﻿Vol. 66.] ON THE GEOLOGY OF NYASALAND. 229 



of it consists of acacia and mimosa, etc., which outlive the scorching. 

 It is not impossible, however, that the dense tropical vegetation. 

 now found only in patches, extended at one time throughout 

 the length and breadth of Xyasaland, and that its destruction 

 was hastened, though probably not initiated, by forest-fires. This 

 change from the leafy tropical vegetation to stunted forest-growth, 

 leafless for many months of the year, would no doubt tend to render 

 the climate less humid. 



The destruction of vegetation may have a marked effect upon the 

 erosion of an area. In Xyasaland villages are often so few and far 

 between, that the clearings in the forest effected by man are almost 

 negligible. Where, however, the population is dense, a considerable 

 area of forest is frequently cut down for firewood and other purposes. 

 This leaves the soil unprotected, and allows of a more rapid run- 

 off of the rain into the neighbouring streams. The gashes cut 

 by the streams in the talus-soil or wash along the western edge of 

 the Dowa Hills may perhaps have originated through the destruction 

 of the trees on the hill-slopes, and the consequent increase in volume 

 and erosive power of the streams during the rains. Even far 

 away from any habitation, the bush-fires, by burning off the long 

 grass and thus laying bare the ground to the force of the rains, 

 may promote a certain wasting away of the surface-soil. On the 

 whole, however, the works of man appear to have had so far but a 

 trivial effect in altering the face of the land. 



IV. Physical Features. 



A brief description of the topography of Nyasaland has already 

 been given. We now propose to treat the subject in a somewhat 

 more systematic way, dealing first with the various plateaux and 

 finally with the great valley in which Lake jNyasa lies (fig. 1, p. 191). 



The Neno Plateau and the Shire Highlands. 



In Southern Nyasaland a narrow strip of plateau-region runs 

 roughly north and south along the Anglo-Portuguese border. The 

 plateau averages, perhaps, 4500 feet in height, but is very undu- 

 latory and carries numerous ridges and mountain -peaks, one of 

 which, Songani byname, reaches 6000 feet above sea-level. Rivers 

 draining eastwards incise the eastern edge of the plateau, forming 

 shallow gorges opening out into the Shire Valley. The drop from 

 the plateau-top to the Shire Valley, some 3000 feet below, is often 

 very steep, but never precipitous. The Shire Valley forms a huge 

 trough over 20 miles across, and is bounded on the east by the edge 

 of the Shire Highlands, which rise steeply above it to a height of 

 over 2000 feet. The Shire Highlands consist of a tract of fairly 

 even undulating country, with the watershed lying to the west 

 and almost on the summit of the steep slope leading down into the 

 Shire Vallev. Rivers flowing in an easterlv direction are mature 



