part 1] .L\D ASSOCIATED ROCKS OF MOZAMBIQUE. 93 



inselberge represent relics of those cupola-like portions of upward- 

 moving granitic magmas whiqh locally rose to a higher level in 

 the crust than the parts represented in the intermont plains. 



There remains to be considered a fourth type of inselberg. It 

 includes all those peaks which cannot be referred to intrusion 

 processes, since no recognizable differences can be discerned between 

 the structure or composition of their rocks and those of the sur- 

 rounding district. It is difficult to understand why these inselberge 

 should not have become disintegrated to the same depth as the 

 neighbouring rocks, for the conditions of weathering at the present 

 time are far from favourable to inselberg survival. The effect of 

 insolation is to remove superficial shells from the inselberge (by 

 exfoliation and radial cracking) much more rapidly than the surface 

 of the plateau can be worn down. 



At the present time much of the Mozambique plateau is traversed 

 by low ridges which follow the prevailing direction of strike. 

 These ridges must not be confused with the gneissic anticlines 

 of elongated turtlebacks, for they are essentially of the nature of 

 low escarpments. 1 If such escarpments were initiated by pluvial 

 and stream erosion, it seems probable that during subsequent arid 

 conditions the escarpment slopes would be worn back and intensified 

 by localized attack at their base, and that the escarpments them- 

 selves would be eaten away laterally, until only isolated remnants 

 remained. Some of the Mozambique inselberge are elongated 

 along the dip, which is always low, and have a steep, often nearly 

 vertical face on one side, Avhich sharply truncates the banding of 

 the gneisses. 2 In several cases, notably Mount Tugwi and a series 

 of peaks north-west of the Mwipi Mountains, the inselberge stand 

 out on low escarpment-like ridges. Mount Tugwi appears to be 

 continued north-eastwards in a series of less prominent inselberge, 

 south-east of which the Monapo River now flows. 



Summing up, it Avould seem that in Mozambique desert con- 

 ditions, involving the attack of slopes at their bases by arid 

 weathering and the removal of disintegrated material by wind, are 

 more favourable for the development and maintenance of an insel- 

 berg landscape than are alternating conditions of deep weathering 

 and rapid erosion. 



Faults and Joints. 



Although block- faulting cannot be appealed to as a cause for 

 the extraordinary altitude of individual peaks and mountain -blocks, 

 yet in the Eibawe-Nrassi district block-faulting superimposed on 

 the earlier structures seems often to correspond with the lateral 

 limits of the elevated masses. The determination of the faults 

 depends on the assumption that the dip and strike of the gneissic 

 banding and foliation can be used as reliable criteria of the 



1 See A. Holmes, Geol. Mag. dec. 6, vol. i (1914) p. 531, for illustration. 



2 For an admirable illustration (photograph by Mr. D. A. Wray), see 

 Geogr. Journ. vol. xlii (1913), opposite p. 146. 



