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, pi. 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 ^nagnitude, 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 bedsin, 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, Ac. 



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 



