﻿Vol. 66.'] ON THE GEOLOGY OF jSYASALAXD. 231 



Lake Nyasa. To the west, and across the border, one looks down 

 upon a tumbled sea of mountains, a land of conical peaks and ridges, 

 crossed by swift-flowing rivers travelling southwards and westwards 

 on their way to the Zambezi. 



Described in more scientific terms, it amounts to this. On the 

 east of the watershed is a wide homogeneous plateau, crossed by 

 senile rivers flowing eastwards and rejuvenated along the eastern 

 edge of the plateau, while on the west is a much dissected plateau 

 incised by streams in an early state of development. 



The Bua serves as a type of the senile eastern rivers. For many 

 miles it flows sluggishly at the bottom of a wide shallow groove in 

 the alluvium, forming in places a long line of marsh. Though 

 subject to curves or flexures, the river can never be said to meander 

 across the plateau. The same is true of the tributaries, which, 

 like the main stream, have a very low gradient. Several of the 

 smaller tributaries are little better than a line of marsh, even 

 during heavy rains. Despite this fact, their valleys open out quite 

 gradually into the main river-depression. This is a clear indication 

 that erosion throughout the area has practically come to a stand- 

 still. Near the eastern edge of the plateau, however, the rivers 

 are confined to narrow V-shaped valleys or gorges which follow 

 sharp zigzag courses, in marked contrast with the wide even sweeps 

 of the upper reaches. The greater part of the plateau is covered 

 with a pale, very sandy loam. Near the eastern edge, however, 

 this may give place to red clays : these clays are almost invariably 

 found resting upon graphitic gneiss, and either consist of wash 

 from the same, or are products of weathering in situ. Owing to 

 the lack of exposures on the .west, the actual floor upon which 

 the sandy loam rests is rarely exposed. The deposit is, however, 

 identical in character and continuous with the wash formed at the 

 base of the granitic Dzalanyama llange, and has probably been 

 derived from acid rocks, including granites, granulites, and gneisses. 

 Several monadnocks rise from its surface, and these are almost 

 invariably composed of such rocks, especially granite. Moreover, 

 there appears to be little or no overlap of the sandy loam upon the 

 clays covering the main mass of graphitic gneiss on the east. We 

 are, therefore, inclined to believe that the loam is mainly derived 

 from the underlying rocks, and has not necessarily been transported 

 far from its source of origin. There appears to be no evidence to 

 support the view that the material was transported by wind. More 

 probably it consists of river-alluvium and wash, the latter forming 

 an ever-broadening sheet round the isolated mountain-masses, as 

 these gradually rot or are eroded away to the level of the plain. 

 The monadnocks which remain at the present day are usually sur- 

 rounded by a low pedestal of wash, but are often remarkable for the 

 steepness of their sides and the comparative freshness of the rocks of 

 which they are composed. These characters are probably dependent 

 upon dry weathering. 



The Central Angonaland and Marimba Plateau is the only one of 

 its kind in Nyasaland that can at all be compared with the huge 



