xc POSTSCRIPT. 



Livingstone has proved, by numerous facts, that there once was 

 a vast lake in the central parts of South Africa ; which has left 

 its traces by deposits of calcareous tufa, some of which run 

 far up the present river-courses, and point out the high levels 

 at which the old central lake once stood. Nor is this all his 

 evidence. He has told us that he has found in the earth-heaps, 

 thrown out by the burrowing animals of the desert, certain 

 species of shells identical with the fresh-water shells of the 

 Lake Ngami. Hence he infers that the old central lake was of 

 fresh water ; and that the present Lake Ngami is nothing but 

 a great pool left in one of the lower hollows of the central 

 region, when its great body of waters, from some cause or other, 

 drained off and disappeared 1 . 



But how had the central waters disappeared ? Not by mere 

 evaporation and absorption. For if so, we might have expected 

 more traces of saliferous deposits than we meet with in the 

 central plain of South Africa. It was almost certain- — before 

 we had been taught by Livingstone — that the brim, which held 

 the great central lake, must, somewhere or other, have been 

 broken through, so as to let off the waters to a lower level. 



We may now affirm that Livingstone has explained this 

 difficulty. The great break of continuity, among the rocks 

 below the Victoria Falls, certainly let off the waters to a lower 

 level ; and a convulsion capable of causing that enormously 

 deep and continuous chasm (above described, supra, p. xlvi.) 

 may well have broken through the north-eastern barrier by 

 which the waters of the Zambesi were dammed back into the 

 great central lake. I believe this to be the true explanation of 

 the geographical fact; and that, by placing the evidence before 

 us, he has thrown a good and new light upon the physical geo- 



1 We are not without an example of this kind in England, of course on 

 a pigmy scale ; but no worse, for comparison, on that account. There was 

 once a lake at Bovey Tracey in Devonshire, which for many ages was fed 

 by the rivulets which descended from the neighbouring granitic hills of 

 Dartmoor. Its waters overflowed, and found their way to the sea, but not 

 by the channels through which they now flow down to Teignmouth. In 

 course of time the lake was partially filled up : and at length came an 

 earthquake and disruption of the strata; and then the rivulets began to 

 drain off, and move along their present channels. 



