180 AY RAS CROLL 
the well, and not so much upon the transmitting power of the porous 
beds between the well and the outcrop area. As indicated by the 
tests in this experiment, as well as in the Madison test above cited, the 
disturbance caused by the water, escaping at a well, soon dies out as 
the distance from the well becomes greater and greater. Seldom in 
the Wisconsin district does the disturbance extend more than a mile 
back from the well. 
The movement of the water in these ponded basins is very slow. 
In the Wisconsin area where the annual precipitation approximates. 
30 inches, probably not more than one fifth or 20 per cent. of this 
amount is added to the ponded sea. On the other hand it may be 
considerably less than 20 per cent., for no accurate determinations 
are at hand which give the exact amount of run-off in the Wisconsin 
region. ‘To the amount of immediate run-off must also be added 
the amount that returns to the surface by evaporation, by vegetation, 
springs, and shallow wells. _* 
Assuming, however, that 20 per cent. is a fair estimate of the 
amount of precipitation that is added to the ponded sea and that 1632" 
is the average pore space of the Potsdam sandstone, we can compute 
the rate of flow. 
In order that 20 per cent. of the precipitation may be added to the 
ponded sea without raising the water level, the increment of the 
previous year must have moved vertically downward 3.6 feet. 
The dip of the Potsdam sandstone as indicated by wells between 
Madison, Wis., and Chicago, IIl., is about 12 feet per mile. It follows, 
therefore, that the lateral movement of the water amounts to 1760. 
feet per year or one-third of a mile. As much of the Potsdam sand- 
stone gathering ground lies 200 miles northwest of Chicago, it is evi- 
dent that the water collected by the catchment area will require in the 
neighborhood of 600 years before it reaches Chicago. If the average 
pore space of the sandstone is larger than 163, the downward move- 
ment of the water as above computed would be less, and if the pore 
space was smaller, the downward movement would have to be greater, 
in order to hold the 20 per cent. of the precipitation. It also follows. 
tC. R. Van Hise, ‘A Treatise on Metamorphism.” Monograph 47. U. S. 
Geological Survey, pp. 585-89. 
o 
