﻿Vol. 66.~] OX THE GEOLOGY OF NYASALAND. 233 



and wide trough-shaped valleys lying east and west of the Nyika. 

 The formation of these troughs seems to have been largely due to 

 the erosion of comparatively soft strips of Karroo Beds let down 

 into the gneiss by trough-faults. There appears to be no reason 

 against supposing that the Karroo once formed a more or less- 

 continuous sheet over the present site of the Nyika. This is 

 the view held by us, for we consider that the Nyika forms but 

 a segment of an old denudation - platform, produced at a time 

 subsequent to the main faulting of the Karroo and prior to the 

 formation of the lake- depression. The apparent absence on the 

 Nyika and Vipya plateaux of Karroo outliers, or even of gravel - 

 deposits representing the detritus of the old basal conglomerates, 

 seems, however, to preclude the idea that the present surface of the 

 plateaux coincides with an old pre-Karroo land-surface brought 

 to light again by denudation. It is probably of entirely inde- 

 pendent origin. 



The Nyasa Trough. 



Our personal knowledge of Lake Nyasa is practically limited to 

 its western shores. We have been able, however, to refer to 

 Dr. Bornhardt's work on German East Africa for a description of 

 the north-eastern border of the lake, and for his general conclusions 

 with respect to its formation. A useful map of the Nyasaland 

 Protectorate, giving some of the sublacustrine contours of Lake 

 Nyasa, has recently been published by the War Office, and this has 

 been used as the basis of some of the maps which illustrate the 

 present paper. 



Lake Nyasa lies in a huge trough-shaped valley, closed at its 

 northern end, but opening southwards into the Shire Valley. This 

 trough-shaped valley is situated at the junction of two similar series 

 of troughs running for many hundreds of miles in a northerly 

 direction. The entire system, including Nyasa, is roughly Y-shaped. 

 It is now very generally admitted, in accordance with Prof. Suess's 

 interpretation, that the whole system of troughs has been determined 

 by comparatively recent faulting. Lake Nyasa lies in a rock-basin 

 which attains the extraordinary depth of over 670 feet below sea- 

 level, the total depth of the lake being no less than 2316 feet. 

 While it is quite obvious that earth-movement has played some 

 part in the formation of the Nyasa trough, it does not necessarily 

 follow that the trough was altogether fashioned by faulting. It 

 might, for instance, have originated as a great river-valley which 

 has suffered subsequent deformation. There are, however, many 

 difficulties in the way of accepting this hypothesis, and these will 

 be detailed below. 



The extraordinary breadth of the Nyasa trough, coupled with the 

 height, steepness, and general straightness of its sides, must at once 

 differentiate it from normal valleys of erosion. We have seen, 

 however, that valleys of somewhat similar type, though on a much 

 smaller scale, have been formed in Nyasaland by selective erosion 

 working along soft strips of Karroo rocks trough-faulted into the 



