34 DE. A. HOLMES OX THE PEE-CAMBR1AN [vol. lxxiv, 



seen that the strike swings round the contours of the hills, and 

 that the foliation dips away from their summits in every direction, 

 occasionally even to the extent of being practically coincident 

 With the actual surface. 1 



Proceeding westwards, we note that the country slowly changes 

 in character. Within the first 50 or 60 miles, isolated hills and 

 linear series of hills of the inselberg type rise up abruptly from 

 the plateau. Climbing one of these precipitous peaks — if a 

 precarious footing can be found on its steep and often convex 

 slopes — one beholds a magnificent panorama displayed from the 

 smoothly-domed summit. The bare rounded inselberg of gneiss 

 stands alone, like an island in an undulating sea of vegetation. On 

 the east the forest-clad plateau stretches unbroken to the horizon, 

 or drops to the coastal plain and the sea, but on the west it 

 is bounded by the dark-blue outlines of an increasingly moun- 

 tainous country. Here and there detached peaks, culminating in 

 gracefully-curved domes, fantastic knobs, or unscalable pinnacles, 

 rise into the sunshine above the gloom of the tropical forest. 

 Rolling across the plain one sees regular alternations of level : low 

 elevations and corresponding shallow depressions — marked by a 

 brighter green — run side by side like the corrugations of a gal- 

 vanized iron roof, following a direction that is parallel to the 

 foliation and banding of the gneisses. Farther west the low 

 ridges broaden into long series of turtlebacks, elongated along the 

 strike of the rock-structures. The small inselberge increase in 

 height and numbers, and grow together in imposing linear series 

 and towering mountainous clusters, all of which aft'ord unrivalled 

 vertical and horizontal sections for the study of the detailed 

 structures and inter-relations of the rocks, gneissose and granitic, 

 which enter into their architecture. 



The characteristic domed outlines of the peaks and their 

 smoothly-curved surfaces are obviously due to chemical denudation. 

 Sharp corners tend to be more rapidly worn away than rounded 

 surfaces, and the dome-topped form that is ultimately attained 

 offers therefore a superior resistance to weathering, and maintains 

 a higher degree of permanenc}". The rounded surfaces are further 

 developed by exfoliation. Owing to wide daily variations in tem- 

 perature, the rocks split along planes parallel to the surface, and 

 almost any inselberg exhibits gently-curved shells of gneiss lying 

 on its steep sides in readiness to fall away so soon as they are 

 liberated by the growth of radial cracks. 2 Occasionally concentric 

 shells lie one over the other like the tiles on a roof, making an 

 ascent difficult, and in some cases even impossible. 



Livingstone stated in 1865 that near Lake Nyasa the rocks, 



1 For other examples of this type of structure see F. D. Adams & A. E. 

 Barlow, ' Geology of the Haliburton & Bancroft Areas ' Mem. No. 6, Geol. 

 Surv. Canada, 1910, p. 14 ; and F. E. Studt, 'Outline of the Geology of South 

 Central Africa' Trans. Geol. Soc. S. Africa, vol. xvi (1913) p. 92. 



2 J. W. Evans, ' The Wearing-down of the Rocks ' Proc. Geol. Assoc, 

 vol. xxiv (1913) pp. 245-49. 



