1865.] 57 [Lesley. 



of XI and the Limestone of XI, taken together, are only 

 300 or 400 feet thick, and the Knobstone is 350 to 550 feet 

 thick, then the top of the black slates under the sandstone 

 might be struck at the depth of the Fint Venango Sandrock 

 (700 feet), and certainly would be at the depth of the Third 

 (1100 feet), or even of the Second (900 feet). 



It is evident, therefore, that all reference to the " Three 

 Sandrocks" of the Oil Creek country is useless for countries 

 to the southwest of it, and will be made only by those who 

 are ignorant of the general bearings of the subject. 



We can only say, that part of the 350 to 550 feet of Knob- 

 stone Form., X and VIII, represents the Oil Creek Forma- 

 tion, and, perhaps, contains one or more like horizons of oil ; 

 but whether in one, two, three, or what number of oil-bearing 

 • sandrocks, separated by oil-preserving shales, nothing but 

 actual experiment can determine. 



It is probable that the wells which penetrate the Lime- 

 stone XI get their petroleum partly from the Conglomerate 

 above, descending with the drainage waters. But it is still 

 more likely that they get their principal amount of petroleum 

 from the Knobstone Formation below, by a system of fissures 

 similar to that of the Venango Oil region. In any case they 

 are bound to prove productive; and I have not the least 

 doubt that wells, sunk 600 to 800 feet along the Paint Creek 

 Valley, will produce reasonably profitable amounts of Upper 

 Devonian Petroleum, steadily, for an indefinite number of 

 years ; and this petroleum will be, of course, light oil, and 

 not the heavy oil of the Paint Creek Valley surface. 



The amount of petroleum capable of being held by rocks 

 themselves is far greater than people imagine. They hold it 

 in three ways : 1. By being more or less gravelly and porous 

 throughout ; 2. By being cracked in systems of cleavage 

 planes throughout ; 3. By being traversed by large fissures, 

 which are, probably, all of them merely enlargements of 

 cracks along the cleavage-planes. 



Every foot of gravel-rock may be considered to consist of 

 three-fourths quartz, &c., and one-fourth cavity, cleaned out 

 by long percolation, and now occupied by water and oil. 



VOL. X. — H 



