314 HISTORICAL GEOLOGY. 



Like water, and with water, it often oozes on the surface 

 as hillside springs. With water, it collects in fissures 

 and cavities of all kinds, from which, through artesian 

 wells, it issues, in some cases, in great quantities as 

 fountains. 



But, unlike water, there is no great, continuous, peren- 

 nial supply ; and also, unlike water, the force by which 

 it spouts is not by hydrostatic pressure alone, but hydro- 

 static pressure transmitted through the elastic force of 

 compressed gas always associated with the oil. As gas, 

 oil, and water are nearly always found together, these 

 arrange themselves in the order of their relative specific 

 gravities ; and therefore in a flowing well the water usu- 

 ally appears only after the gas and oil are exhausted. 

 The flow of oil wells is not perennial like water, because 

 the oil is not continually re-supplied. The accumulation 

 of ages is now being exhausted with a rapidity propor- 

 tioned to the abundance of the flow. What is true of 

 oil is much more true of gas. A gas well is necessarily 

 short-lived. 



Age of Petroleum-bearing Strata. Petroleum has 

 been found in strata of nearly all ages, but under the 

 two conditions of local abundance of the organic matter 

 from which this substance is formed, and the absence of 

 metamorphism, which always changes it into asphalt. 

 At one time it was supposed to be characteristic of newer 

 rocks, having been found in foreign localities, mostly in 

 Tertiary strata. But in the Eastern United States it is 

 confined to the Paleozoic rocks, while in California it 

 is again found only in the Tertiary. 



The great petroleum area of the Eastern United States 

 is the Paleozoic basin. In this basin it is found on sev- 

 eral horizons, but always below the coal-measures. The 

 most celebrated, viz., the Pennsylvania horizon, is in the 

 Upper Devonian. The Canada horizon is in the lowest 

 Devonian. In West Virginia it is in the sub-Carbonifer- 



