188 



ME. G. W. LAMPLTIGH ON THE 



[May 1907. 



Fig. 4. — Section about half- 

 way down in the cleft at the 

 eastern end of the Chasm at 

 the Victoria Falls, showing 

 the vein to which the chasm 

 is clue. 



[The vein, as a whole, has a 

 decided hade to $he south. The 

 unshaded spaces are hidden by 

 talus.] 



a = "Vein-stuff, partly vertical ribs 

 of crystalline calcite, and partly 

 red and purple decomposed rock 

 and earthy material ; 4 feet 

 seen, but probably 4 feet more 

 in places, hidden under debris. 



b-A small 'stringer' of crystal- 

 line calcite, 1 to 3 inches in 

 width, cutting off a 'horse' or 

 wedge of basalt. 



yy= Massive basalt, amygdaloidal 

 in places, with strong east-and- 

 west jointing which is cut at a 

 low angle by the vein. 



dry or maintaining a very feeble flow. 

 Hence, when a gully is formed in 

 the stream-bed it soon serves to trap 

 the whole flow except during tem- 

 porary floods ; the erosive activity 

 of the stream is thus concentrated 

 along the trench ; and it is steadily 

 enlarged until even the flood-waters 

 cannot escape from it, so that the 

 rest of the bed is abandoned. 



The grandest illustration of this 

 method of erosion is afforded by the 

 weird zigzags of the great gorge 

 immediately below the Victoria 

 Falls ; but these have been so fre- 

 quently described that it is unneces- 

 sary for me here to enter into details 

 regarding them. 



I have elsewhere shown l that 

 the mile-long transverse Chasm into : 

 which the waters of the Falls descend 

 has been scooped out along one of 

 the vertical fractures, as I found on 

 scrambling down its eastward termi- 

 nation, where the section illustrated 

 in fig. 4 was revealed. 



The deep cleft which notches the 

 lip of the Falls on Cataract Island, 

 illustrated in PI. X, is due, as Mr. 

 Molyneux has pointed out, 2 to the 

 erosion of an oblique fracture by 

 a small overflow-channel from the 

 upper river. It exemplifies the de- 

 velopment of a trench along a weak 

 plane, which, if it should happen to 

 strike diagonally up-stream above 

 the present Chasm, may eventually 

 capture the whole river and lay dry 

 the broad lip from which the water 

 now plunges. 



At many spots within the great 

 gorge high above the present river 

 I saw traces of abandoned channels 



1 ' Notes on the Geological History of 

 the Victoria Falls,' in the ' Official Guide 

 to the Victoria Falls ' compiled by F. W. 

 Sykes, Conservator (Bulawayo 1905) ; re- 

 printed in Geol. Mag. dec. v, vol. ii (1905) 

 pp. 529-32. 



2 ' The Physical History of the Victoria 

 Falls' Geogr. Journ. vol. xxv (1905) p. 51. 



