200 C. SCHUCHERT THE XOETH AMERICAN GEOSYNCLINES 



lines. Oceans, however, are not geosynclines. Suess in 1909 wrote 

 Euedemann : 



"I do not believe in oceanic geosynclines. No existing ocean has a synclinal 

 structure, except by superimposed sediments, and the Pacific troughs [deeps] 

 are not synclinals.*' 



CoXTIJvTEXTAL FOUXDERIXG 

 RIFl'S AXD GRABEN 



Rifts, even when filled with marine waters, are not geosj^nclines. They 

 are, Gregory says,^* "^'formed by the subsidence of strips of the earth's 

 crnst between parallel faults." The Great Eift Valley is the grandest 

 example. This series of fault-formed valleys is continuous from Pales- 

 tine to south of the Zambesi of southern Africa, and branches of it extend 

 from the Gulf of Aden westward to the central Congo. The Red Sea is 

 one of its marine invaded parts, with depths down to 6,000 feet, but the 

 rift has "a total downthrow of 11,000 feet." 



The Great Basin country of the United States is another example, with 

 a much wider area of downthrow, which is something like 1,000 feet in 

 Utah. 



The rifting of eastern Africa is generally believed to be due to settling 

 of an uplifted arch of epeirogenic extent. KrenkeP^ has shown, however, 

 that the rifts cut all the geologic structures, none of which give unmis- 

 takable evidence of an extensive arch. He states that Africa is being 

 squeezed between the southwardly moving mass of Asia and the north- 

 wardly pushing mass of the Antarctic Ocean. In consequence, Africa is 

 moving northwestward. On the other hand, the Indian Ocean resulted 

 in the foundering of eastern Gondwanaland, or, better, Lemuria, that 

 began in Cretaceous time. Because of all these interacting movements, 

 the outer crustal portion of eastern Africa has been widened about 5 per 

 cent, the compensation for this enlargement giving rise to rifting on a 

 grand scale. This increase in area and faulting through tension has gone 

 on in compensation for the orogeny elsewhere. Therefore fafrogenesis 

 (from the Greek for rifts or graben) is the counterpart of orogenesis, 

 and East Africa is the type area for tafrogenic structures. Rifts also 

 develop in mesogeosynclines, and one of the best examples is the Ethio- 

 pian mediterranean of Neumayr. It began to develop as a marine trough, 

 about Permian time, between Africa and Lemuria, and persisted as such 



•*^ J. W. Gregory : The rift valleys and geology of East Africa. An account of the 

 origin and history of the rift valleys of East Africa and their relation to the contem- 

 porary earth-movements which transformed the geography of the world. London. 1021. 



■*^ E. Krenkel : Die Bruchzonen Ostafrikas. Berlin, 1922. 



