ORIGIN OF ROCK PRESSURE OF NATURAL GAS. 159 
or even 150 feet in thickness. Sometimes the dolomite is found in a 
continuous body, but oftener in interrupted beds. This part of the for- 
mation has outcrops in the Manitoulin islands of Lake Superior and in 
the Galena limestone of Hlinois and Wisconsin. In the gas and oil fields, 
it is found lying in terraces and monoclines, or flat arches, 800 to 1,500 
feet below the surface; and these several features effect the separation 
of the varied contents of the porous rock. The boundaries of gas, oil, 
and salt water are easily determinable and are scrupulously maintained 
in the rock, except that as soon as development begins the salt water 
is always the aggressive and advancing element. When the drill de- 
scends into the gas rock proper dry gas escapes; when into the contig- 
uous and lower-lying terrace, oil accompanied with gas appears, as al- 
ready described; but at a little lower level salt water is struck, and this 
rises promptly in the well, sometimes to the point of overflow. Far out 
from the narrow ridges or restricted terraces where gas and oil are found 
the salt water reigns undisturbed, and wherever reached by the drill it 
rises in the wells as in those already described. It would be in the high- 
est degree absurd to count the little pockets of gas that are found in 
the arches the cause of the ascent of this ocean of salt water a score 
or a hundred miles away. The rise of the salt water is unmistakably 
artesian. It depends on hydrostatic pressure, as does the flow of all 
artesian wells, and its head must be sought, as in other like flows, in 
the higher portions of the stratum that are contiguous. 
The nearest outcrops of this porous Trenton have been already 
named, They are found in the shores of Lake Superior at an altitude 
of about 600 feet above tide. It is certainly significant that when an 
abundant flow of salt water is struck in a boring in northern Ohio or 
in Indiana, no matter at what depth, it rises generally about to the level 
of Lake Superior; or, in other words, about 600 feet above tide. If the 
mouth of the well is below this level, as is the case in the Wabash Val- 
ley, the salt water overflows. On the shore of Lake Erie the water 
rises to within 20 feet of the surface; in Findlay, to within 200 feet. 
The height to which the salt water rises in any portion of the field is 
one of the elements to be used in measuring the force which can be 
exerted on the gas and oil that are caught in the traps of the terraces 
and arches of the porous Trenton limestone. 
Why, then, is not the rock pressure of the gas the same in all por- 
tions of the new horizon? For the obvious reason, I reply, that there 
is a varying element involved, viz., the depth of the rock below sea level. 
The surface elevations at the wells vary greatly, and the wells of the 
same depth consequently find the gas rock in very different relations 
to sea level. 
THE TEST OF THE HYDROSTATIC THEORY. 
It is obvious that if an explanation of the rock pressure of the Tren- 
ton limestone gas is attempted on this basis, there are facts enough 
now at command to substantiate or overthrow it. By the facts it must 
