part 



i] 



AND ASSOCIATED HOCKS OF MOZAMBIQUE. 



59 



minerals : soda-lime felspars, hornblende, garnet, and scapolite. 

 The loss of quartz is a necessary result of the interaction between 

 a granitic magma and limestone, and calls for no further comment, 

 except that it may be pointed out that the increase of quartz 

 in the garnet-rich rock relative to the amphibolite is to be cor- 

 related with the fact that garnet is an orthosilicate and that horn- 

 blende is a metasilicate. Thus, if each rock had for its formation 

 an equal supply of silica, the garnet would fix much less than the 

 hornblende, lea vino; the o-arnet-rock relatively richer in free silica 

 than the amphibolite. The disappearance of potash is more 



Table VI. 



Mineral Composition (by Volume) of Bocks fkom the Ampwihi District. 



(To illustrate the transition types between biotite-gneiss and 

 crystalline limestone. See tig. 5, p. 39.) 



Jl/j/erals 

 present. 



Biotite- 



Gneiss. 



Xo. 196. 



Bintite- 



Homblende- 



Gneiss. 



Xo. ?n.\ 



Hornblende- 

 Gneiss. 



Xo. ISO. 



„ .. c Onartz- 



(jr« met if emus , , 



. 7 ., ,., ha met- 



Amphibolite. Scapolite . Bock 



Xo. 104. Xo. 103. ^ 



Quartz 



Orthoclase . .. ~) 

 Microrline ... ) 



j Oligoclase 



Andesine | 



Labradorite ... ) 



Garnet 



Scapolite 



Calcite 



Biotite 



Hornblende 



Accessories 



Specific | 



gravities ... ) 



'£1 



55 



14 



24 



43 



6 



17 



7 

 3 



12 

 14 



9 



13 



28 



12 



— 



— 



11 



61 



— 



— 



16 



— 



— 



3 



o 



— 



— 



41 



63 



— 



3 1 



5 



7 



2-68 



271 



3-04 



3-10 



3-18 



Badium 

 -12 



in 

 per 



gms 



gm. of rock . 



2'61 



1-77 



2*13 



n.d. 



n. d. 



^ 



6 



1 No. 189 was crusbed and panned in tbo field, and the heavy residue collected is 

 found to contain sphene as the most abundant mineral. Apatite is also plentiful, 

 and garnet and cpidote are well represented. Entile and zircon are comparatively 

 rare. The opaque constituents, which make up about a third of the whole 

 residue, consist mainly of ilmenite together with a little magnetite. 



significant. It is clear that the incoming of calcic plagioclase and 

 garnet (and probably that of hornblende also) implies the addition 

 of alumina as well as of silica. The chief source of such alumina is 

 certainh' to be found in the material in the magma that, away from 

 the contact, crystallized as potash-felspar and biotite. Evidently, 

 under the physico-chemical conditions that prevailed, calcium 

 carbonate had a claim on alumina superior to that of potash. 

 Whether orthoclase molecules or potassiuin-aluminate molecules 

 had formed before the interaction with calcite, does not matter. 



