440 GEOLOGY. 



the adjacent region to the north, do the Devonian strata appear to 

 have been notably disturbed at the close of the period. Here the 

 Devonian strata are overlain unconformably by the Carboniferous. 

 Elsewhere the formations of the younger system rest on those of the 

 older without stratigraphical break. 



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 oil being derived chiefly from 

 porous beds. The Middle Devonian is the oil-producing series of 

 Ontario (Enniskillen). 



Within the regions of their occurrence, oil and gas are more likely 

 to be found under low anticlines than in other positions, for the apparent 

 reason that these furnish an inverted basin capable of holding these 

 light substances against the superior pressure of subterranean waters 

 which tend 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, and in this there is a certain 

 similarity to the conditions requisite lor 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. Both require a porous 

 bed as a reservoir which, in the one case, ideally — but not always 

 actually — forms a basin concave above, in the other, concave below. 

 It is the general opinion of geologists that the oil and gas of the Devonian 

 beds were derived from organic matter entrapped in the strata below, 

 though distinguished chemists suggest an inorganic origin for many 

 or most such products. 3 



The lower part of the Chattanooga shale of central Tennessee is 

 the horizon of black phosphates, which are of considerable impor- 

 tance commercially. 4 



1 For report on the oils and gas of Pennsylvania, see Carll, Rept. I, 5 Penn. Geol. 

 Surv., 1890. For statistics on the production of oil, gas, etc., see Mineral Resources 

 of the United States, an annual publication of the U. S. Geol. Surv. 



2 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 Orton, 8th Ann. Rept., U. S. Geol. Surv., Pt. II, and White, op. cit. p. 180 et seq. 

 * Hayes and Ulrich, Columbia (Tenn.) folio, U. S. Geol. Surv. 



