﻿part 3] 



VOLCANIC ROCKS OF MOZAMBIQUE. 



257 



With the data thus ascertained the specific gravity of the rock 

 is calculated to he 267. The value actually found was 2 - 68. , 



The phenocrysts of plagioclase average andesine in com- 

 position. They are often strongly zoned, and the interior is, as is- 

 usual, richer in the anorthite molecule than the outer zones. 

 Inclusions are rare, except for occasional wisps of chlorite. 

 Alteration is common to a macrocrystalline aggregation in which 

 kaolin, perhaps sericite, epidote, and a very little calcite can be 

 distinguished. Green hornblende is present in good idiomorphic 

 crystals modified by slight resorption. Patches of ground-mass, 

 shreds of biotite, grains of magnetite, and small crystals of apatite 

 are found as inclusions, and in one case a rounded crystal of zircon. 

 Biotite also exhibits slight resorption, and its fibrous texture 

 suggests a certain degree of alteration. Magnetite and apatite 

 are present sparingly as inclusions. The ground-mass contains 

 small laths of andesine or oligoclase surrounded by fan-like inter- 

 growths of colourless minerals having refractive indices respectively 

 above and below that of Canada balsam. The minerals in question 

 are probably quartz and orthoclase : for the intergrowths exhibit 

 an approach to microgranophyric texture, and the chemical 

 composition. . of the ground-mass (tabulated below) leads to the 

 same conclusion. Wisps of biotite largely altered to chlorite are 

 distributed through the ground-mass, and irregular grains of horn- 

 blende are also present. 



Chemical Composition (Hornblende- Anrlesite). 



An analysis of part 

 following resiilts : — 



Percentages. 



Si0 2 , 60-76 



ALO, 16-81 



Fe 2 0., 2-07 



FeO 2-34 



MgO 3-27 



CaO 6-57 



Na,0 3-32 



K 2 0 2-42 



Loss above 110° C. 1-19 

 Loss at 110° C. ... 0-48 



Ti0 2 0-31 



MnO 0-22 



Total 99-76 



of specimen 126 was made, with the 



Molecular 

 Proportions. 

 1-013 



•165 



•013 



•032 



•082 



•117 



•053 



•026 



•004 

 •004 



Specific gravity — 2'68. 



Mineral Composition 

 (Norm). 



Quartz 14 , 16"1 



Orthoclase 14'46 I Salic 



Albite 27-77 f =80-30, 



Anorthite 23-91 J 



Diopside 6-891 



Hypersthene ... 7*28 I Femic 



Magnetite 3-02 ( =17-80. 



Ilmenite 0 - 61 J 



Total 98-10 



Loss on ignition 1*67 



Total 99-77 



Class II Persalane. 



Order 4 Austrare. 



Rang- 3 Tonalase. 



Subrang 4 . . . Tonalose. 



The analyses of some other andesites from other East African 

 localities are quoted below (B, C, D, E ) for comparison with the 



