of tht Natural Gas of t/u Trentoti Limestone, 227 



4. The gai and oil are separated in every division of the 

 tield where both occur by lines of geographical Level. Proper 

 relief of the porous rock in the shape of arches or terraces is 

 indispensable to gas fields and oil fields. The more pro- 

 Dounced the forms and the amount of the relief, the greater 

 the accumulation. 



5. Salt water or bittern is invariably found at a depth, con- 

 stant for each subdivision of the field, in the lower levels of 

 the porous rock, constituting a dead line for the oil and gas. 

 The water is directly in contact with the oil or in the absence 

 of oil, with the pis that is held in the arches or terraces, as is 



? roved by the behavior of wells on the margins of the held, 

 t is highly mineralized, having a specific gravity of lvl. 



6. When a well is drilled outside the dead line, the salt 

 water ri>es more or less promptly as soon as the porous bed of 

 the Trenton is penetrated. These salt water tracts are very 

 extensive as compared with the areas of gas and oil, occupy- 

 ing hundreds of square miles to one of the former. 



7. The height to which the salt water rises in the various 

 portions of the new gas and oil fields, appears to be nearly 

 constant. It reaches an elevation of about 600 feet above tide. 

 If the elevation of the surface is less than 600 feet, as in por- 

 tions of the Wabash Valley, the salt water flows from the well 

 mouth. 



8. The rise of the salt water to the level named is unques- 

 tionably artesian in origin. It implies a head of water, estab- 

 lished through continuously porous rock, from some more or 

 less distant outcrop. The fact that the outcrops of the Tren- 

 ton limestone on the shores of Lakes Superior and Huron are 

 dolomitic and consequently porous, and that they are also 

 about boo feet above tide, suggests this region as the source of 

 the pressure which is shown in the ascent of the salt water in 

 the gas and oil fields. The Trenton limestone is dolomitic as 

 far as it has been followed under the surface of the State of 

 Michigan, both southward from Lake Superior and northward 

 from the Ohio boundary, the facts as to the latter being ob- 

 tained by the drill. 



Hock Pressure. 



By the rock-pressure of gas is meant the pressure in a well 

 which is locked in so that no gas can escape. The tubing of 

 the well becomes in this way a part of the reservoir and shares 

 its pressure. This rock pressure varies greatly in different 

 fields and to a less extent in different wells of the same field. 

 It sometimes reaches the enormous figures of 800, 900, or even 

 1000 lbs. per square inch. Pressures of 400 to 600 pounds to 

 the square inch are not unusual. The phenomena connected 



