28 PROCEEDINGS OF THE OTITOAGO MEETING 



1. The Northern Basin. 



2. The Southwestern Basin. 



3. Tlie Ordovician-Silurian Arch. 



4. Tlie Mississippian Rift Zone. 



Northern Basin: Considering the Silurian as the floor of the basin, it is 

 occupied by about 400 feet of Devonian, Mississippian, and Pleistocene de- 

 posits. 



Southwestern Basin: Contains Devonian, Mississippian, Pennsylvanian, and 

 Pleistocene rocks measuring a total thickness of 2,500 feet. 



Ordovician-Silurian Arch: Separates the two basins; is a northward exten- 

 sion of the Cincinnati Arch; bedrock surface of the arch composed of Ordo- 

 vician and Silurian strata, the former in the southeastern and the latter in 

 the central and northwestern parts of the State. 



Mississippian Rift Zone : Lies on the southwest side of the arch and roughly 

 parallels it ; in a general way follows the western border of the outcrop of the 

 Knobstone (Keokuk) division of the Mississippian; is characterized by several 

 structural features, namely. Mount Carmel Fault, Heltonville Fault, Denniston 

 Anticlinal. 



Mount Carmel Fault : Probably extends from the Ohio River northwestward 

 to the Illinois-Indiana boundary ; amount of throw, 300 feet ; direction, east 

 and north ; evidence of two periods of movement. 



Heltonville Fault: Minor displacement one mile west of the Mount Carmel 

 Fault at Heltonville ; parallel with the major fault ; Warsaw faulted against 

 Keokuk ; may pass into the Mount Carmel Fault and into the Denniston Anti- 

 clinal Structure. 



Denniston Anticlinal : Produced by downthrow of southwestward dipping 

 strata toward the east and north ; characterized by a short, steeply dipping 

 limb toward the fault and a more gradual dip on the longer opposite limb. 



Discussion 



Prof. W. H. HoBBS : The glacial anticyclones to which Mr. Visher has re- 

 ferred must, during special glacial periods, be enormously increased in vigor. 

 As I have already shown, such great fixed whirls in the atmosphere constitute 

 an essential part of the circulatory system. They are windpoles of the earth, 

 which gather in and pull down the upper currents of the atmosphere, bringing 

 them to the surface and returning them equatorward. Any increase in their 

 vigor will, therefore, accentuate the zonal contrast. 



TVTVILA, SAMOA, AXD THE CORAL REEF PROBLEM 

 BY ROLLIN T. CHAMBEKLIN 



{Abstract) 



A description of the coral reefs of the island of Tutuila, American Samoa, 

 and their bearing on the various theories which have been advanced in ex- 

 planation of barrier reefs and atolls. 



The island of Tutuila is bordered by a submerged shelf which has an average 

 width of about two and one-half miles. Near the outer margin of this shelf is 



