248 MINERAL WATERS AND MEDICINAL SPRINGS OF ESSEX.
a high cliff of London Clay, of an horizon very near the base of
the series. I must demur to Dr. Thresh's description of this as
a gravel water. Apart from its position, there is apparently no
gravel within the possible range of supply. There is some
beyond the summit of Beacon Hill, and a patch northward of
the high road 500 yards to the west of the Spa. There may,
of course, be pockets or channels in the clay surface, which have
retained portions, more or less displaced, of the once-continuous
sheet of gravel ; and it must be remembered that nothing under
30 yards across can be mapped on the one-inch scale. But all
the ingredients are such as occur in the London Clay."
IV.—GENERAL REMARKS ON THE MINERAL
SPRINGS OF ESSEX FROM THE GEOLOGICAL
POINT OF VIEW.—All water derived from wells, springs, or
streams is primarily supplied by rain. The porosity of the
surface on which it falls determines the amount carried off by
streams or by re-evaporation. The streams, again, often lose
into porous beds some of the water flowing from less pervious
ground. The degree of porosity is of very wide range, from
coarse gravel to clay of such density that a very thin seam will
stop all percolation.
From the practical point of view, beds containing any notable
proportion of clay may be regarded as impervious. The sources
of true springs, or of yield to wells, are confined, therefore, in
Essex, to Gravels, Sands, and Chalk.
In the case of thin seams of sand (e.g. in the London Clay),
such probably absorb water from gravel beds or streams at one
point and discharge it at another, often with mineral additions
acquired in its transit.
Essex waters are derived, as stated above, from various
sources—from the Sands and Gravels deposited by rivers in
Post-glacial times at higher levels than their present channels ;
from the Glacial-sands and gravels underlying the Boulder Clay ;
occasionally, in small quantities, from the Boulder Clay itself ;
from sandy seams in the London Clay ; from the Lower London
Tertiaries (the Reading and Thanet Beds) ; and from the
Chalk.
The River-gravel occasionally yields a water containing a
small amount of iron (derived, in all probability, from the sub-