ipart 1] AND ASSOCIATED ROCKS OF MOZAMBIQUE. (35 



radium-content of an invading granitic magma (now represented 

 by the gneisses) was diluted by the intimate penetration of the 

 magma into less radioactive rocks. The abundance of biotite in 

 many of the gneisses, combined with the known existence of older 

 .arenaceous and calcareous rocks, leads to the conclusion that the 

 rocks penetrated were probably mica-schists derived from argil- 

 laceous sediments. 



If this view be correct, then we may interpret some of the 

 inselberg peaks as foci of granitic intrusions, indicating, like the 

 'batholitic foci' of F. D. Adams & A. E. Barlow, 1 the axes of 

 the greatest upward movement, and along Avhich the magmas 

 were most rapidly supplied. W. Gr. Foye has recently put forward 

 .a similar hypothesis in explanation of the gneisses and so-called 

 'batholiths' of the Haliburton- Bancroft area : 2 



' The granitic gases and fluids must have had their origin at certain definite 

 points. At these points they were pushed upward and sideways along planes 

 of easy parting and a pine-tree structure was produced. In general, the 

 increase of material due to the addition of granite would produce a doming at 

 the centre of intrusion with quaquaversal dips away from these points. 

 However, the subsidence of the magma on cooling might very possibly cause 

 ■a collapse of the dome, and irregular dips would result.' 



A corollary from this conception of the mechanism of intrusion 

 and composite nature of the gneisses, is that the foliation and 

 banding of the latter are due essentially to intimate penetration 

 or lit-par-lit injection, accompanied by complete recrystallization 

 of the invaded formations. The frequent arrangement of inselberg 

 peaks in linear series parallel to the prevalent strike of the foliation 

 and themselves constituting gneissic anticlines (see map, PI. XI), 

 points to the existence of tangential pressure acting at right 

 .angles to the north-east and south-west or north-and-south 

 directions, and sufficiently intense to cause the uprising magmas 

 to elongate themselves along the direction of strike. This pres- 

 sure, however, was not so severe as to prevent the local forma- 

 tion of closed curves of foliation due to the superior pressure 

 •of the upward flowing magmas. Crystals grow most rapidly 

 in the directions at right angles to that of the pressure, 3 and 

 thus biotite, which is the most conspicuously elongated mineral, 

 generally follows the prevailing direction of strike controlled by 

 moderate tangential pressure. In the neighbourhood of uprising 

 magmas, however, the 'shouldering pressure' would assume control 

 and lead to tangential elongation around the dome-shaped granite 

 foci. 4 Moreover, if tangential pressure had been the only con- 

 trolling influence, the biotite-flakes should be frequently in 

 approximately vertical positions, so that the prevailing dips of the 

 foliation-planes would be high. On the contrary, the dips over 



1 Geol. Surv. Canada, Mem. 6, Pub. 1082 (1910) p. 19. 



2 Journ. Geol. Chicago, vol. xxiv (1916) p. 790. 



3 G. F. Becker & A. L. Day, Proc. Wash. Acad. Sci. vol. vii (1905) p. 283. 



4 W. J. Miller, Journ. Geol. Chicago, vol. xxiv (1916) p. 596. 



*Q. J. G. S. No. 293. r 



