ROCK PRESSURE. 99 
from the Trenton limestone in that region exhibits the unmistakable 
characteristics of shale gas. One anomalous and thus far inexplicable 
fact, however, comes into view in connection with it. namely, its amaz- 
ing rock pressure. I have already stated the initial rock pressure of a 
single well as 1,525 pounds to the square inch, a higher figure than ever 
recorded before in gas wells of any description, so far as my knowledge 
goes. 
This rock pressure bears a certain relation, thus far undetermined and 
unexplained, to the depth at which the gas is found. I do not propose 
to consider this subject now, but it is evident that the extraordinary fig- 
ures found in this field require us to add an important qualification to the 
artesian theory of rock pressure. 
Trenton limestone gas, whether found in reservoir rocks associated with 
salt water or in thin bedded limestones or shales which carry no water, 
has the same chemical and physical properties, and the compression 
under which it is found must have the same root in both cases. That 
root, as the later facts show, is not the weight of a salt-water column, as 
I once believed, but the expansive power of the gas itself, modified in 
some unexplained way by the thickness of the overlying section. In 
cases where the artesian theory of rock pressure is applicable, the com- 
pression due to the expansive power of the gas is less than that which 
the weight of the salt water gives, and is therefore marked and measured 
by the latter elements. This is the qualification which the new facts 
oblige us to add to the artesian theory. 
DuRATION OF PETROLEUM SUPPLY 
[have now completed the task which I set for myself. I have pointed 
out some of the principal geological probabilities as to petroleum and 
its derivatives. I hope that you will not find reason to complain of an 
undue amount of subjective color in my statements. 
Tam well aware that there are within the limits of our ever widening 
science many larger and more important subjects than that which I have 
brought before you, but this, too, deserves its “day in court.” In fact 
it has some peculiar claims on geologists. Petroleum is a form of stored 
power, and geologists know better than other men the priceless value of 
such accumulations. They know that on them the well being and 
progress of the race largely depend, and that without them civilization 
cannot long maintain the pace which the nineteenth century has set. 
They know that these stocks of buried ight and heat and power are 
small at the best and demand the most careful husbandry. They know, 
too, that petroleum and its derivatives, by virtue of their essential char- 
