130 PECKHAM — THE GENESIS OF BITUMENS. [April 1, 



portion filled with petroleum and the upper portion filled with gas ', 

 both originally under an enormous pressure. In Warren county, 

 farther to the southwest, the drill reaches petroleum not in the Mc- 

 Kean county sand, but in a different sand, higher in the series. 

 Still farther southwest, in Venango county, the surface rocks are 

 still higher in the series and the drill reaches petroleum in a pebble 

 conglomerate that outcrops at the surface to the northeast. These 

 pebble conglomerates, known as the ''Venango Oil Sands," 

 formed great riffles in the currents of the primeval ocean. They 

 are several miles long and a few rods wide, level on the upper sur- 

 face, and rounded on the under surface to a feather edge at the 

 sides. One is above the other and they are covered, when they 

 contain petroleum, with a solid, impervious shell of silica, that the 

 drill penetrates with difficulty. The uppermost of these conglom- 

 erates consists of spherical pebbles of yellow quartz, about as large 

 as cranberries ; the lowest consists of lenticular pebbles of very 

 white quartz. In both cases the pebbles are cemented together at 

 their points of contact leaving large open spaces. These conglom- 

 erates are sometimes replaced by coarse, porous sandstones ; neither 

 of these contain fossils of any kind. Still farther southwest, on 

 Slippery Rock creek in Mercer county, and at Smith's Ferry in 

 Beaver county, another sandstone, that is barren where it occurs in 

 Venango county, yields petroleum above the pebble conglomerate. 

 If a line be followed farther to the left, across western Pennsylvania 

 and into West Virginia, the outcrops of the formations would rise 

 successively in the scale until the oil would be found in the Mahon- 

 ing sandstone, which lies at the top of the Lower Productive Coal 

 Measures. Since the development of the Lima oil fields the range 

 of rocks holding the petroleum reaches in Ohio, Canada and Penn- 

 sylvania from the Lower Silurian, Trenton limestone, to the Lower 

 Coal Measures. These rocks embrace nearly the entire palaeozoic 

 formations of North America. Very few wells have been sunk 

 below the petroleum-bearing sandstone, for the obvious reason that 

 it involved a useless expense. One of the deepest wells ever drilled 

 in the oil region of western Pennsylvania was Jonathan Watson's deep 

 well nearTitusville. This well went down through all of the oil sands 

 and the Devonian shales beneath them, to a depth of 3553 feet, when 

 just as it was abandoned a hard rock was struck which was supposed 

 to be the Corniferous limestone, which is the oil-bearing rock of 

 Canada. The interval between the oil sands and the bottom of the 



