﻿Vol. 66. ] 0N THE &EOLOGY OF NYASALAND. 213 



of normal valleys of erosion. As a result of this, the Nyika edge 

 is broken up on the north into the lofty Mpanda mountain-group. 



Moreover, from comparison with other areas, we know that the 

 Karroo sandstones and shales are much more susceptible to 

 weathering and erosion than the harder gneiss against which they 

 are faulted. Thus, rivers which cross the adjacent gneiss in deep 

 and narrow gorges give rise to broad and open valleys in the 

 Karroo. There are a few exceptional cases, such, for instance, 

 as the depression formed by the horst of gneiss in the area here 

 described. In this case, however, the gneiss probably was much 

 shattered by faulting and so rendered non-resistant. 



The Rukuru trough west of the jSTyika may, therefore, be 

 considered as a normal valley eroded out of a faulted strip of 

 sedimentary beds. 



(E) Mount Waller Area. — The Karroo of the Mount Waller 

 area (fig. 7, p. 214) occupies a tract, some hundred square 

 miles in extent, lying east of the Nyika Plateau. Mount Waller, 

 from which the series has received its name, is a flat-topped moun- 

 tain carved out of a platform of sedimentary rocks rising to about 

 3000 feet above the lake. The descent to the lake-shore from this 

 platform is often abrupt, and the almost horizontal beds exposed in 

 cliff-sections form a conspicuous feature, as seen from the lake. It is 

 therefore hardly surprising to find that the Karroo of this district 

 has received special attention from earlier investigators, beginning 

 with Stewart in 1879. Facing Mount Waller, almost on the oppo- 

 site side of the lake, in the Ruhuhu area, Karroo rocks are also 

 found, and have been described by Dr. Bornhardt as dipping south- 

 wards against a roughly east-and-west fault-line which throws the 

 beds against the gneiss. Very probably, as has been suggested by 

 Mr. Moore, the two formations were at one time continuous, 

 stretching right across -what is now the site of Lake Nyasa : their 

 present isolation being due to the rift-faulting which gave rise to 

 the Nyasa depression. Before this severance was made, however, 

 the Karroo of both regions was evidently traversed by an earlier series 

 of faults, probably synchronizing with those bounding the isolated 

 troughs and patches already described as occurring at Nkana, 

 Mpata, and west of the Nyika. In the last-named areas the 

 Karroo was found to fall into three groups, and the same division 

 is applicable to the Mount Waller beds. 



In a section taken from the lake-shore, immediately south of 

 Port Stewart, and thence over the north-eastern face of Mount 

 Waller, the following succession of beds dipping gently westwards 

 is observed : — 



Thickness in feet. 



3. Upper Group. Mostly inudstones 1500 



2. Middle Group. Sandstones with shales and coals 200 



1. Lower Group. Sandstones *. 1300 



The Lower Group consists of coarse- to medium-grained white 

 to yellow felspathic sandstones, with pebbly sandstones and fine 

 conglomerates. 



