﻿Vol. 66.~] OX THE GEOLOGY OF NTASALAND. 223 



1. Deposits within the Lake-basin. — Littoral deposits 

 consisting of sand or silt are found in many places along the 

 western margin of Lake Nyasa. These were evidently formed at a 

 time when the lake-level was from 10 to 30 feet higher than it is 

 at present. Recent lacustrine deposits also occur at higher levels, 

 and are especially well developed on the north-west between the 

 Songwe Eiver and Mount Waller. 



Travelling southwards from the Songwe, roughly along the 

 junction of the western hills of gneiss with the coastal plain (fig. 2, 

 p. 193), we encounter a series of isolated patches of gravel. These 

 gravels may be observed near Kasante, north of the Lufira, and 

 a few miles south of the same river. They usually consist of 

 pebbles of vein-quartz set in a soft, sometimes felspathic matrix. 

 The deposits run more or less parallel with the trend of the hills 

 on the west, and give the impression of an old stretch of beach 

 formed against a steep shore-line. 



Farther south, from Mpata (fig. 4, p. 206) to Mount Waller (fig. 7, 

 p. 214), high-level deposits are of frequent occurrence, and are some- 

 times found at very considerable heights above lake-level. 



Near Mpata, immediately east of the Karroo outcrop already 

 described, these may be divided into a probably lower series of 

 white, slightly calcareous sands containing pebbles and boulders of 

 quartz, and an upper, partly consolidated, ferruginous conglomerate, 

 formed of boulders of quartz weighing sometimes as much as 5 lbs. 

 The beds rest upon a floor of gneiss or of Karroo rocks, and are 

 found at a height of about 400 feet above lake-level. The dip is 

 roughly 3° eastwards. 



Some miles to the south of Mpata, somewhat finer red con- 

 glomerates were noted. These are roughly 100 feet thick, lie at 

 about 700 feet above lake-level, and usually dip very gently 

 eastwards. Locally, however, the beds are turned sharply up, 

 apparently near a fault, and dip eastwards at an angle of 35°. 



About 3 miles west of Chiwondo (fig. 1, p. 191) a series of marls, 

 sands, and pebble-beds are found resting upon the gneiss, at about 

 350 feet above lake-level. At Chisali recent beds also occur, and 

 have been cut up by streams into a series of low hills. The 

 beds, as usual, dip gently eastwards, and may be divided up as 

 follows : — 



(3) Dark red conglomerates with friable, sometimes micaceous sandstones. 

 — About 400 feet above lake-level. 



(2) Shelly marls consisting of comminuted shells of Vivipams, overlying 

 grey limestones with grains of quartz and felspar. The lower lime- 

 stones also contain casts of Viviparus, probably identical with species 

 now existing in Lake Nyasa (see Appendix II, p. 239). — About 400 feet 

 above lake-level. 



(1) Friable grey and greenish sandstones and mudstones. — From 250 to 

 350 feet above the lake. 



Near Masiunjuti, some 15 miles from Lake Nyasa, a series of 

 dark-red conglomerates and white sands occur. These form a sort 

 of terrace 700 feet above the lake. The deposits are faulted in 

 places, the displacement produced in one case amounting to 100 feet. 



Q. J. G. S. No. 262. q 



