388 



PALAEOZOIC SYSTEM OF ROCKS. 



much more true of gas- wells ; they must be very short-lived. In the 

 case of less productive but more permanent wells, the oil is contained 

 in more numerous but smaller fissures and pores. In all cases of col- 

 lection in large fissures and cavities, these reservoirs are occupied also 

 by water and gas ; and the three materials arrange themselves in the 

 order of their relative specific gravities, as in Figs. 517 and 518. 



These facts easily account for the many curious phenomena con- 

 nected with oil-wells. Thus, if the well a (Fig. 517) taps the reser- 

 voir, only gas will escape, and oil 

 and water can be got only by 

 pump. But if the well be at b, 

 oil will spout ; and afterward, 

 when the gas has escaped, oil 

 and water may be pumped. If 

 the well be at c, then water will 

 spout first and afterward oil. If 

 the cavity be irregular, with 

 more than one chamber contain- 

 ing compressed gas (Fig. 518), 

 and the well be at a, then gas will escape first, and afterward oil and 

 water will spout. 



Kinds of Rocks which bear Petroleum. — As already stated, petro- 

 leum, like water, is found principally in pores and fissures and cavities. 

 The same kinds of rocks, therefore, which are water-bearing are also 

 oil-bearing, viz., limestones and sandstones. In Canada it is found in 

 limestone, in Pennsylvania in sandstone. The intervening shales are 

 usually barren. In Pennsylvania there are three oil-bearing sandstones, 

 separated by about 200 feet of intervening shales. If a well reaches 

 the first sandstone without obtaining oil, the boring is continued to 

 the second, or even to the third. Fig. 519 (taken from Lesley) rep- 

 resents a section through the Pennsylvania oil-regions, showing the 



Fig. 518. 



DUNKAROS CR 



BRAOYS BEND 



OIL CR 



Fig. 519. 



three principal oil-horizons of the United States, viz., the Venango 

 County (Pennsylvania) horizon with its three sandstones ; theVirginia 

 sub- Carboniferous horizon above ; and the Canada horizon below. 



Petroleum (especially the lighter oils) is usually found only in hori- 

 zontal or gently-folded strata, because strongly-folded and crumpled 

 strata are always metamorphic, and the heat which produced meta- 

 morphism has also concreted the oil into bitumen or asphalt. Also the 

 outcropping of the edges of highly-inclined strata favors the escape of 



