326 Reviews— Dr. J. W. Gregory's Great Rift Valley. 



crossed from Arabia to Africa there was a land connection between 

 tbe two, across the Straits of Bab-el-Mandeb. The natives of Ujiji, 

 at the southern end of the line, have a folklore that goes back to 

 the time when Lake Tanganyika was formed by the flooding of 

 a fertile plain, rich in cattle and plantations. And at the northern 

 end of the valley we have the accounts of the destruction of the 

 towns of Sodom and Gomorrah. 



"There is geological evidence to show that great earth-movements 

 have happened along this Eift Valley, as it may be termed, at a 

 recent date, which makes it distinctly probable that these traditions 

 are recollections of the geographical changes. 



"The structure of the Rift Valley has, therefore, very varied 

 interests — geological and geographical, on account of its connection 

 with the history of the eastern basin of the Mediterranean, and 

 ethnographical, on account of its explanation of some of the best- 

 known stories in our folklore. But it comes in contact with the 

 problems of science on yet another side." " If all the air and water 

 were removed from the earth, then the Rift Valley would present 

 much the same aspect to the inhabitants of the moon as some of the 

 larger of the lunar rills present to us. So the exploration of the 

 Rift Valley has the additional attraction of offering the possibility 

 of explaining the nature of some features in the surface of the 

 moon." 



From the days of Murchison in 1852 to the publication of Prof. 

 Suess's memoir in 1891 writers in general on Africa seem to have 

 held to the view of the great simplicity of Tropical Africa, and the 

 long conservation of its ancient terrestrial conditions. If we turn 

 to the Geological Map of British East Africa (p. 217) we shall 

 see that a vast area is here covered with lava-flows on so gigantic 

 a scale, that those of Vesuvius and Etna sink into insignificance 

 beside them ; they can only be compared with the great lava-sheets 

 of America and the Deccan traps in India ; the latter cover an area 

 of 200,000 square miles, and those of the Western States of America 

 have been estimated to occupy a tract as large as Great Britain and 

 France combined. Richthofen and Geikie have suggested that these 

 great lava seas were discharged from subterranean reservoirs through 

 fissures possibly hundreds of miles in length, instead of through 

 simple circular vents. 



Dr. Gregory suggests that instead of a long line of fissure 

 eruptions the plateau was probably traversed by a double series 

 of lines of weakness crossing one another like a network, and that 

 from the intersection of these lines numerous flows of lava would 

 occur which would coalesce into continuous sheets. He describes 

 these as plateau eruptions, rather than as fissure eruptions (p. 219). 

 He then goes on to describe the mode of formation of the Great 

 Rift Valley, contrasting it with the sinuous course and rounded 

 slopes produced by denudation such as we are familiar with in 

 England. " On emerging from the Kikuyu forests, we entered one 

 which was straight in direction, and was bounded by parallel and 

 almost vertical sides ; its characteristic features were that its lines 





