THE LIBRARY TABLE. 63
depth at which this formation lies from district to district is therefore a matter
of primary importance. We do not, in saying this, ignore the speculative
element involved. That a boring may pass very near to a water-charged
fissure in the Chalk, and yet obtain no yield from it, is demonstrated by the
few cases in which enlightened engineers have used explosives (preferably
roburite) to shatter the rock around the lower part of a boring, with the
result of effecting communication with such fissures, and securing their yield.
This very usual practice for petroleum-wells, is, however, but little in vogue
in England, though the Chalk is a rock pre-eminently suitable for such pro-
cedure. The proportion of fruitless borings in the Chalk is, moreover, very
small, in comparison with the successful operations, even if we include in the
failures those in which the water has in course of time become too salt for use,
as mentioned above.
On pp. 20, 21 is given a table of the level of the Chalk-surface, referred to
Ordnance Datum, at over 100 localities. Unfortunately this contains many
clerical errors, perceptible only by reference to the records published in various
works, since there are given here merely the differences between depth to
chalk and elevation of site above datum. Though the author solicits correction
it is impossible to deal here with more than two or three salient instances.
Any traveller by the Tilbury line can see that the Chalk-surface at Purfleet is
not 30 to 40 feet below sea-level, but some 50 feet above it. At Mistley, on
the other hand, it is 75 feet below datum, but is given as 25 feet above it. At
Bishops Stortford it is given as 328, the surface-level at the Waterworks,
instead of 213, 115 feet down the well. These and many similar instances
largely detract from the value of the table, a corrected re-issue of which is
emphatically desirable, as is the recording in the Essex Naturalist of
details of all wells not yet published. Our pages will constitute the recognised
storehouse and source of information for future labourers in the same field,
and there are many wells mentioned in the work before us of later date than
Mr. Whitaker's last series (No. iv), published in 1896 in the E.N.
In dealing with the gradual lowering of the hydrostatic level in various
parts of the county, Dr. Thresh regards the bulk of water in the chalk as
stationary ("stagnant" suggests unpleasant ideas), and as having little or no
free communication or definite flow. The area of each "hydrological unit,"
represented by a single water-charged fissure, or connected group of fissures,
is a matter for speculation. It does not follow that because the Southend
wells are such-and-such distances apart, therefore intervening wells would be
either futile or detrimental to those existing. Everything depends on the
general trend of the Chalk-fissures. If, for instance, this is north-westerly,
the Southend and Oakwood wells might be on one fissure of three miles length,
and the Prittlewell, Eastwood, and Noble's Green wells on another and wholly
independent fissure, of about the same length and half a mile distant from the
first. With a different general trend each well might be a separate fissure,
and in no case can it be asserted positively that wells at intervening points
would affect them.
Nor, in so fortuitous a matter as fissures in chalk, can it be laid down as
established that headings driven from a shaft will of necessity exhaust a larger
area than the shaft itself or even than a boring on the site. If the Chalk there
is sufficiently fissured, the yield is a mere question of pumping power, and the