64
THE LIBRARY TABLE.
shaft and headings are wasted labour; whilst if the rock is compact the
fissures may be too far apart to be reached by headings. On the open chalk-
downs, when the whole rainfall is absorbed to fill the fissures, and where the
trend of the main joints is visible, the conditions render the system of shafts
and headings far less speculative than would be the case in South Essex at
least, where several hundred feet of Tertiary deposits overlie the Chalk. But
the system would answer well enough in North Essex, and may be decreed in
the Book of Fate as the ultimate general means of supplying the whole county
with water, all taken above sea-level, and of incontestable purity.
For the saline ingredients of the Chalk-water, as already suggested, the
Tertiary sands are probably responsible. These are of insignificant thickness
on the northern outcrop, from Roydon to Sudbury, but reach 150 to 180 feet
in the Southend district, and necessarily crop out in the estuary above Thames
Haven, The faults traversing the southern part of the county are of
insufficient magnitude to cut off the connection, with the exception of the two
that, meeting near Romford, form an angular projection of what may be
termed unsalted chalk, into the area susceptible of marine influence.
A comparison of Dr. Thresh's map of calcareous and saline waters (p. 34)
with that of the sub-Tertiary contour of the Chalk (p. 16, taken from the Essex
Naturalist, vol. v, pl. iii., 1891) will shew that the boundary, except at the
projection referred to, is approximately along the line at which the Chalk-
surface rose above sea-level before the latest movement of depression. In
other words, the limit of marine influence is on or near the line—70 of the
contour of the Chalk-surface, with the exception indicated. This projection is
bounded by a triangle of faults of considerable magnitude, the enclosed mass
having been compressed into anticlinal arch and synclinal trough, discordant in
strike with the surrounding less-disturbed areas, from which it is separated
hydrologically by clay crushed into impervious walls along the faults. Dr.
Thresh has thus, on purely chemical evidence, confirmed deductions from
geological premises. But we must point out that the boundary is not a fault,
except at that part of its course, but an old subterranean shore-line, and that
faults, though not rigidly straight lines, are more angular than the gracefully-
sinuous dotted line on Dr. Thresh's map, more or less diagrammatic in its
character.
In the useful table of analyses (p. 37) from over forty different wells, the
initials "T.S." stand for Tertiary sands, in many cases beds in, or immediately
at the base of, the London Clay ; in only a few instances is the supply from
Thanet Sands, which the initials might equally represent.
After these generalisations, Dr. Thresh takes in succession the several
Water Companies and District Councils, describing their works and the nature
of private supplies utilised within their respective areas. For the clay
districts generally the supply may be said to be deficient in quantity and
quality, repeated mention being made, with skilful avoidance of tautology, of
rainwater, ponds, ditches, shallow wells little better than cesspits, cartage for
a mile or two from source, sale by the pailful, &c.
But the charges of the Companies, occupying pp. 145-167, though perhaps
justified by the heavy cost of providing proper supplies, largely explain the
reluctance of local authorities to close even badly-contaminated sources, and