282 American Geologist. ^^^' ^^^^ 
few years of use; second, they are all within the uppermost 
portion of the St. Peter sandstone, and are below, that is, 
south of the valley of Bassett's creek, and wells which would 
be depended upon in that area would necessarily pene- 
trate much deeper than these sewers and would obtain a water 
from the stratum below the shale which is in the St. Peter 
sandstone, as already described ; third, the St. Peter sandstone 
operates as a filter to abstract any impurities which might re- 
sult from the sewers, a filtration which would be many, many 
times thicker and more effective than any filtration through a 
layer of sand mechanically formed for the purpose of filtra- 
tion; fourth, these impurities would necessarily be drained 
from the surface of the sandstone, if they once entered it and 
were carried along, into the Mississippi river. They could not 
permeate the general body of water contained in the sandstone. 
A current of water goes where it is freest to flow. The surface 
is the freest and the lightest portions rise to the surface. The 
body of pure water lying below has the greatest specific gravity 
and is under hydrostatic pressure and would reject impurities 
that would have a tendency otherwise to enter the St. Peter 
sandstone and compel the drainage into the sewers rather than 
out from the sewers. 
This can be illustrated. Did you ever see a peat swamp 
through which ran a stream? Have you seen the enormous 
peat swamps in the northern part of the state, particularly that 
one which is northwestward from Duluth, drained by the St. 
Louis river? Any bog is permeated by water constantly, and 
in this great swamp for a width of several townships, but the 
river flows in its own channel, without reducing the water of 
the bog in general. It enters at the upper end, follows the chan- 
nel, passes out at the lower end. It has but little effect upon the 
water standing in the bog, but, on the contrary, it is affected by 
the waters of the bog; they pass slowly into the river instead 
of the river into the bog. In the same manner, a channel of 
flowing water, as in a sewer in the St. Peter sandstone, would 
abstract water from the St. Peter sandstone instead of giving 
water to it. It might, however, be answered here that in the 
spring of the year, or at flood time, the water does encroach 
upon the bog and raises the water in the bog, and that there- 
fore the bog is affected by the stream. This, however, is more 
