THE STRUCTURAL CLASSIFICATION 



567 



in this country is the Cincinnati anticline, in which immense reservoirs 

 of oil and gas have been developed and exhausted from the Trenton lime- 

 stone. Owing to the broad areas under which oil is found in the Cin- 

 cinnati anticline, the chances of success in drilling were originally much 

 better than in other fields. The pools in the Clinton sand in Ohio are 

 situated along the eastern flank of the Cincinnati anticline, but these 

 pools belong under Class III. A cross-section of the Trenton limestone 

 field appears in figure 2. Another great geanticline, which is important 

 for natural gas development, is that in western Canada, extending north 

 from the International Boundary to the Athabasca River.^'^ 



Subclass 11(d) — Overturned folds. — Examples of oil and gas occurring 

 in connection with overturned folds are not common, but instances are 



Figure G. — IdcaJ Section in a pinching Sand 

 Showing the rehitions of gas, oil, and water according to Subclasses II (e) and III (d) 



conspicuous in California, as shown by Arnold and Johnson (figure 5).^^ 

 In this case the overturned formations are the retaining ones, while the 

 oil is contained in the synclinal portion of the sand. Other instances are 

 reported from Galicia and Eoumania. 



Subclass 11(e) — Lenticular nature of the sands. — In all types of struc- 

 ture there are numerous instances where the sandstones or other porous 

 oil-bearing beds are locally too hard or close grained to hold the oil or 

 they pinch out laterally between shale beds. It is necessary, therefore, 

 to add a new subclass under Classes II and III to include these lenses. 

 A typical example is shown in figure 6, where the sand pinches out toward 

 the anticlinal crest, causing the gas to collect on the side of the anticline 



^ F. G. Clapp and L. G. Huntley : Petroleum and natural gas resources of Canada, by 

 F. G. Clapp and others, vol. ii, 1915, pp. 271-272. 



32 Ralph Arnold and Harry W. Johnson: Bull. 406, U. S. Geol. Survey, 1001, p. 97. 



