THE STRUCTURAL CLASSIFICATION 591 



Subclass V(d) — Contact of secUmentaries tvith older igneous rocJcs. — 

 While there are no positively known occurrences of oil according to this 

 type, gas does exist in small quantities in this way in the provinces of 

 Quebec and Ontario and in northern New York State, where it is under 

 great pressure. So far as the writer has been able to learn from his in- 

 formants, it is contained in the lower zone of the Potsdam sandstone, of 

 arkose structure, resting directly on the underlying granite or gneiss. 



Figure 15. — Hypothetical Cross-section of an intrusive Bed in the Coastal Plain of 



Mexico 



Illustrating occurrence of oil according to Subclass V (c) 



CLASS VI — STRATA DIPPING UNCONFORMABLY AWAY FROM AN OLD 



SHORELINE 



A good example of oil in an unconformity on a monocline is brought 

 by Thompson^^ from the Maikop field of Russia, where inclined strata of 

 Upper Oligocene and Lower Neocene age, dipping 7 to 10 degrees, rest 

 unconformably on an overlap Cretaceous strata, as shown in figure 16. 

 The unconformity has sealed up the upper end of the Neocene sands. 

 The structural position of some pools in the vicinity of the Arbuckle and 

 Wichita Mountains, in southern Oklahoma, may be similar. 



71 Trans. Instn. Min. and Met., vol. 20, 1910-1911, p. 229. 



