Theories of Origin of Kock Pressure. 



Considering its importance, the main question has received less considera- 

 tion than would naturally be expected. The known literature of the subject 

 is very meagre. Professor J. P. Lesley, in the Annual Report of the Penn- 

 sylvania Survey for 1885, discussed the question at greater length than any 

 other geologist, so far as I know. In a paper published in the American 

 Manufacturer May 27, 1887, I threw out a few suggestions as to the cause 

 of rock pressure, and these suggestions I afterwards expanded into a more 

 extended statement, in the sixth volume of the Geology of Ohio, page 96. 

 Professor I. C. White reminds me that he suggested an explanation in the 

 journal named above at an earlier date than either of those given. 



The men who are engaged in the practical development of gas and oil 

 fields make great account of rock pressure. It is the first fact that they 

 inquire after in a new gas field. They appreciate its importance in whatever 

 utilization of the gas they may propose, knowing that the distance of the 

 markets that they can reach and the size of the pipes that they can employ 

 are entirely dependent upon this element. These practical men, so called, 

 are, as is well known, among the most venturesome of theorists, and a ques- 

 tion like this would not be likely to be left unanswered by them. A certain 

 rough correspondence that exists between the depth and the rock pressure 

 of wells is made of great account in explanations that they offer. In other 

 words, the pressure is supposed to be due to the weight of the overlying 

 rocks ; and next to this we find among them the expansive force of gas the 

 favorite explanation of the phenomenon. 



In the paper of Professor Lesley, already referred to, the learned author 

 suggests the two possible explanations of rock pressure already named, and 

 to this he adds a third, viz., hydraulic pressure ; but he adds this explanation 

 only to reject it as a true cause of the phenomenon under discussion. The 

 absurdity of the more commonly received explanation of rock pressure, as 

 due to the depth of the well— in other words to the weight of the overlying 

 country, — he sets in such clear light in his discussion that no further con- 

 sideration of this is required on the part of those who are open to reason. 

 Until we can prove, or at least render it probable, that the gas rocks have 

 lost their cohesion and that they exist at the depths of storage in a crushed 

 or comminuted state, no explanation can be based upon the weight of the 

 overlying rock in accounting for the force with which the gas escapes from 

 its reservoirs when they are penetrated by the drill. Professor Lesley throws 

 the whole weight of his authority in favor of the view that the gas "produces 

 its own pressure, like gas generated by chemical reaction iu a closed vessel." 

 This explanation certainly leaves something to be desired, for it fails to 

 -account for the most significant and important tacts in this connection, viz., 

 the differences of rock pressure in different localities and at different depths- 



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