﻿Vol. 66. ,] OX THE GEOLOGY OF NYAS ALAND. 227 



obviously formed during periods of abnormal rainfall not necessarily 

 far removed from the present time. The soil of Nyasaland is, as a 

 rule, sufficiently compact and bound together by vegetation to pre- 

 vent much transport by wind. Sand-dunes are occasionally formed 

 along the shore of the lake, as to the south of Chintetchi (fig. 1, 

 p. 191), but never inland. 



Weathering plays a very important part in disintegrating the 

 rocks, and thus rendering them more susceptible to erosion. 

 Weathered products are spread broadcast over the country, and 

 owe their present extent and thickness to the absence of recent 

 glaciation. Following, however, a fairly widespread rejuvenation 

 of the rivers, exposures of solid rock are of comparatively frequent 

 occurrence. Moreover, even in the heart of the plateau-regions, 

 where the mature topography is still preserved, there are a con- 

 siderable number of hills and ridges formed out of solid rock. In 

 that these hills constitute rock-islands surrounded by weathered 

 soil, they may be compared with the tors of Devon, but generally 

 differ from the latter in their smoother pseudoglaciated outline and 

 in their greater altitude. 



There are certain kinds of rock which, on exposure to atmospheric 

 agencies, tend to break on their surface into large lenticular 

 fragments. By the splitting-off of these flakes a rocky hill will 

 assume a dome or whale-backed shape. The curious dome-shaped 

 hills of gneiss, round Songani Mountain on the Neno Plateau, 

 for instance, have possibly received their finishing touches in this 

 manner. 



lliver-erosion in Xyasaland is governed by somewhat different 

 conditions from those of Northern Europe, since the rainfall, though 

 heavy, is limited to four or five consecutive months of the year. 

 The rains are followed by a dry season, during which an enormous 

 amount of evaporation takes place. In consequence, numerous 

 streams, and even rivers, which are active in the rains dry up later 

 on in the year. During the hot months the conduct of a river 

 depends largely upon the presence or absence of neighbouring uplands 

 of sufficient height to provide a continuous supply of water. It 

 often happens that a river flows in its upper reaches, but becomes 

 dry lower down in its course, although occasionally these conditions 

 are reversed when the high land lies down-stream. It thus follows 

 that, during the long dry season, erosion comes to a standstill in the 

 low-lying parts of a river, but may yet be in continuous operation 

 on all those tributaries which come from the well-watered uplands : 

 a condition of things obviously favourable to the production of 

 peneplains by selective erosion of the upland tracts, although, as the 

 bulk of the erosion is performed in the wet season, this effect cannot 

 be regarded as of more than secondary importance. There are 

 certain of the larger rivers in Nyasaland which appear too small for 

 their present valleys. The Kasitu and the Lower Southern Rukuru 

 of Xorthern Angonaland are cases in point. The latter river, before it 

 reaches the Xyika Plateau and receives thence an abundant supply 

 of water, occupies a wide sandy bed traversing a region of hills 



