52 THE PRE- CAMBRIAN ROCKS OF MOZAMBIQUE. [vol. lxxiv. 



The last view is the one specialty favoured, but further discus- 

 sion will be reserved until the gneisses themselves have been 

 described. 



It remains to be considered why the limestones should persist at 

 all. The probability is that argillaceous sediments are easily 

 granitized ; whereas limestones, by taking up a large amount of 

 silica, become enclosed within a blanket of hornblende- or syenite- 

 gneisses, or of less extensive contact-deposits no longer over- 

 saturated 1 in silica, which preserve the interior of the limestones 

 from further change. Limestone can always continue to exist, 

 even at high temperatures, provided that the pressure of overlying 

 rocks is greater than the dissociation pressure of calcium carbonate, 

 and provided that free silica does not come into direct contact 

 with it. 3 



V. The Gneisses and G-neissose Granites. 



(1) Biotite-Gneisses and Gneissose Granites. 



Biotite-gneisses are by far the dominant rocks of the country. 

 The prevailing directions of foliation have already been stated, and 

 attention has been drawn to the important fact that in certain 

 inselberge the foliated structure gradually passes into 

 granitic texture. 



The essential minerals usually present are quartz, orthoclase. and 

 rnicrocline (sometimes perthitic), soda-lime felspar (generally oligo- 

 clase), biotite, and in rare cases muscovite. Accessory minerals 

 are apatite, zircon, sphene, garnet, and iron-ores, including magne- 

 tite, ilmenite, and pyrite, and occasionally mispickel. Among the 

 alteration-products, sericite derived from potash-felspar, and epi- 

 dote derived from soda-lime felspar, are the chief ; their dis- 

 tribution is purely local, and from a majority of the specimens 

 collected they are absent. The rocks vary widely in structure, 

 some being very coarsely banded, others finely foliated ; occasionally 

 augen-gneisses are found, and in other cases the disposition of the 

 biotite is that expressed by Loewinson-Lessing's term glomero- 

 plasmatic. 3 By increase in the proportion of biotite, the gneisses 

 pass locally into types that structurally are biotite-schists. By 

 decrease in the proportion of biotite, accompanied by gradual loss 

 of directive structure, the rocks pass into indubitable granites. 



The following brief descriptions of characteristic types cover 

 the whole range of specimens collected. For convenience, the per- 

 centage mineral compositions (determined by Rosiwal's method of 

 micrometric analysis) are collected, together with the respective 

 specific gravities and radium contents, in Table V, p. 53. 



1 S. J. Shand, Geol. Mag. dec. 5, vol. x (1913) p. 510. 



2 J. Johnston, Jonrn. Geol. Chicago, vol. xxiii (1915) p. 730, & J. Kcenigs- 

 berger, Econ. Geol. vol. vii (1912) p. 699. 



3 Trav. Soc. Nat, St. Petersb. vol. xxx (1900) p. 208. 



