MINERAL WATERS AND MEDICINAL SPRINGS OF ESSEX. 249
ajacent London Clay), but never any other substance which could
confer upon it medicinal properties.
The Boulder Clay covers a very large area in the central and
northern parts of the county. The waters draining from it into,
and those directly entering, the glacial gravels lying beneath it
are often charged with considerable quantities of the salts of
lime and sometimes of magnesia. Some which contain large
amounts of these salts may be classed as "Mineral Waters," but
those occurring in this county do not appear to possess any
valuable medicinal properties. Those from certain of the
localities indicated above have been valued for their chalybeate
properties, but the iron which gives them these properties has
been derived from the London Clay.
The London Clay is a fairly homogeneous formation which
extends over the greater part of the county and attains a
maximum thickness of, perhaps, four hundred feet, decreasing to
nothing at its northern and southern edges. The waters from
the occasional seams of sand in it, especially near the base and
at the upper limit, frequently contain a considerable amount of
magnesium sulphate (Epsom salts) and calcium sulphate.
These may be classed as "Mineral Waters" and they form a
majority of those Essex waters which have acquired, in the
past, a reputation for possessing medicinal properties. The last-
named salt has, however, little or no medicinal value.
In the coastal parts of Essex, the Thanet sands, and possibly
also the upper beds of the Chalk, yield a water containing
considerable quantities of sodium chloride (common salt) and
sodium carbonate. Possibly, if these waters occurred on the
Continent, they would be charged artificially with carbon-
dioxide gas and sent over here for use as "Table-waters" ; but
they are so common in this county that they are regarded as
potable, rather than as mineral waters.
" The presence of sodium carbonate, even in wells penetrating
some distance into the chalk, is to be attributed to infiltration
from the Thanet Sands. True chalk-waters are always hard,
from the abundance of calcium bicarbonate they contain.
Sodium chloride is generally attributed to marine percolation,
and is most noticeable on the sea-board—increasingly so in late
years, owing to the largely-extended demand for water, due to
the development of the London invasion of Essex.
R