MICHIGAN ACADEMY OF SCIENCE. 
103 
which is present in large amounts in many of the deep well waters, 
both from drift and rock, and is met with all over the State. This 
actively corroding agent probably does much of the damage that so com¬ 
monly is referred to iron in the water. 
Sulphuretted hydrogen especially when associated with calcium sul¬ 
phate renders water objectionable, but there are only limited districts 
in which the amount is sufficient to be a source of much inconvenience. 
There are limited districts in which oil and inflammable gas have 
accumulated in the water bearing beds that underlie compact clay, 
a notable instance of gas accumulation being the district between-Milan 
and Ypsilanti, while oil is especially abundant in wells west of Howell. 
The oil and gas besides being objectionable in the water give rise to wild 
speculation as to their occurrence in paying quantities underneath the 
water bearing formation. The hopes thus aroused are seldom if ever 
likely to be realized. The gas of the Milan district appears to be largely 
from the underlying Devonian shale, while the oil near Howell appears 
to have been distilled from limestone which there immediately underlies 
the drift. 
Turbidity. The underground waters are seldom turbid, for as a rule 
the supplies come from beds of sand and gravel too coarse to be held in 
suspension. There are, however, occasional flowing wells which con¬ 
stantly bring up sediment because of the fineness of material in which 
the water is obtained. For example, flowing well districts near Pickford 
and near Rudyard in Chippewa County penetrate large amounts of very 
fine sand termed “slush” by the drillers, in which some of the wells that 
terminate in the slush are troubled with turbid water, but the majority 
reach coarser material. There are also a few places where wells -termi¬ 
nate in a marsh mud or in very clayey beds in which water is fre¬ 
quently if not continually turbid. 
GEOLOGICAL CONDITIONS FROM SANITARY STANDPOINT. 
The leading question in determining a public water supply is that of 
the healthfulness of the water to the consumer. A study of the sources 
of supply in use in Michigan cities and villages brings out numerous 
instances in which the choice of a public supply has been unwise. The 
individual cases cannot be taken up, but a few general observations will 
be presented. 
It has already been pointed out that the surface waters contain much 
less mineral matter than the underground waters and on that account 
they are preferable for drinking and for boiler use, but the chances for 
contamination are greater than in underground waters. It would seem 
wise, therefore, wherever surface waters are to be turned into a public 
supply to have well equipped filtration plants connected with the water¬ 
works system. By this means greater safety will be insured to the 
000,000 people who are already using such water and the much larger 
number which will use it in the future. 
The underground waters are in most cases practically free from dan¬ 
ger of contamination. There are, however, certain situations in which 
the deposits from which water is drawn are liable to become contami¬ 
nated. The coarse deposits of gravel and sand along water courses that 
lead through villages are thus exposed and yet are drawn upon in a 
