156 ORIGIN OF ROCK PRESSURE OF NATURAL GAS. 
cities which it supplies with the unspeakable advantages of gaseous 
fuel. Itis the same cause that lifts the oil from the rock in all flowing 
wells. 
By rock pressure is meant the pressure which a gauge shows in a 
well that is locked in after the drill has reached the gas reservoir. The 
iron tubing of the well becomes by this means a part of the reservoir, 
and the same conditions as to pressure are supposed to pertain to it 
that are found in the porous rock below. 
The rock pressure of gas varies greatly in different fields and to a 
less, but still an important, extent in different portions of the same 
field. The highest rock pressure recorded in the Trenton limestone is 
about 650 pounds to the square inch, while there are considerable por- 
tions of the gas territory that never reach 3800 pounds pressure per 
squareinch. The original pressure in the Findlay field was 450 pounds, 
varying somewhat in wells of different depths. In the Wood County 
field, from which the largest amount of gas is now being conveyed to 
Ohio cities, the original pressure ranged from 420 to 480 pounds, the 
general pressure being counted 460 pounds to the square inch. There 
were occasional records made of still higher pressure in single wells, 
but of such cases the number is very small, and the existence of these 
anomalous pressures was short lived. 
Passing to the westward, the gas wells of Auglaize and Mercer coun- 
ties show a decided reduction in original rock pressure aS compared 
with Findlay, though the depths of the wells remain the same as in that 
field. The highest pressure recorded in Mercer County is 390 pounds 
to the square inch, but no gauge was applied to the wells until they, 
had been allowed to discharge without restraint for several months, 
while 375 and 350 pounds mark the extreme limit of other portions of 
this district. 
In the Indiana field a still further reduction of rock pressure is to be 
noted. The range of the principal Indiana wells is between 250 and 325 
pounds to the square inch. The Indiana gas wells, as compared with 
Ohio gas wells, are marked by a reduction in total depth, as well as in 
rock pressure, the figures for depth in the productive territory seldom 
or never passing 1,000 feet. 
How can these variations be accounted for? Back of this question 
is a larger one, viz: What is the origin of the rock pressure of natural 
gas? 
THEORIES OF ORIGIN OF ROCK PRESSURE. 
Considering its importance, the main question has received less con- 
sideration than would naturally be expected. The known literature of 
the subject is very meager. Prof. J. P. Lesley, in the Annual Report 
of the Pennsylvania Survey for 1885, discussed the question at greater 
length than any other geologist, so far as I know. In a paper pub- 
