﻿210 THE GEOLOGY OF NYASALAND. [May 191a. 



Viewing the district as a whole, it is clear that faulting has- 

 played an important part in isolating the Karroo Beds into the 

 various troughs and basins just described (Section I, fig. 9, p. 218)-. 

 Subsequently, or during this period of faulting, denudation must 

 have levelled off the inequalities and reduced the area to something 

 approaching a peneplain. The drainage-system was then rejuven- 

 ated, owing perhaps to earth-movements connected with the 

 formation of Lake Nyasa. The rejuvenation of the rivers gave rise 

 to the present gorges in the gneiss and the cutting out of the softer 

 Karroo Beds in the troughs and basins. No other explanation seems 

 adequate to account for the general disregard which the Rukuru 

 and its tributaries show to the present topography. 



(D) Western Nyika Area. — The Karroo, here, occupies an area 

 of about 70 square miles. It forms the floor of a deep trough, 

 through which the Rukuru River passes on its way to Mpata and 

 Karonga. The trough runs almost due north and south, and is 

 bounded on the west by hills of gneiss, above and behind which 

 rise the lofty mountains of the Kayuni Range. A great mountain- 

 mass known as Mpanda lies on the east, followed on the south by 

 the western edge of the Nyika Plateau. The eastern border so 

 constituted runs in a nearly straight north-and-south line for 

 upwards of 20 miles. The edge of the Nyika is at first fringed by 

 lower ranges of gneiss, but finally drops as a huge grassy wall, 

 3000 feet in places, down to the tree-covered valley below. The- 

 western mountains run parallel with the Nyika edge at a distance 

 of some 5 miles, but die out eventually southwards, leaving a gap 

 which is filled in, about 8 miles farther south, by a series of north- 

 and-south ranges. These abut on the east against the edge of the 

 Nyika Plateau, and constitute the southern boundary of the Karroo. 



In its northerly extension the Karroo forms a simple faulted 

 trough, separated from the gneiss by long lines of fault. The 

 eastern and more important of these runs nearly due north and south 

 along the edge of the Nyika Plateau. The western lies roughly 

 parallel, and towards the south coincides with the eastern side of the 

 Sudje Range. The Karroo, here, consists of a great thickness of 

 felspathic grits and sandstones, obviously belonging to the Upper 

 Division. The beds dip, as a rule, eastwards at angles greater 

 than the general easterly slope of the valley-floor. 



Farther south the series is divided into an eastern and a western 

 area by a narrow north-and-south horst of gneiss, some 4 miles 

 long and having a maximum width of perhaps half a mile. This 

 lenticular strip of gneiss serves as a connexion between the Sudje 

 Range on the north and the Muoma ridge of gneiss on the south. 

 The remarkable point about this horst lies in the fact that it 

 forms a depression occupied by a limb of the Rukuru River, ancr 

 bounded on each side by low scars of sandstone. 



The general dip of the Karroo, as in the north, is in an easterly 

 direction. The basal conglomerates are consequently exposed on 

 the west, and can be traced for about 7 miles in a north-and-south- 



