98 EK. ORTON—-GEOLOGICAL PROBABILITIES AS TO PETROLEUM. 
\their history when the artesian theory can be fairly applied or tested. 
That date is at the very opening of the field. There are great differences 
in the degree of permeability in different portions of what is essentially 
a porous rock, and vast periods of time have been available for the press- 
ure of the distant head to make itself felt in every pore of the water- 
bearing stratum. The first wells drilled ina district may be able to avail 
themselves of the normal pressure and to exhibit the normal rise of the 
salt water, but it is conceivable that after wells have been drilled in the 
same field by the score, the hundred, or the thousand, the original con- 
ditions may be materially interfered with, at least for the time being. 
The gas and oil, in the storage and adjustment of which ages have been 
used, may have been released and brought to the surface in weeks or even 
in days, and the salt water may be altogether unequal to the task of 
occupying the new territory open to it on such short notice. Years may 
be required for its movement where minutes are allowed. 
So far as known, but a single important gas field has been taken in 
time to furnish a thoroughly reliable record. In the Trenton fields of 
Ohio and Indiana all the facts were noted at the beginning of their de- 
velopment, and the record accompanied the development, pari passu. 
From the facts thus gathered the artesian theory of gas pressure finally 
grew. I find it impossible to believe that the remarkable coincidences 
between observation and theory in these cases can be fortuitous. That 
the theory does not find support in the later experience of the Pennsyl- 
vania fields is no ground for surprise. Two explanations of this want 
of agreement are possible, as we shall presently see. The theory would 
not find support in the later stages of the very field from which the in- 
itial facts for its foundation were gathered. The salt water still rises in 
some wells to the same height as formerly, namely, 600 feet above tide, 
but the rock pressure of the gas ranges from a quarter or an eighth of the 
original figure to zero; but every step of the decline has been observed 
and noted. The figures that entered into the original theory were facts 
at the time they were used, and a rational explanation can be given of 
the conditions that have since supervened and are now existing. That 
another element can enter into this rock pressure I have always recog- 
nized. This element is the expansive power of the gas itself. It is seen 
especially in shale gas fields, where no water, fresh or salt, is associated 
with the gas. In such cases the artesian theory, of course, is not appli- 
cable. 
The highest pressure that I have noted in shale gas wells previous to 
the last summer was less than 200 pounds to the square inch. During 
the present year I have had an opportunity to study the gas production 
of central New York, and to my surprise I found that the gas derived 
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