THE TRENTON LIMESTONE As A GAS ROCK. 91 



Bearing these several sources of ambiguity or uncertainty in mind, we can 

 consider the records of pressure, depth, and the other factors that are ac- 

 cessible. The figures as to pressure have already been summarized in a pre- 

 ceding paragraph, but they will be repeated in an accompanying 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 Tren- 

 ton 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, five, fifty, one hundred, or even one 

 hundred and fifty feet in thickness. Sometimes the dolomite is found in a 

 continuous body, but ofteuer in interrupted beds. This part of the formation 

 has outcrops in the Manitoulin islands of Lake Superior, and in the Galena 

 limestone of Illinois and Wisconsin. In the gas and oil fields, it is found lying 

 in terraces and monoclines, or flat arches, eight hundred to fifteen hundred 

 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 advaucing element. When the drill descends into the gas 

 rock proper, dry gas escapes ; when into the contiguous and lower-lying 

 terrace, oil accompanied with gas appears, as already 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 undis- 

 turbed, and wherever reached by the drill it rises in the wells as in those 

 already described. It would be in the highest 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 arefouud in the shores of Lake Superior at an altitude of about six 

 hundred 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 six hundred feet above tide. If the mouth of the 

 well is below this level, as is the case in the Wabash valley, 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 i n 



