﻿Vol. 66, J ON TECE GEOLOGY OP NYASALAND. 225 



of the rock is easily weathered, certain of its constituents may prove 

 highly resistant : in this case, if in sufficient quantity, they will be 

 found scattered on the surface-soil. Kyanite, for instance, occurs 

 at times in large, well-formed, crystals in a gneiss consisting of 

 felspar, quartz, white mica, and graphite. This gneiss weathers 

 rapidly, leaving the kyanite littered on the surface. 



An equally striking case of differential weathering is very oc- 

 casionally exhibited by certain porphyritic granites or syenites, 

 especially when exposed on steep slopes. These granites contain 

 large felspar crystals measuring over an inch in length : the 

 larger crystals are less easily attacked than the finer ground-mass, 

 and concentrate upon the surface. The crystals of this concen- 

 trate are sometimes quite angular, but more often become consider- 

 ably rounded through solution and attrition. The best instance 

 of this type of weathering is found at Chipara Hill (fig. 1, 

 p. 191), a little north of Kasungu, on the Marimba Plateau. This 

 is a roof-shaped monadnock, practically bare of vegetation and 

 subject to the rapid diurnal changes in temperature which occur in 

 high plateau-regions. Its steep slopes afford a ready run-off for 

 rain-water, so that altogether the conditions are peculiarly favour- 

 able to dry weathering. Chipara is built up of a foliated syenite 

 with large well-shaped felspars, 1 to 3 inches long. While the 

 top of the hill is formed of solid rock, its lower slopes are covered 

 with huge boulders which disintegrate, leaving a concentrate of large 

 and remarkably angular felspars. 



A feature about Nyasaland is the absence of typical scree- deposits 

 below the steep slopes and scarps of the plateau-country. This is 

 due to several causes : in the first place, the rock forming the 

 slopes of the ridges or scars tends to break up into huge boulders 

 rather than small angular fragments. Again, owing to rapid 

 weathering of the scar-top, the streams which course down the 

 slopes in the rainy season may have a sufficient supply of loose soil 

 to form a talus-deposit at the foot of the scar. Any loose rocks and 

 stones carried down by gravity are, in consequence, gradually 

 smothered by the wash from the slopes. In time, moreover, the 

 enclosed boulders disintegrate in situ. If the talus is of a sandy 

 character, as at the foot of the Mchisi range, near Port Manning, 

 water is able to percolate freely downwards, carrying iron in 

 solution, which may cement the boulders into a hard conglomerate- 

 like deposit. In other cases, the iron-bearing solutions become more 

 concentrated, and on evaporation deposit limonite — thus converting 

 the sands and clays into an impure ironstone, which is brought to 

 light on the removal by erosion of the overlying wash. The porosity 

 of these ironstones is due, in some cases at least, to the imperfect 

 cementation of the sand about which the limonite is deposited. 

 Pockets of sand are left, which afe rapidly removed by weathering. 

 Ironstone of a similarly porous character, and sometimes pisolitic, 

 is found far out in the wide plains of the Central Angonaland plateau. 

 Near Nkongoni, on the Bua River, limonitic iron-ore occupies con- 

 siderable areas on the low slopes on each side of the marshy valley. 



Q. 2 



