Geology. 353 



n. Geology. 



1. The Bift Valleys and Geology of East Africa; by J. "W. 

 Gregory. An account of the origin and history of the rift val- 

 leys of East Africa and their relation to the contemporary earth- 

 movements which transformed the geography of the world. Pp. 

 479, 20 pis., 44 diagrams, 5 maps. London (Seeley, Service & 

 Co., Ltd.), 1921. — The author first became acquainted with the 

 geology of British East Africa in 1892-3, and published his con- 

 clusions in a work entitled ''The Great Rift Valley" (1896). 

 In 1919 he returned to the region, and after a study of about five 

 hundred papers and books on the geography and geology of the 

 Great Rift Valley, now presents this interesting book. The 

 Great Rift Valley extends more than one sixth the circumference 

 of the earth, "from Lebanon to the Sabi River, with branches 

 eastward to the Gulf of Aden and possibly to the Pangani Valley, 

 and westward to Tanganyika, the Upper Nile and the rift valleys 

 of Lakes Moero and Upemba in the Central Congo" (p. 359). 

 The rifts are long and narrow sunken blocks between parallel 

 faults. In Palestine the downthrow is as much as 5,000 feet. 

 The Red Sea lies in a rift formed by repeated faulting, having a 

 total downthrow of 11,000 feet. Lake Tanganyika also lies in a 

 rift, its waters having a depth of 4,190 feet, and its floor being 

 1,664 feet below sea-level. These fault-formed valleys are con- 

 tinuous from Palestine to south of the Zambezi, except for 80 

 miles in southern German East Africa. The rifting "has 

 broken through an especially stable part of the earth's crust, 

 which consisted of a pre-Paleozoic mountain chain that extended 

 from Asia Minor to Natal. ' ' 



It is commonly held that this rifting took place during Pleis- 

 tocene time, but it is evident that Gregory is much nearer the 

 truth in his conclusion that these valleys "had a long and 

 broken history." In fact, their geologic history dates from the 

 Oligocene to modern times, with an earlier preparatory stage in 

 the Upper Cretaceous. The author says that the first stage was 

 the formation, with much outpouring of lava, "of a long, low 



arch with the axis trending N. and S The second stage 



in the formation of the Rift Valley was the rupture of the sides 

 of the arch as the lateral supports gave way. The top sank as 

 the keystone of a bridge sinks if its buttresses slip or settle. The 

 sinking of the keystone of the East African arch into the plastic 

 layer below forced some of it up the adjacent cracks, through 

 which the material was discharged in volcanic eruptions. Each 

 renewal of the subsidence was followed by fresh eruptions" (pp. 

 361-2). 



"The geographical features in Eastern Africa are arranged 

 along three main directions — E. to W., N. to S., and diagonally, 



