158 ORIGIN OF ROCK PRESSURE OF NATURAL GAS. 
operators in the new gas and oil fields are coming to the same ground. 
They have become thoroughly satisfied by their own experiences that 
the root of rock pressure is to be found in the water column that stands 
connected with the porous rock in which the gas and oil are contained. 
In the present paper I desire to present to the Geological Society 
a few facts and conclusions bearing upon the subject. 
THE DATA FOR THE HYDROSTATIC THEORY. 
The first question is, What are the facts as to the rock pressures of 
the gas rock in question and what relations do they bear to the depth 
of wells and other conditions in the Trenton limestone? The answer 
is not as full and definite as may be expected, certainly not as may be 
desired. There is but one datum in the development of a gas field in 
which the normal gas pressure can be ascertained, and that is when 
the first well reaches the reservoir and releases the long-imprisoned 
and greatly compressed gas. But often this favorable opportunity is 
lost, and gauges are not applied to wells until the energy of the first 
flow is somewhat abated. Again, different wells in the same field, as 
Findlay, for example, give different results. The wells vary with the 
depth at which the gas rock is found. This factor is found to be an 
essential one, as will presently be shown, in connection with rock pres- 
sure. Moreover, gauges are sometimes inaccurate and their errors 
come in to confuse the study of the subject. Furthermore, the exact 
depth of the wells and the exact altitude of the surface where they are 
located can not be ascertained in all cases. Small errors of this sort 
must be provided for, and there also enters into the discussion a ques- 
tion as to the specific gravity of the water which is to be made the moy- 
ing force of gas and oil. The water found in association with these 
substances is never fresh. It is always saline and often highly miner- 
alized. The weight of fresh water to the square ineh is 0.43285 pound 
for 1 foot in height (I use Professor Lesley’s tables). The average 
weight of sea water is 0.445 pound to the square inch for 1 foot; but 
the mineral waters with which we find the Trenton limestone saturated 
often reach a much higher figure. An examination of several speci- 
mens Shows that a column 1 foot high would weigh to the square inch 
0.476 pound. In fact, some of these waters are more like bitterns, and 
their columns would equal or exceed 0.5 pound per foot. 
Bearing these several sources of ambiguity or uncertainty in mind, we 
can consider the records of pressure, depth, and the other factors that 
are accessible. The figures as to pressure have already been summa- 
rized in a preceding paragraph, but they will be repeated in an accom- 
panying tabular statement. Before coming to this, however, let me in 
the briefest terms review the conditions under which gas, oil, and salt 
water exist in the Trenton limestone. The uppermost beds of the great 
Trenton formation in northwestern Ohio, central and northern Indiana, 
Michigan, Illinois, and Wisconsin consist of a porous dolomite 5, 50, 100, 
