THE DEVONIAN PERIOD 



571 



Economic Products 



Gas and oil. The Upper Devonian is the chief source of oil 

 and gas in western Pennsylvania 1 and southwestern New York, 

 and is one of the sources in West Virginia. 2 The Middle Devonian 

 is the oil-producing series of Ontario. Within the regions of their 

 occurrence, oil and gas are more likely to be found under low anti- 

 clines than in other positions, apparently for the reason that anti- 

 clines furnish an inverted basin capable of holding these light sub- 

 stances against the pressure of the heavier subterranean water 



Fig. 412. Section showing the relations of the Devonian and other Paleo- 

 zoic systems in the vicinity of London, Tenn. ~G = Cambrian; O 

 Ordovician; S = Silurian; D = Devonian; au = age unknown. Length 

 of section, about 7 miles. (Keith, U. S. Geol. Surv.) 



which tends to force them to the surface. In all cases there must 

 apparently be an impervious bed or combination of formations 

 above, to prevent the escape of the oil and gas. In this there is a 

 certain similarity to the conditions requisite for artesian wells, but 

 with the difference that the artesian wells receive their supplies 

 from above and must be closed below, while the oil and gas wells 

 receive their supplies from below and must be closed above. 



The lower part of the Chattanooga shale of central Tennessee 

 is the horizon of black phosphates, which are of some importance 

 commercially. 3 



THE FOREIGN DEVONIAN 



The continent of Europe. At the close of the Silurian there 

 seem to have been more considerable geographic changes in Europe 

 than in America, for the Devonian system is there more commonly 



1 Carll, Kept. I, 5, Penn. Geol. Surv., 1890. For statistics on the produc- 

 tion of oil, gas, etc., see Mineral Resources of the United States, an annual 

 publication of the U. S. Geol. Surv. 



7 White, West Virginia Geol. Surv., Vol. I, Oil and Gas, pp. 208 and 212. 

 This volume is an important contribution to the literature of oil and gas. 



3 Columbia (Tenn.) folio, U. S. Geol. Surv. 



