﻿part 3] 



VOLCANIC KOCKS OF MOZAMBIQUE. 



259 



the lack of material, ferrous oxide was not separately determined, 

 nor were several important minor constituents. The analysis 

 gives a low summation, but is sufficiently full for the immediate 

 purpose. 



Percentages. 



SiO, 65-32 



Al.,0, 16-08 



Fe 2 0 3 .... 



EeO 



MgO .... 



CaO. ...... 



Na 2 0 .... 



K 2 0 



Loss on 

 ignition. 

 TiO, 



2-89 

 n.d. 



2- 13 



3- 47 



4- 33 

 3-62 



1-06 

 0-17 



Molecular 

 Proportions. 

 1-089 

 •158 

 •018 



•053 

 •062 

 ■069 

 •038 



Mineral Composition 

 (Norm). 



Quartz 16-86"] 



Orthoolase 21-13 I Salic 



Albite 36-15 f =88-32. 



Anorthite 14-18 J 



Diopside 2-38 1 



Hypersthene ... 4-20 I Femic 



Haematite 2-72 f =9-60. 



Ilmenite 0*30 J 



Total 97-92 



Loss on ignition l - 06 



Total 98-98 



Class I Persalane. 



Order 4 Brittanare. 



Rang 2 Toscanase. 



Subrang 4 ... Lassenose. 



Analyses of African rocks of similar composition and associated 

 in the field with andesites are quoted on the preceding page 

 <G & H). 



•002 



Total 99-07 



Specific gravity = 2-61. 



XVII. Pyroxene-Andesite. (PI. XXI, fig. 6.) 



Pebbles of pyroxene-andesite (No. 144) were found by Mr. Way- 

 land in the bed of the Monapo River and on the alluvial flats to 

 the south, at distances of 3 to 4 miles from the mouth of the 

 river. The rock is dark grey in colour, and is sj>eckled with 

 phenocrysts of felspar, some of which are transparent, others being 

 opaque. It lacks the prevailing reddish tints of the basalts, but 

 weathers to a dark brown. The rock is singularly compact and 

 free from vesicles — a feature suggesting that, like the neighbour- 

 ing hornblende-andesite, it may be derived from a dyke. No 

 known lava from the Mozambique volcanic area is entirely free 

 from vesicles or amygdales. The specific gravity is 2 - 72. 



Under the microscope the rock presents no unusual features. 

 It consists essentially of phenocrysts of andesine, augite, and 

 hypersthene, and occasional pseudomorphs of serpentine after 

 olivine, embedded in a fine-grained ground-mass of hyalopilitic 

 texture, in which tiny laths of oligoclase, shreds of biotite, grains 

 of pyroxene and iron-ores, together with a little interstitial glass, 

 are the chief constituents. 



The phenocrysts of plagioclase are generally clear glassy 

 andesine, with characteristic twinning and zoning, and an average 

 refractive index of about P556. The opaque felspars of the hand- 

 specimen owe their colour to the presence of large inclusions of 



