THE LIBRARY TABLK. 63 



depth at which this formation Hes 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 been 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 difterent 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 



