THE STRUCTURAL CLASSIFICATION 



579 



Topography and structure. — In southern Louisiana are five prominent 

 elevations known as the "Five Islands/' or the "South Islands/' which 

 constitute the most conspicuous landmarks in hundreds of miles along 

 the coast of the Gulf of Mexico.^^ They rise from a few feet to 200 feet 

 above marsh level and in area range from 200 to 1,500 acres. They have 

 been frequently discussed in literature. Salt is found in all the Five 

 Islands except Cote Blanche. 



It should not be supposed, however, that every structure which is a 

 saline dome geologically is evinced on the surface by a topographic dome. 

 While instances like those mentioned above exist of the occurrence of 



Figure 11. — Cross-section of a typical Saline Dome Oil Field in Texas 

 After Hager. Subclass IV (c) 



mounds or small hills overlying the geological domes, the topographic 

 dome is not an essential of the type, and many saline dome pools are situ- 

 ated where the surface lies practically flat. 



The configuration of the strata in saline domes is a matter of interest 

 and great importance, as the structure is very different from the normal 

 southeastward dip of the Cretaceous and Tertiary beds underlying the 

 Gulf Coastal Plain of Louisiana and Texas. Whether or not there is any 

 particular surface topography indicative of a dome in the locality, there 

 is a very marked geological protuberance consisting of a sudden upward 

 bending of the strata as they approach the edge of the dome, so that they 



^ A F. Lucas : Trans. Am. Inst. Min. Engrs., vol. 29, p. 464. 



