October 6, 1923] 



NA TURE 



515 



Sotme 



lower 

 Jurassic 



SM 



has recently been supported by the work of E. O. 

 Teale and W. Campbell Smith from the Zambezi. 

 Some lavas which these authors correlate with the 

 Kapitian are shown to be Cretaceous ; they remark 

 {Geol. Mag., May 1923, p. 228), "... the close similar- 

 ity between the specimens from the Lupata Gorge just 

 described, and the Kapitian phonohtes, seems to afford 

 very striking confirmation of Prof. Gregory's view that 

 the latter are of Cretaceous age." 



This evidence establishes the suggested date for the 

 beginning of the East African part of the Rift Valley 

 by fixing the age of the oldest associated lavas as 

 Cretaceous. That the Rift Valley faults had begun 

 by the Oligocene has now received further confirmation 

 from the Gulf of Suez. In a lecture to the Royal 

 Geographical Society in 192 1 (Geog. Journ. vol. Iviii. 

 pp. 267-271) Dr. Hume threw doubt on the fault origin 

 of the Gulf of Suez, and attributed itjto folding. This 

 conclusion would have been difficult to reconcile with the 

 successive maps of the area issued by 

 the Geological Survey of Egypt had 

 not that Survey also published a 

 diagram of one of its folds (Petrol. 

 Research Bull. No. 6, 1920, before 

 p. i). The structure represented is 

 what in ordinary geological nomen- 

 clature is termed a fault. In 

 answer to Dr. Hume's view that 

 the Gulf of Suez was formed by 

 folding, it is only necessary to refer 

 to the two last publications on the 

 area by the Survey of which he is 

 director. The valuable account of 

 the geology of the Gulf of Suez in 

 No. 10 of the Petroleum Research 

 Bulletins, by Messrs. Moon and 

 Sadek, includes two sections which 

 illustrate the structure of the Gulf. 

 The essential parts of these sections 

 are here reproduced (Figs, i and 2). 

 They both represent the Gulf of 

 Suez as in a typical fault-formed valley. The 

 second figure (after PI. IX. D) is especially instructive, 

 as it shows that the faults which formed the Gulf 

 of Suez were post-Eocene and pre-Miocene. It there- 

 fore shows that the conclusion that the Rift Valley 

 faulting began in the Oligocene, which was first 

 based on evidence from Lake Nyasa, holds for the 

 Gulf of Suez. A further Petroleum Research Bulletin, 

 No. 12, has just been issued, in which part of the 

 eastern shore of the Gulf of Suez is described. The 

 authors, Messrs. Moon and Sadek, conclude that the 

 position of the shore is determined by " a very import- 

 ant fault," and they show that the faults in this area 

 were in part pre-Miocene and partly post-Miocene. 

 One of the sections, PI. I. D-H, shows a series of 

 vertical and steeply inclined fracture planes which are 

 marked as faults and not as folds. 



Suess's view that the Great Rift Valley is tectonic in 

 origin has been supported by an overwhelming balance 

 of opinion ; but his view that it was a sudden rupture 

 due to the contraction of the crust has been less widely 

 adopted than the writer's hypothesis that it was due to 

 a series of infalls along an upraised belt. That pre- 

 liminary uplift has been accepted under various names 



NO. 2814, VOL. I 12] 



- — arch, anticline, or mountain ridge along the axis of 

 the valley — and it is consistent with the gravity survey 

 by Kohlschiitter, of the results of which ah excellent 

 summary is given by Prof. Krenkel. Tanganyika Ter- 

 ritory is under three different conditions. Along the 

 coast gravity is in excess. The central area, along the 

 south-western branch from the Great Rift Valley 

 through Lake Eyasi to Tabora, includes a broad basin, 

 with gravity less than the normal. Along the western 

 branch of the Rift Valley is a long narrow band in 

 which the gravity is also less than normal ; Krenkel 

 describes it as a Dichterinne or density-trough. 



The majority of recent authors have adopted the 

 view that the Great Rift Valley was due to lateral 

 tension. That the faults which bound the valley 

 might be due to compression has been several times 

 suggested. The occurrence of reverse faults in the 

 older rocks beside the Great Rift Valley appeared to 

 support this possibility. This view was suggested by 



I. — The structure of the Rift Valley of the Gulf of Suez according to the Egyptian Survey. 

 From Petroleum Research Bull. No. lo. (Cairo, 1921.) 



/^.£. 



Carioni/erous yPrz-Miocena 



Jgmaua Rocks 



Fig. 



2. — Another section by the Egyptian Survey of the valley of the Gulf of Suez. 

 From Petroleum Research Bull. No. 10. (Cairo, 1921.) 



Uhlig in 1907, but he has abandoned it. An over- 

 thrust fault— which has since been rejected — was de- 

 scribed from German East Africa, but Suess remarked 

 that he knew of no other anywhere along the Great 

 Rift Valley system. Dr. Hume inserted a reversed 

 fault on the western shore of the Gulf of Suez. His 

 section was reissued last year " slightly altered " 

 (Petroleum Research Bulletin, Geological Survey 

 Egypt, No. 10, PI. VIII. Fig. 2) ; but the only noticeable 

 modification, except in colouring, is that the fault is no 

 longer drawn as a reverse fault. 



The main advocate of the compression theory is 

 Mr. E. J. Wayland, the director of the Geological 

 Survey of Uganda, for the Great Rift Valley near the 

 Albert Nyanza (Geog. Journ. vol. Iviii., 1921, pp. 

 344-359). The suggestion is more probable for that 

 area than for those places where the Rift Valley is 

 associated with immense lava fields, and in Unyoro it 

 has some abnormal features. Mr. Wayland's view is 

 based on general considerations, and he does not appear 

 to have seen any reversed fault along the Rift Valley. 

 All the numerous faults that have been recognised in 

 the Great Rift Valley series are normal. Any reversed 

 faulting that may be found will probably prove to be 



