ces are
usually absent.

Limestone rock, on the other hand, is commonly laid down in horizontal
strata, and while a succession of strata may frequently give rapid
slopes, marshes are very common, existing even on the tops of the hills.
The drinking water is always to be suspected as to quality because, in
the first place, it is hard from absorption of lime, and in the next
place, cavities and seams in the rock allow polluting material to travel
for long distances.

Sandstone, being porous, may be considered a healthy foundation, and
sands and gravels of all sorts are usually free from marshy land.

Gravel has always been assumed to be the healthiest soil on which a
house could be built, provided the ground water reaches its highest
stage three or four feet below the cellar bottom.

Sand is equally desirable except in the cases where vegetable matter has
been mixed with the sand, rendering decay imminent. Water drawn from
such sands in the form of springs will contain large quantities of
nitrates which may lead to excessive development of vegetable life and
may have on the human system the same laxative effect as comes from
drinking swamp water.

Clays and heavy alluvial soils are not usually considered desirable
soils on which to build. Water does not run from such soils; they hold
moisture, and hence are always damp, and marshes are very apt to exist
in the vicinity.

_Effects of cultivation._

It was formerly thought that extensive cultivation was objectionable
from the standpoint of health, that manured fields in the vicinity of a
house were undesirable, and that the turning up of a well-manured field
with a plow in the spring was a very likely source of fever. It is a
very common belief to-day that when water pipes are to be laid in city
streets, thereby disturbing the soil and bringing fresh earth to the
surface, typhoid or other fevers may be expected. There is, however, no
ground for this belief, and the fact that laborers and their families
live healthily in the midst of the thousands of acres of
sewage-irrigated fields near Berlin, where the heavily manured fields
are constantly being plowed, is a sure proof of this. The earlier
text-books on hygiene all assert, however, the contrary; Parkes, for
instance, says that irrigated lands, especially rice fields, which give
a great surface for evaporation and also exhale organic matter into the
air, are hurtful, and in northern Italy the rice grounds are requ