62
SULPHATE OF LIME IN ESSEX SOILS AND
SUBSOILS.
By T. S. DYMOND, F.I.C.
[Read January 28th, 1905]
IN his last Presidential address to the Essex Field Club, Mr.
F. W. Rudler traced the existence of Selenite in certain clay
strata to the oxidation of Pyrites in the London-clay, and the
reaction of the resulting sulphuric acid with carbonate of lime,
by which sulphate of lime is formed. The sulphate of lime
crystallizes slowly from solution at the ordinary temperature in
the form of selenite. I wish to suggest in this note that there
is another possible source for the selenite, and at the same time
briefly to discuss the practical bearing of the question upon Essex
agriculture.
The selenite crystals appear to be most abundantly found in
certain superficial clay strata overlying the London-clay. In
sinking shallow wells at Upminster and other places in South
Essex, deposits of clay loaded with selenite are sometimes met
with.1 The crystals are found in star like masses, each ray of
which consists of characteristic twin crystals superimposed
on each other. In the same part of the County, a dry bank
under a thick hedge is often found to be encrusted with a white
efflorescence of sulphate of lime, pointing to its abundance in the
surface soil ; if a drying wind occurs after rain, the surface of an
arable field will appear white for the same reason, and the farmer
finds the soil "capped" and hard. The water obtained from
wells sunk into gravelly pockets' of the London-clay is often
excessively hard, the hardness being partly due to sulphate of
lime. In one such water from Wickford I found the permanent
hardness was equal to 93 parts of sulphate of lime per 100,000,
and another from Ingrave to 112 parts. In such waters, however,
part of the hardness is invariably due to sulphate of magnesia.
That such excessive quantities of sulphate of lime are not
found associated with the Boulder-clay is sufficiently explained
by its permeability to water. The rain water draining through
the Boulder-clay dissolves from the surface and carries with it
the sulphate of lime, and the water issuing from springs at the
outcrop of the underlying gravel, contains appreciable, but not
excessive, quantities of the salt. Through the London-clay
water cannot easily percolate, and percolation is rather upwards
1 Similar deposits were struck in cutting the new Woodford and Ilford railway.