91 DR. A. HOLMES ON THE PRE-CAMBRIAX [vol. lxxiv, 



structures. Viewed from various points, the Ribawe Mountains 

 present the appearance of a rectangular block with steep precipitous 

 sides. In the surrounding country on the south and east, the 

 foliation frequently is nearly horizontal and, when otherwise, the 

 prevailing dip is at a low angle towards the west. In the moun- 

 tains themselves, however, the dip is northwards. 1 This striking 

 divergence suggests that at one time the rocks of the Ribawe block 

 were elevated by an upthrust acting chiefly along the southern edge, 

 a process made possible by two sets of fractures, one nearly due 

 east and west, parallel to the southern front, and the other nearly 

 due north and south, parallel to the eastern walls (see PL XI). 

 No exposures of horizontal sections giving direct evidence of 

 faulting have been seen, for around the high peaks such evidences 

 are buried under rock-debris and hidden by thick vegetation. The 

 Bwibwi River gives no help, as in man}* places vegetation grows 

 abundantly down to the water's edge, and the stream itself has 

 a bed that generally is thickly covered with coarse gravel and 

 transported boulders. 



Joints cannot be said to be common in the gneisses, but two 

 systems have been identified parallel to the north-south and east- 

 west fracture-lines. The north-and-south direction is also followed 

 in the Nrassi Basin and elsewhere by a series of pyroxenite-dykes. 

 The same direction is represented by parts of the Mozambique 

 coast, by the small faults in the Cretaceous beds, and by the 

 boundary-fault and basaltic dykes between the Cretaceous and the 

 Pre- Cambrian. 3 On a still larger scale, the north-and-south line 

 of fracture appears to have determined many of the controlling 

 features of this part of the earth's crust. Parallel to it are the 

 faulted eastern coast of Madagascar on the one side, and the 

 Frontier fracture -lines south of the Zambesi on the other, 3 while 

 several sectors of the rift-valleys of Eastern and Central Africa 

 follow the same direction. 4 * 



XII. Summary. 



(a) The succession of rocks in eight of the betler-known 

 districts of Mozambique is summarized, and it is shown that the 

 general sequence in order of age is — 



(5) Dykes of pyroxenite and picrite. 



(4) Granites and massive pegmatites. 



i 3) Granulitic granites and pegmatites. 



(2) Biotite-, hornblende-, and garnetiferons gneisses. 



(1) Crystalline schists and limestones. 



(b) The schists are, with few exceptions, the representatives of 

 a differentiated series of detrital deposits including felspathic, 



1 See photograph in Geol. Mag. dee. 6. vol. i (1914) pi. xxxvii. 



2 Q. J. G. S. vol. lxxii (1916 17) pp. 225-26. 



:{ E. 0. Teale & E. C. Wilson. Geogr. Jonrn. vol. xlv (1915) p. 16. 

 4 E. Suess, 'La Faoe de la Terre' vol. iii, pt. 3 (1913) %. 221. p. 971 ; 

 A. Holmes, Geogr. Jonrn. vol. xlviii (1916) p. 149. 



