﻿226 MESSES. A. B. ANDBEW AND T. E. G. BAILEY [May I9IO,. 



It has at times a curious bedded appearance, and occasionally contains 

 angular fragments of vein-quartz. Owing to its impervious character, 

 it not infrequently holds up the surface-water in small pools. The 

 pisolitic varieties of this rock may be compared with the derived 

 laterites of India ; but it would, nevertheless, hardly be correct to 

 describe this ironstone as a laterite. 



Among other accumulations worthy of note is travertine or cal- 

 careous tufa. This is forming at the present day at the base of 

 small rock-falls, along the course of dry stream -beds cut through 

 rocks rich in lime. As a rule, the sandy beds of these streams, a 

 few feet below the surface, contain a sufficient supply of moisture 

 to allow of an intermittent seepage across any rocky barrier in the 

 stream-course. The streams fill with water only in times of excep- 

 tional rain, and so the deposits are more or less protected from 

 erosion. In one instance, however, a stream lias partly removed 

 the travertine in its bed, leaving it as a terrace 8 feet deep. 



Another form of calcareous deposit may be compared with the 

 kunkur of India and East Africa. 1 This is typically developed in 

 the alluvium of the Lower Shire district, and in the weathered soil 

 derived from the Karroo Beds of this area. It here forms small 

 irregular concretions scattered within, and, to a certain extent,, 

 concentrated upon the surface of the soil. 



Processes of Erosion. 



Rain, w r ind, weathering, and river-erosion all play a more or less 

 important part in the denudation of ISTyasaland. Vegetation, on the 

 other hand, has a conservative effect, and by binding the soil together 

 tends to protect the surface of the land from the ravages of wind and 

 rain. The greater part of Nyasaland is covered with stunted forest- 

 growth. The trees lose their leaves during the hot season, but 

 partly regain them a little before the rains have set in. In con- 

 sequence, the forest-country is to a certain extent protected from 

 the torrential storms which usher in the wet season. There are,, 

 however, tracts covered only with a sparse grass, and here a very 

 considerable part of the ground is bare and at the mercy of the 

 rains. In many places, the surface-soil consists of a sandy clay 

 covered with fragments of quartz. The hard fragments shield the 

 underlying soil from the beat of the rain, and so tend to stand out 

 above the general surface on tiny earth-pillars an inch or two high. 

 Earth-pillars of much larger size, but of imperfect development, 

 are occasionally found along the banks of streams. 



Where the rain has a rapid run-off, as down a steep stream- 

 bank cut in soft alluvium, it may sometimes give rise to a narrow 

 groove. Under favourable circumstances this will widen rapidly 

 and eventually form a deep trench, or wadi, 100 yards or so long. 

 These wadis are found in various parts of the country, and were 



1 H. B. Maufe, ' Reports relating to the Geology of the East Africa 

 Protectorate ' : Colonial Reports — Miscellaneous, No. 45, 1903, p. 23. 



