, as also 
for the measurements of the well at the Cavalry-barracks. 
t See the Author’s paper ‘On some natural Pits on the Heaths of Dorsetshire,’ 
Quart. Journ. Geol. Soc., vol. xv. p. 187. 
