118 
REVIEWS. 
The base of the Gault was marked, as at Folkestone and other places, 
by the occurrence of phosphatic nodules, which left no doubt of its true 
geological horizon. 
The agreement between the predicted and actual thickness of the 
strata is very close, and is the highest testimony that can be borne to 
Mr. Prestwich’s accuracy and knowledge of the details of the London 
strata: but the unfortunate results of the boring, as shown in the fol¬ 
lowing statement, prove how little aid the most skilful geologist (in the 
present imperfect state of the science) can render to the practical engi¬ 
neer. Every engineer is a practical geologist, but every geologist is not 
an engineer:— 
“ We are indebted, however, to the Hampstead Water-Works Company for the first 
attempt to solve this problem practically; but, as the surface of the ground at their 
Works at Kentish Town is 174 feet above Thames high-water-mark, the situation is not 
so favourable as might have been wished. A few years since this Company sunk a well 
through the Tertiary strata (at that spot 324 feet thick), to a depth of 215 feet in the 
chalk, making a total depth of shaft of 539 feet. The supply of water from this source 
being found insufficient for their purpose, the Directors of the Company, in 1852, con¬ 
sulted MM. Degousee and Laurent, the eminent well-engineers of Paris, on the advisa¬ 
bility of sinking through the Chalk into the Lower Greensand. In November of that 
year these gentlemen came to London, and I accompanied them to those places in the 
neighbourhood of Merstham and Reigate where the outcrop of the chalk and underlying 
clays and sands is best exposed. The conclusion to which they arrived was precisely 
similar to my own, and on their report the Directors resolved to undertake the work. 
Accordingly, on the 10th of June, 1853, boring was commenced in the chalk at the bottom 
of this well. 
“ At a depth of 569 feet from the surface the chalk with flints ended; grayish chalk, 
without flints, becoming more argillaceous in descending, was then traversed for a thick¬ 
ness of 294 feet. The chalk-marl next succeeded, and continued for 47^ feet. This 
would give a total thickness to the chalk of 586 feet. The chalk-marl passes so insen¬ 
sibly into slightly sandy marls representing the Upper Greensand, and these into the 
Gault, that it is difficult to draw any satisfactory lines of division. I have taken as the 
representative of the Upper Greensand the more arenaceous and chloritic beds. They 
are 72i feet thick. These strata, however, were here, on the whole, so argillaceous that 
they were not permeable, and they consequently afforded no additional supply of water. 
The Gault was found underlying the Upper Greensand in the usual order, and presented 
the ordinary character of a fine gray calcareous clay, 1301 feet thick. At the base of tins 
mass of clay a layer full of the phosphatic nodules, so common at the base of the Gault 
at Folkestone and elsewhere, was met with. 
“ Thus far all the strata were in regular succession, and there was every reason to 
believe that the same order which prevailed at their outcrop, and with which there seemed 
to be nothing to interfere, would be continued underground; and that after traversing 
this band of phosphatic concretions, the light-coloured siliceous sands of the Lower Green¬ 
sand would succeed. The ordinary probabilities of the geological sequence being main¬ 
tained throughout this central area seemed then so strong, that when the works were at 
that point, just a year since, having occasion to speak on the subject at the Institute of 
Civil Engineers, I did not hesitate to express my conviction that a very few more turns 
of the auger would tap these sands. This opinion has unfortunately proved incorrect. 
Instead of meeting with loose sands, the next bed which presented itself was one of red 
argillaceous sand and sandstone, 1 foot thick: 12 feet of red clays (some mottled light 
bluish-green) and sandstones then succeeded; followed by a singular conglomerate, 2 feet 
thick, containing pebbles, of a considerable size, of various old and crystalline rocks: 
amongst these were dark gray syenites, greenstones, red clay stone-porphyry, trap-rock, 
a grey semitranslucent quartz or hornstone, and a granular schist with traces of fossils. 
Then came 26 feet of red clays, underlaid by red sand and a bed of small rolled pebbles. 
