214 " FRESH WATER CHALK " AT HALSTEAD
or upon the Glacial Drifts. Such a deposit might be formed in
three ways :—
(1) By deposition of Mollusca and other organisms from a
fresh-water lake.
(2) By deposition of carbonate of lime from spring-water
super-charged with carbonic acid and in consequence super-
saturated with the bicarbonate of lime.
(3) By segregation of carbonate of lime from water charged
with the bicarbonate in passing from a permeable stratum into
clay.
The first suggested method of formation is quite untenable
for three reasons. The existence of a lake in such a position
and upon such formations is extremely improbable ; secondly,
the substance contains no trace of any organic remains ; and
lastly, the chemical composition does not correspond with that
of a recently formed organic deposit, for instance the chalk mud
of the Atlantic Sea-bottom (see Appendix).
The second suggestion is much more likely to be correct,
especially since there are springs of hard water on the very spot
where the deposit occurs, and since the existence of crystals in
the substance shew that the chalk has been deposited from
aqueous solution. Deposits of chalk, in the form of the incrus-
tation of sticks, etc., in the bed of a stream, are. frequently
observed, and it is possible that they would occur on a larger
scale.
It has been observed that under the Boulder-clay a hard
crystalline deposit of chalk 3 or 4 inches thick sometimes occurs
(see G. S. Memoir Geology of North West Essex, etc., Sheet 47, p. 60),
and is supposed to be formed from water, containing dissolved
chalk, diffusing from below into the Boulder-clay, in which
process the chalk is dialysed out. It seems to us that the
deposit at Greenstead Green may be the outcrop of a similar
deposit, existing, however, in a much thicker stratum, and at
the outcrop thoroughly disintegrated by natural processes. Its
chemical composition is quite in accordance with this explana-
tion.1
In conclusion it may be pointed out that this chalk-like
substance, if it existed in larger quantities, would be a valuable
1 Mr. T. V. Holmes, F.G.S., remarks as follows:—"The spring at Greenstead Green
evidently results from the saturation with water there of the Glacial Sand and Gravel under-
lying the Boulder-Clay and overlying the London-clay. All the chalky matter thrown down
there must be derived from the overlying chalky Boulder-clay, the underlying gravel being
lull of water to the top. Below the gravel the London-clay is from 80 to 100 test or more
thick so that there can be no doubt as to the source of the chalky materia! in this case."