THE STRUCTURAL CLASSIFICATION 585 



volume is many cubic miles. The theory of Harris, if not modified, requires a 

 supply of roughly 50 times as many cubic miles of salt. 



"Yet the 'concretionary' theory of Harris has much in its favor, since it 

 meets the mechanical requirements of the problem better than any other. The 

 chemical difficulty can be met if the salt domes have been the loci of the .escape 

 of solutions carrying a common ion, either of sodium or of chlorine. Analysis 

 of some volcanic waters shows an abundance of sodium chloride, and others of 

 sodium sulphate or carbonate, but these do not meet the requirements of the 

 present problem. From the character of the water and from the presence of 

 se("ondary lenses of dolomite, one may infer that the precipitation was pro- 

 duced by the intermingling of concentrated salt solutions with brines rich in 

 magnesium and calcium chloride. The former were probably derived mainly 

 from the sedimentary strata, as suggested by Harris, but the latter probably 

 rose from underlying plugs of olivine basalt which failed to reach the surface." 



A late paper on the sul)ject is by Norton.''^ The doming seems to have 

 appeared first in late Cretaceous time and to have continued to a greater 

 or less degree ever since. There is no doubt that the salt, sulphur, and 

 gypsum are of later origin than the overlying sediments. Dumble states®^ 

 that "the Sun mounds near Waller and Damons Mound are part Lafay- 

 ette." The salt domes of Transylvania, which belong to the same type, 

 are still forming, as evinced by studies of the present writer. 



The great similarity between the bosses of salt and gypsum in Texas 

 and Louisiaiia and those of basalt in Mexico has been mentioned by Gar- 

 fias.*'-' An ingenious European theory is that the saline domes of Ger- 

 many, Transylvania, and Eoumania have been caused by the lateral flow- 

 ing of beds of salt into the domes owing to pressure in the overlying and 

 underlying strata. So many theories, all with their advocates, and ac- 

 companied by equally as good arguments, go a long way to support the 

 view that all our material theories must in time give way to something 

 more tangible than materialism. 



Subclass IV (d)— Volcanic plugs. — The best known examples of the 

 volcanic neck type of quaquaversal structure come from the Coastal Plain 

 of Mexico, which contains oil fields connected with many types of geo- 

 logic structure, several of which are quaquaversal. The type in question 

 consists of plugs or necks of basalt and similar rocks which rise through 

 the Cretaceous and Tertiary sediments in the Coastal Plain to elevations 

 of several hundred feet. While little drilling has as yet been done in the 

 vicinity of the necks, and the geological structure is therefore somewhat 



«i E. G. Norton : Origin of the Louisiana and east Texas salines. Am. Inst. Mining 

 Engrs., Bull. No. 97, Jan., 1915, p. 93. 



82 E. T. Dumble : The occurrences of petroleum in eastern Mexico as contrasted with 

 those in Texas and Louisiana. Fuel Oil Journal, Oct., 1915, p. 86, 



^ v. R. Garfias : The effect of igneous intrusions on the accumulation of oil iu north- 

 eastern Mexico. Jour. Geol., vol. 20, no. 7, Oct.-Nov., 1912, p. 666. 



