November 12, 1920] 



SCIENCE 



457 



The article, although published in a journal 

 from which geological discussions are usually 

 excluded, devotes its second part to a strictly 

 geological discussion of the " age and history " 

 of the African rift valley, with the result of 

 assigning it a relatively modem date. The 

 article closes vidth an inquiry into the orgin 

 of the rift valley and its relation to contem- 

 porary earth movements ; and here speculation 

 is given free rein. The following extracts 

 are from the second and closing parts: run- 

 ning comments are added. The length of the 

 valley is " about one sixth of the circumfer- 

 ence of the earth. It must have had some 

 world-wide cause." It is " dependent in the 

 main on the volcanic history of the country; 

 the two are connected, as the subsidence of 

 the earth blocks doubtless forced up the lavas 

 along the fractures ; " and from this it would 

 appear that volcanic eruptions are to be re- 

 garded as caused by the sinking of the rift 

 blocks, although the opjxisite view, that the 

 eruptions caused the sinking, deserves con- 

 sideration. 



The first stage . . . was the uplift of a long, low 

 arch [by lateral pressure] with the axis trending 

 north and south. . . . The second stage was the 

 cracking of the aides of the arch as the lateral 

 pressure was reduced, and the top sank as the key- 

 stone of an arch sinks if the end supports give 

 way. 



But it may be questioned whether there is 

 any true homology between the crest of a very 

 broad and low earth-crust anticline with 

 plenty of subcrustal material crushed up be- 

 neath it, and the keystone of an arch of 

 masonry which spans an open space; and 

 moreover the lateral pressure here assumed 

 contradicts the state of tension, mentioned be- 

 low. " The sinking of the keystones . . . into 

 the plastic material below forced some of it 

 up the adjacent cracks, through which it was 

 discharged in volcanic eruptions." The possi- 

 bility that the primary impulse was not lateral 

 pressure in the superficial crust, but an up- 

 ward pressure of deep-seated and crowded 

 lavas is not considered. 



The si)eculative character of the views here 



exposed reaches its climax on the closing 

 page: 



The essentially different character of the con- 

 temporary earth-movements of Africa and of the 

 Pacific borders is explained by their antipodal 

 position. Africa was antipodal to the Pacific, and 

 it is in accordance with the well-known antipodal 

 relation of ocean to continent that while the Pa- 

 cific was sinking and the crust beneath it under- 

 going compression, its antipodal land should be 

 rising and subject to tension. . . . The subsidence 

 of the AraJbian sea and the outflow of the vast 

 quantities of lava to the east and west [in India 

 and AlDyssinia] left the east African arch insuffi- 

 ciently supported, and the top of it sank between 

 parallel fractures. . . . Africa was in tension, and 

 torn by north and south fractures, along whick 

 the sinking of a strip of the crust formed the- 

 longest meridional land valley on the earth. . . ^ 

 There are two great regional disturbances in the: 

 Eastern Hemisphere with which they [the rift-val- 

 ley faults] may be correlated; the foundering of 

 the Indian Ocean . . . and the movements which 

 raised the Alpine-Himalayan Mountains. . . . The 

 Great Rift Valley . . . owes its unique character 

 to its position antipodal to the Pacific, and its 

 course to the wrench in the crust of the Eastern 

 Hemisphere ibetween the segment pressing north- 

 ward against Europe and that pressing southward 

 in Asia toward the deepening basin of the Indian 

 Ocean. 



These are truly far-reaching generaliza- 

 tions, but whether they will endure is un- 

 certain. If they are correct their prophetic 

 power is great, for the evidence to demon- 

 strate them is not yet fully collected. 



The doubts that are aroused by a reading 

 of Gregory's article are re-enforced by the 

 protest published a month later by Ball, of 

 the Geological Survey of Egypt.^ He points 

 out that, as large sections of the supposed rift 

 valleys are submerged in the Eed Sea and the 

 Gulf of Aden; and as not a hundredth part 

 of the rest has been examined by competent 

 geologists, much uncertainty must still re- 

 main as to their structural interpretation and 

 physical origin. He adds that, as a result of 

 Gregory's early work on the subject, some of 



2 "The African Eift Valleys," by John Ball 

 Geogr. Jour., LVI., 1920, 234-238. 



