PETROLEUM 367 



ably covers a vastly greater area than the portions that are 

 productive. This is the case also in some instances where the 

 structural conditions may favor its accumulation, and the 

 surrounding slates and shales are similar. When productive 

 rocks can be traced to their outcrop there is invariably no 

 trace of petroleum or asphaltum to be found in them. The 

 wells drilled outside of these favored localities usually find no 

 trace of })etroleum or natural gas, althougli numerous sands 

 may have been pierced which are productive elsewhere and 

 are sufficiently open and porus to be receptacles. 



Petroleum, in both its liquid and soHd forms, is largely 

 made up of carl3on and hydrogen. It ranges from 77° Baume, 

 or 0.6763 specific gravity, in the case of the lightest naphtha, 

 condensed naturally from natural gas, up to between 1.1 and 

 1.3 in its more dense and solid form, such as natural asphalt, 

 which usually contains from 50 to 80 per cent of carbon, 6 to 

 10 per cent of hydrocarbon, and 8 to 10 per cent of sulphur. 

 These solid forms are widely distributed, vary greatly in com- 

 position, and are usually associated, in varying proportions, 

 with more or less silica and carbonate of lime. 



There is a vast difference in the character of petroleum 

 produced in the United States and in foreign countries, and 

 a great variety in the character of the strata which contain 

 it. The infiltration of the petroleum into different sands and 

 limestones has had much to do with imparting to it the varied 

 physical and chemical properties. However, the structural 

 conditions of these strata and the position they must occupy 

 in order to bring about a concentration of the petroleum are 

 similar in all of the productive fields. Its occurrence in small 

 quantities in strata where the conditions necessar}^ for conden- 

 sation are wanting has in many instances been the cause of 

 financial loss and disappointment. 



The intimate association of the Hquid and gaseous hydro- 

 carbons has made it difficult to separate them when one or 

 the other is discussed. The great deposits of petroleum in the 

 Appalachian field are in a series of sand rocks reaching up from 

 the lower Devonian to the upper Barren measures or ^lonon- 

 gahela formation, less than 100 feet above the Pittsburg coal, 

 and embraces not less than 50 distinct horizons. The greater 



