46 
The Chalk area is by far the largest in this district, and, exclusive 
of the Isle of Thanet, equals about 296 square miles. While the 
area of the Tertiary beds is about 158 square miles. 
The rainfall of the district is subject to considerable variation, and 
more statistics on this head are needed. At Sheldwich the average 
rainfall for three years 1883 to 1885 equalled 25:99 in. At Canter- 
bury for 1884 it was equal to 20°83 in. At Ramsgate for three years 
from 1883 to 1885 = 21:81 in. While at Horton Park I have for 
1883 above 30-00 in. It must be remembered that the last three 
years were exceptionally dry ones. 
One inch of rainfall represents about 100 tons of water to the 
acre. A portion of this is lost by evaporation, and some passes away 
underground to sea; the remaining portion constitutes our available 
supply in rivers and wells. 
A consideration as to the permeability of different beds, and their 
capacity for retaining water, will form an important element in our 
calculation. We may consider the Chalk area as the most important, 
not only as the chief water reservoir, but as constituting by far the 
largest portion of the district, and it will be found that it is through- 
out its extent extremely permeable when not saturated with water, 
down to a short distance from the Gault. The large supplies of 
water required for the Canterbury Waterworks, the Chartham 
Asylum, the Canterbury Breweries, the wells for drinking water at 
Ramsgate, Margate, and Dover, are all derived from this formation. 
In the case of the deep boring for the Convict Prison at Dover, made 
during this year, when strata were pierced to the depth of over 900 
feet (penetrating both Gault and Lower Greensand), the water- 
supply was chiefly, if not entirely, derived from the upper three 
hundred feet, all in the Chalk. The time taken by the water to 
reach the line of saturation in this Chalk area appears to be about 
three or four months, and the height of the springs varies in propor- 
tion to the rainfall. 
The Chalk area is in some places capped by impermeable beds of 
clay, as at Swingfield Minnis, but the clays and gravels over the 
Chalk are for the most part permeable. North of Canterbury, over 
the Tertiary area, the permeable beds are the sandy portions of the 
Old Haven, Woolwich, and Thanet beds; the lower parts of the 
latter, however, are impermeable. Above these the London Clay 
and the clay and gravel beds of the Blean district are impermeable. 
A narrow tract of land below the escarpment of the Chalk Downs, 
stretching from Folkestone to Wye, is composed of impervious 
Gault and pervious beds of the Lower Greensand. In many of the 
wells, after piercing the Gault, the water flows up to, or over the 
surface. In this area the water from the Sandgate beds of the Lower 
