270 American Geologist. ***y' ^^^^^ 
the porous gravel and sand until it came to clay, which it would 
saturate less rapidly, and it would appear along the margins of 
creeks and lakes in the form of springs along the upper surface 
of this clay. After lapse of sufficient time, however, and owing 
to many breaks in the continuity of the clay, water gets below 
the clay and penetrates in the same way the next lower layers 
and so on to the bottom of the scale until, not only the whole 
drift, but all of the rocky strata become filled with water. 
Now this experiment is only the epitome of what actually 
occurs in nature. The rains and snows of the year serve as a 
constant supply to little creeks and rivulets which, uniting, 
form the rivers. The earth has become saturated to its full 
depth, including the rocky strata. It is the surplusage of the 
precipitation of the year which appears first in the form of 
streams and then in little creeks and then in rivers, which final- 
ly reach the ocean. This is a very elementary principle of me- 
teorology and would need scarcely to be mentioned were it not 
for the fact that the ground supply has been said to be less 
constant and abundant than the river supply. It is only neces- 
sary to say that in case of sufficient drouth the rivers would 
dry up before the rock reservoirs would, and it would be abso- 
lutely necessary to resort to the rock strata for a supply of city 
water, as is now done in many parts of the western country. 
2. Is It Suitable for the Uses of a City? I will here call 
your attention to an important general fact as to the varying 
qualities of the water derived from deep wells within the 
city limits,* or sunk in the surrounding country into the same 
formations. For this purpose it is only necessary to mention 
four principal rock horizons from which water is obtained. 
The deepest is the Hinckley sandstone, No. 12 of the foregoing 
section, in which wells are 700 to 800 feet deep ; and, secondly, 
the Dresbach sandstone, 600 feet down ; third, the Jordan sand- 
stone, 400 to 450 feet down ; and fourth, the bottom of the St. 
Peter sandstone, about 200 feet down. It is true that water 
can be obtained, though in less quantities, at many intervening 
depths, but all of the wells of the city can be roughly classified 
in these four classes. It has been found that the water coming 
from the greatest depth is the most highly mineralized, and the 
water from the least depth is the most nearly soft and pure; 
hence, the deep wells which reach 700 to 800 feet and those 
