FEOM THE CHALK OF HEKTFOKpSHXRE. 
155 
at the present time is no doubt greatly due to the small rainfall we 
have had during the last few years, that cannot account for the 
gradual reduction in its height during the last twenty years or 
more. We have seen that for the last half-century the mean rain¬ 
fall in Hertfordshire has been about 261 inches per annum, and the 
deviation from the mean in each of the five decades comprised in 
this period has been as follows:—-1840-49, 5 per cent, less than 
the mean; 1850-59, 1£ per cent, less; 1860-69, 1 per cent, less; 
1870-79, 5 per cent, more than the mean; and 1880-89, 2\ per 
cent. more. For the first 30 years of the half-century, therefore, 
the rainfall was considerably less than the average for the whole 
period, and for the last 20 years it was considerably more. If 
we take the last 90 instead of the last 50 years, the increase in 
the rainfall in recent years is still more striking, as will be seen by 
referring to the table on p. 140. 
The process of depletion is certainly a slow one ; but if we let it 
go on until we are seriously affected by it, when we do stop it, if 
we can, which is very doubtful, the process of replenishment will 
be still slower. One inch of rain yields about half a gallon of 
water over every square foot on which it falls, and, as a cubic foot 
of chalk in its natural moist state will absorb two gallons of water 
(one-third of its bulk), it will take a depth of 4 inches of rain to 
saturate it. Assuming that we can rely upon an average percolation 
through our ordinary soil of 6 inches of rain per annum, each year’s 
rainfall would saturate 18 inches of the underlying chalk. The 
Chalk of the London Basin varies in thickness from 620 to upwards 
of 700 feet. Taking it at only 600 feet, if it were exhausted of the 
available water it contains, it would require 400 years’ rainfall 
to saturate it again. We have seen that the water-level under 
London, and in some parts of Hertfordshire, is being lowered at 
the rate of at least a foot per annum. If this reduction in height 
were general over our Chalk area, and all pumping from the under¬ 
ground reservoir were stopped, it would be forty years before the 
rainfall (assuming a percolation of six inches per annum) brought up 
the water-level to the height it was sixty years ago. 
In 1868 a Royal Commission was appointed to enquire into the 
best means of preventing the pollution of rivers. In their Sixth 
Report the Commissioners expressed the opinion that the rivers 
Thames and Lea ought to be abandoned as sources of water-supply 
for London as early as possible, and that recourse should be had 
to deep borings in the Chalk for the entire water-supply of the 
metropolis. To the examination of this report Dr. John Evans 
devoted a considerable portion of his presidential address to the 
Geological Society of London in the year 1876, in the course of 
which he said : 
“ It can hardly be believed that a proposal such as this, involving 
the diversion of the whole of the water from the natural springs and 
streams over an area of not less than 440 square miles (an area larger 
than that of several English counties), should have been brought 
forward without the slightest reference to what would be the result 
