
ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 365 
its way down the bedding-planes, and poisoning the under- 
ground water-supply of adjacent districts. It is, of course, 
often necessary to carry drains in some given direction other 
than that which geological considerations might dictate—for 
questions of safe outlet and the expense of excavation cannot 
be ignored. In cases of this kind, all the engineer can do is 
to see that the drains are made as water-tight as may be, 
and to insist on the closing of every common well in the 
immediate neighbourhood. Flat lands in the neighbourhood 
of considerable towns are sometimes laid out as sewage-fields, 
with satisfactory results, it may be, to the towns, but often 
much to the prejudice of a scattered rural population, whose 
water-supply may come entirely from common wells. What- 
ever excuse there may be for carrying the drainage of a 
city in some particular direction, irrespective of geological 
considerations, there can be none for discharging sewerage 
over the surface of the ground without preliminary inquiry 
as to what may happen in the event of leakage into the rocks 
below. Cases could be cited to show how neglect of this 
precaution has been followed by disastrous results. Some 
low-lying fields were selected by a certain town for sewage 
irrigation. The subsoil was boulder-clay, a deposit supposed 
by the engineers to be impervious. But there are boulder- 
clays and boulder-clays; many are practically impermeable, 
others are only relatively so. This particular boulder- 
clay was one of the latter class, somewhat sandy in texture, 
and only two or three feet at most in thickness, Unfor- 
tunately, also, the sewage-field spread over the back of a low 
anticlinal arch. Under those conditions the inevitable result 
followed in due time. Ina year or so the subsoil of boulder- 
clay become water-logged, leakage took place into the 
underlying strata, and the foul liquid made its way down the 
bedding-planes and poisoned all the springs and common 
wells in the surrounding neighbourhood—typhoid fever, of 
course, ensuing. 
In villages and rural districts, where no general system of 
drainage is provided, cesspools are often sunk. When these 
are carried down through a thick bed of clay into underlying 
gravel and sand, they may suit the purpose of their owners 
well enough. ~ It is obvious, however, that they are a menace 
