THE STRUCTURAL CLASSIFICATION 575 



in southern Oklahoma and the Petrolia pool in northern Texas owe their 

 position to distinct doming of the strata seems to have been first men- 

 tioned by Gardner/^ the structure of the Petrolia pool being originally 

 worked out by Udden and Phillips.* The anticlines and domes have since 

 been mapped for the United States Geological Survey and by the author 

 of this paper, as well as many other geologists in private work. The 

 doming is supposed to have taken place both before and after the deposi- 

 tion of the Permian red beds, which, near the Arbuckle and Wichita 

 Mountains, lie unconformably on the Pennsylvanian series. Oil exists 

 both in the Pennsylvanian and Permian. 



Subclass IV(b) — Monoclinal bulges. — This type is frequently confused 

 with the anticlinal-bulge type, but is quite distinct in structure. Anti- 

 clinal bulges are expansions and elevations in the crest of definite anti- 

 clines or continuous folds, while monoclinal bulges are domes that rise 

 with apparent irregalar spacing on a monoclinal slope in which struc- 

 tures of Class III also exist. In Subclass IV (b) the monoclinal structure 

 gives place locally to a quaquaversal structure. On the great monocline 

 of central Ohio few domes are kno^vn in the Clinton sand and are fre- 

 quently absent in the Berea for long distances. In Kansas and Okla- 

 homa, however, monoclinal bulges form one of the commonest forms of 

 structure, an illustration of which is given in figure 10. Since in that 

 part of the country the sands are commonly saturated with water, the 

 oil and gas both occur on the dome itself. 



Subclass IV(c) — Closed saline domes. — History of saline dome devel- 

 opments. — The credit of discovering that this form of domes contains oil 

 is due largely to Captain Lucas,** who in 1901 drilled a well at Spindle- 

 top, Texas, and discovered a famous field. As early as 1894 diamond- 

 drill borings had been made by him at Jefferson Island, Belle Isle, Weeks 

 Island, and Anse La Butte, Louisiana, discovering salt masses of limited 

 area, but of great depth. In 1899 a paper on this subject was first pub- 

 lished by Lucas*^ and the discoveries were confirmed by a paper published 

 by Hill in 1902.*^ The last-mentioned writer says : 



"Before tlie discovery of Spindle Top there was only one man whose ideas — 

 although not yet coordinated into a theory — approximately fitted the observed 

 conditions. Of course, I refer to Captain Lucas, who, in his explorations of 

 the Coastal Plain, seeking successively salt, sulphur, and oil, had observed the 



43 .James H. Gardner : P^con. Geol., vol. 10, no. 5, 1915, pp. 422-434. 



* J. A. Udden and D. McN. Phillips : Tex. Univ. Bull. 246, 1912. 



*4 A. F. Lucas : The dome theory of the Coastal Plain. Science, n. s., vol. 35, no. 912, 

 June 21, 1912, pp. 961-964. 



^ Rock-salt in Louisiana. Trans. Am. Inst. Min. PJngrs,, 1899 ; also Journ. Ind. and 

 Eng. Chemistry, vol. 4, no. 2, Feb., 1912. 



*6 R. T. Hill : Journ. Franklin Inst, vol. 154, Aug, and Oct., 1902, pp. 143, 225, 263. 



