April 27, 1905J 



NATURE 



619 



THE PHYSICAL HISTORY OF THE 

 VICTORIA FALLS.' 

 XJU HEN Dr. Livingstone discovered the Victoria Falls in 

 *' 1855, he sought to explain their origin by calling 

 in volcanic agency, and stated that 

 they were " simply caused by a 

 crack made in the hard basaltic rock 

 from the right to the left bank of 

 the Zambezi, and then prolonged 

 from the left bank away through 30 

 or 40 miles of hills." All subsequent 

 travellers support the same idea ; 

 but in his article Mr. Molyneux, in 

 the Geographical' Journal, claims 

 that, as at Niagara, the combination 

 of canon, gorge, chasm, and falls is 

 due to erosion and the constant re- 

 ducing action of the Zambezi River 

 (Fig. i). 



In explaining his theory, the 

 author first refers to present-day con- 

 ditions of the river, and divides it 

 into three portions; the coastal, 

 stretching 360 miles up as far as the 

 Kebrabasa Range — a portion of the 

 mountain axis of South Africa — 

 through which it runs in a gorge 

 35 to 40 miles in length. The 

 middle reach is 600 miles long, in 

 low-lying country, and is divided 

 from the upper regions of the river 

 by the high Victoria Falls, 1000 

 miles from the coast. 



The geology of the country around 

 the falls is then sketched briefly. 

 During what was probably the 

 Tertiary period. South Central 

 Africa was subject to vigorous vol- 

 canic action, the concrete forms of 

 which can now be seen in the de- 

 nuded and exposed lava-flows of the 

 Limpopo and Zambezi valleys. In 

 the vicinity of the falls, the' Batoka 

 country, the basalt is interbedded 

 with the soft forest sandstones, but 

 the Zambezi River, in draining the 

 ancient lake regions of Central 

 Africa, has eaten into the overlying 

 sediments until it has reached the 

 hard and almost level igneous sheet 

 in which the falls occur. This sheet 

 extends from the end of the canon. 

 40 miles east of the falls, to bevond 

 the Gonye Falls, 120 miles north- 

 west. 



On reaching the top of this sheet, 

 the erosive action of the river was 

 checked ; but conditions were more 

 favourable in the middle regions of 

 the river, which had no protective 

 covering, and where the rocks are 

 unresisting sandstones and Coal- 

 measures. .\ difference of level be- 

 tween the two regions came into 

 existence — defined by the eastern 

 edge or fringe of the basalt sheet. 



It may be understood that the 

 eastern edge would be thin, and the 

 backward erosion of the Zambezi 

 from its middle reaches would 

 quickly break into it. But as the 

 thickness of the basalt increased as 

 the river receded westwards, the 

 cutting action became slower, until 

 the rate of deepening of the middle 

 reach and Kebrabasa gorge far out- 

 stripped the .slower process of form- 

 ing the Grand Canon of the Victoria Falls. The difference 

 between the river bed where running on the basalt sheet 



' Abstract of a paper by Mr. A. J. C. Molyneu.\ in the Gcogro-phiccil 

 Journal for January. 



NO. 1852, VOL. 71] 



and the altitude of the central reach became more ex- 

 aggerated as time passed. Including the height of the 

 falls (400 feet), this difference is now about 1000 feet. 



Of the process by which the river cut back this Grand 

 Canon and shaped the falls as they are seen to-day, the 



Fig 



From the Geographical Jo\ 



author states that, as is common to all rocks of this 

 nature, it is full of cracks and fissures due to contraction, 

 generally assuming a columnar form. These columns can 

 be seen at low water along the lip of the falls, more or 



