THE STRUCTURAL CLASSIFICATION 565 



been described by White/^ Andrews/^ and Evans." A map of a typical 

 portion of_it is shown in figure 3. The pools of the La Salle anticline 

 in Illinois and some of the California fields belong to this class, as does 

 at least onei.,field in Oklahoma and several in Texas. 



Subclass 11(h) — Wliere well defined alternoAing anticlines and syn- 

 clines exist. — This may be considered as a composite of Subclass 11(a). 

 With minor exce|)tions, it includes the pools of the Appalachian field in 

 Pennsylvania and West Virginia, some in southern Indiana and Illinois, 

 certain Oklahoma fields, the Caddo field of Louisiana, and certain fields 

 in Wyoming. The anticlinal crests in this subclass range all the way 

 from 2 or 3 miles apart, as in Trinidad, to the great geosynqline of the 

 Ohio Valley, which is at least ^00 miles across. 



The strata in the fields' of Subclass II(&) are folded into alternating 

 anticlines and synclines having dips seldom more than 30 degrees from 

 the horizontal. This is the subclass to which the anticlinal theory was 

 originally applied. It is illustrated in figure 4, where the sand is or has 

 been wet ; but the oil field occupies the syncline, where the sands are prac- 

 tically dry in a region. 



The Caddo field has geologically nothing in common with the Spindle- 

 top, Humble, Jennings, and other fields in the Coastal Plain of Louisi- 

 ana and Texas, but it has certain similarities in structure with the fields 

 of Pennsylvania, West Virginia, and Illinois. In northern Louisiana the 

 great oil-accumulating structure is the Sabine uplift, and the local dis- 

 tribution of oil and gas is due to minor anticlines and synclines, accom- 

 panied by differences in porosity of the Upper Cretaceous formations 

 which exist there. 



Several of the California oil fields also belong. in this class, namely, 

 the Coalinga field and the Los Angeles field, according to descriptions by 

 Eldridge^^ and by Arnold and Anderson.^^ The Baku and Surakhany 

 fields of Eussia and the Yenangyuang field in Burma apnear to come in 

 this subclass. The Negritos and Lobitos fields in Peru are reported to 

 lie on the eastern flanks of an extensive series of anticlinal structures, the 

 axis of which is almost parallel to the Pacific Ocean. 



Subclass 11(c) — Broad geanticlinal folds. — This is an extreme type of 

 11(a). By a geanticline is meant an anticline which is extremely long 

 and broad and constitutes more than a local feature, extending over 

 thousands or tens of thousands of square miles. One of the best examples 



27 L C. White: Bull. Geol. Soc. Am., vol. 10, 1899, p. 29. 



28 E. B. Andrews : Am. .Jour, Sci., 2(1 ser., vol. 32, 1861, pp. 85-9.3. 

 * E. W. Evans : Am. .Tour. Sci., 2d ser., vol. 32, 1866, pp. 334-343. 



29 Geo. H. Eldridge : Bull. 213, U. S. Geol. Survey, 1902, pp. 306-321. 



30 Ralph Arnold and Robert Anderson : Bull. 357, U. S. Geol. Survey, 1908, pp. 70-71. 



