116 CLASSIFICATION OF THE PUBLIC LANDS. 



sands may bring about communication between the different sands 

 and have a notable -effect on local production. In a series of uni- 

 formly dipping beds an oil sand which would normally crop out at 

 the surface may be cut off by a strike fault and sealed beneath imper- 

 vious beds and thus retain oil which would otherwise migrate to the 

 surface and be dispelled. Moreover, faulting may produce fractured 

 zones along which the oil or gas can migrate and in which it may 

 collect. In a number of localities, as in some of the fields in Mexic®, 

 where intrusive dikes have pierced oil-bearing strata and conse- 

 quently arrested the movement of the oil in certain directions, the 

 petroleum has accumulated in apparent disregard of the structural 

 features of the sedimentary series. 



In many fields there is little or nothing at the surface to indicate 

 the presence of valuable hydrocarbons below, but in many other fields 

 there is ample indication of oil at the surface. The oil-bearing 

 stratum itself may crop out and the oil ooze from it, giving to the 

 rock a dark, greasy appearance and the odor of petroleum, or the oil 

 may find its way to the surface from the oil pool below through 

 some fracture of the overlying rock. Water charged with various 

 salts or with sulphur may rise with the oil, so that a spring is formed, 

 the oil floating as a brown scum on the surface of the water or in 

 smaller quantity producing the brilliant iridescent sheen character- 

 istic of petroleum. Gas may find its way to the surface and appear 

 in " gas springs " or under certain conditions may produce the phe- 

 nomenon of mud volcanoes. The place at which oil has come to the 

 surface and evaporated through long periods of time may be marked 

 by a deposit of asphaltum. In certain localities oil-bearing shales 

 have been burnt to a pink or deep brick-red color or altered to a 

 hard vesicular rock resembling scoriaceous lava. This metamor- 

 phism is due to the burning of the hydrocarbons that have impreg- 

 nated the rock, and the presence of such rock therefore becomes an 

 important surface indication of petroleiim. 



The stratigraphic occurrence of hydrocarbon minerals in the United 

 States is by no means limited; on the contrary, petroleum in the 

 solid, liquid, or gaseous form is found in greater or less quantity 

 throughout the range of strata from the Cambrian to the yoimger 

 members of the Tertiary series. 



In general the commercially important accumulations of oil through- 

 out the central and eastern portions of the United States are found 

 in strata belonging to the Paleozoic era. In the great Appalachian 

 field, which extends from the southern portion of New York along 

 the western slope of the Allegheny Mountains to northern Tennessee, 

 the accumulations of oil occur in strata ranging in age from early 

 Devonian to late Carboniferous. In Ohio and Indiana petroleum is 

 derived chiefly from rocks of' Ordovician age, and in Indiana mainly 



