226 Orton — On the Origin of the Rock Pressure 



above, while an extraordinary volume of manufactures has been 

 established on the new source of power. 



The exploitation, upon which the remarkable developments 

 above indicated' are based, has furnished better opportunities 

 in some respects for studying the phenomena and deducing 

 the laws of the accumulation of petroleum and gas than any 

 other American field has supplied. In the first place, the sur- 

 face of the country in which the drilling has gone forward is 

 so nearly level that all of the wells can be easily referred to a 

 common base, as for example, to mean tide ; in the second 

 place, the series of strata penetrated by the drill is so uniform 

 and so well marked that sharp and accurate determinations of 

 all the important facts of stratigraphy are possible, and in the 

 third place, the facts of structure are free from all complica- 

 tions, so that the effects due to them can be clearly and readily 

 followed. 



But a single one of the laws of the production of gas and 

 oil as brought to light by this recent experience will be con- 

 sidered in the present paper, that, namely, pertaining to the 

 rock pressure of gas ; but before taking up this subject, a few 

 preliminary statements, involving some of the laws already 

 recognized, are required. 



Preliminary Statements. 



1. The gas and oil of the Trenton limestone are held in 

 porous portions of the stratum. Neither caves nor fissures 

 have been found by the drill and none are necessary. 



2. The porosity of the limestone is due to the dolomitization 

 which it has undergone. The portions of the stratum thus 

 replaced seem to have been originally crinoidal limestone of a 

 good degree of purity. The spaces left by imperfectly inter- 

 locking crystals give the rock great storage capacity, as great 

 probably as the coarsest sandstones possess. 



3. The porous beds are distributed through the uppermost 

 portions of the stratum. None have been found as low as 100 

 feet below the surface, and almost all occur at less than half 

 this depth. Several consecutive beds of dolomite, separated by 

 ordinary and generally impure limestone, are occasionally found 

 in the section. These petroliferous beds commonly range from 

 seven to ten feet in thickness : they rarely rise to fifteen feet. 

 Without dolomite in the Trenton limestone there is no petro- 

 leum. The dolomitized regions of the Trenton limestone are 

 exceptional. They appear to extend from central Ohio north- 

 ward through Michigan and they certainly extend westward 

 through Indiana. To the south and east of these regions, the 

 Trenton limestone lacks this character and is unproductive as 

 to gas and oil. 





