THE SHALLOW AND DEEP WELL WATERS OF ESSEX. 31
beds again yield us samples ; but at Great and Little Braxted,
Great and Little Totham, Wickham Bishop, and Tiptree Heath,
where the Glacial gravel is raised and exposed so as to be beyond
the influence of the Boulder clay, we also obtain water almost
destitute of carbonate of lime. In the Chelmsford and Maldon
districts these are the only parishes yielding such waters. In all
others, we find that most of the water in the gravel must have
percolated through the Boulder clay which, in some places, is far
more pervious than is generally supposed. The springs at Writtle,
Roxwell, Chignall, and Ford End are at the edge of patches of
gravel, most of which is covered with Boulder clay. As this latter
contains a considerable amount of chalk, the water, in percolating
through, dissolves a certain amount, and its temporary hardness, due
to the dissolved carbonate of lime, is increased (vide analyses). I
have also given a few analyses of waters obtained from wells sunk
through the Boulder clay to the gravels and sand lying between this
and the London Clay, and it will be observed that they contain from
ten to thirty grains of carbonate of lime per gallon.
At present, therefore, I have no evidence that there is any
difference between "Bagshot" and "Drift" waters, except such
as can be accounted for by the influence of the Boulder clay,
or the proximity of villages with defective sanitary arrangements.
In certain localities, I have observed a peculiarity of the Boulder
clay water which I find other analysts and medical officers have
noted, viz., that the water, when freshly drawn, has the odour of
rotten eggs. This smell is due to a trace of sulphuretted hydrogen;
but how the gas is produced, and why peculiar to localised areas, I
cannot explain. It is very probably formed by the reduction of a
sulphate by some peculiar organism, or by dead organic matter,
vegetable humus. The general public, however, invariably ascribe
the smell to sewage pollution ; but my analyses do not confirm this
opinion. The smell very rapidly disappears if the water is left in an
open vessel, the gas being oxidized by the exposure to air.
In other districts, again, we meet with waters containing much
sulphate of magnesia. In such cases the water seems to be derived
from small beds of sand in the London Clay. On the east of
Galleywood Common we recently made a few trial bores, finding
water at a depth of about twenty feet ; but it contained so much
sulphate of magnesia (Epsom Salts) as to be unfit for any domestic
purpose. At Margaretting Tye there is a well (now closed) yielding