THE SHALLOW AND DEEP WELL WATERS OF ESSEX. 35
The East Donyland water contains about fifteen grains of carbonate
of soda per gallon, the Colchester water No. I. about six grains, and
No. II. none, the whole alkalinity being accounted for by the
lime salts present. In all respects the Donyland water closely
resembles many of the sand waters. Amongst this group of moder-
ately soft waters we find the amount of chlorides varies very
considerably ; but the wells yielding the largest quantity are sunk
near the sea and close to a tidal river, the Blackwater. It will be
observed that the waters in these two groups are all obtained from
wells at and south of Colchester. North of that town all the deep
well waters I have examined are from the chalk and contain over
ten grains of carbonate of lime per gallon. They therefore fall into
group III. (Table V.).
The first eleven waters in this group, and five out of the last six,
are derived from near the outcrop of the chalk or from places in
which the London Clay and Tertiary sands are thinning out. In
nearly all, the chlorides are very small in amount. The exceptions
are at Mistley and Grays, both near the sea or a tidal river. In nearly
all the alkalinity is due to carbonate of lime, the exceptions being the
Mistley water, which contains probably about four grains of carbonate
of soda per gallon, and the Stratford water (Phoenix Works), which
contains about ten grains. The only other sample from Stratford
(Howard's Chemical Works), on the other hand, contains little, if any
carbonate of soda. Many of the Tertiary sand waters are very hard
(due to both magnesia and lime), but contain very much more
chlorides, sulphates, etc. (of soda chiefly), and some are so brackish
as almost certainly to suggest infiltration of sea water. This is more
probable, since all of them come from sources near tidal rivers or the
sea. The last water on this list, that from the Beckton Gas Works,
resembles some of these waters in being brackish; yet it is sunk well
into the chalk, and I am told that many wells in that neighbourhood
have been sunk to considerable depths into the chalk, yet have
yielded waters so brackish as to be useless, and the wells have been
closed.
Reviewing the whole three groups, we find that most of the chalk
waters in the south of the county and in the north-east resemble
each other in containing considerable amounts of carbonate of lime
and very little chloride and carbonate of sodium; but differ from the
chalk waters in other portions of the county, where the chalk lies
at a much greater depth, since the latter contain very little lime and
D 2