﻿spart 3] 



VOLCANIC BOCKS OF MOZAMBIQUE. 



247 



.are common. Otherwise, the felspars are clear and glassy, and 

 show no traces of alteration. 



While most of the felspar came early in the order of crystalli- 

 zation, there is another member of the felspar family which is 

 allotriomorphic, and was the last of the minerals to crystallize. 

 It is free from twin lamellae, exhibits lower polarization-colours 

 than labradorite, and has a refractive index of l - 522. Its identity 

 is thus determined as orthoclase, which is further verified by 

 the relatively high percentage of potash contained in the rocks. 



Augite is pale yellowish-green (iron-stained round the borders 

 in the Mochelia rock), and exists in rounded grains or sharply- 

 defined wedges determined by the disposition of the felspar-laths. 

 It is probably a member of the en statite -augite series. The 

 intersertal texture thus produced is a marked characteristic of 

 nearly all the Mozambique rocks. 



Magnetite, perhaps accompanied by ilmenite, is evenly 

 distributed in small octahedra, irregular grains, and skeleton- 

 crystals. It tends to cling round the augite-crystals, but rarely 

 foi'ms inclusions in them, and never does so in the labradorite. 

 In the Mochelia rock, magnetite is very abundant inside dark 

 patches of glass or palagonite. The mineral certainly belongs to 

 a late stage in the crystallization of the rock. 



Dark interstitial matter occupies nearly a quarter of the volume 

 of the rocks, being, however, much less abundant than in the lava- 

 flows. 'Some of this material is an isotropic glass packed with 

 globulites, or penetrated by a network of iron-ores. In other cases, 

 microlites of felspar and obscure aggregates of dark augite are 

 buried in a pale glass. Palagonite is also present in deep-brown 

 or greenish patches, which under crossed nicols are seen to have 

 a fibrous cryptocrystalline texture. 



By means of a diffusion-column of methylene iodide and benzine, 

 the constituents of the Sokoto-Hill rock (128) were separated into- 

 the following groups. The approximate proportions of each 

 mineral, measured by micrometric analysis, are added : — 



Mineral Composition of No. 128. 



Specific Gravity. Mineral. Percentage by weight. 



2-20 Opal 3 •/ 



2'35 1 



V Glass and palagonite , 20 



2-56 Orthoclase : 6 



Chalcedony and chlorite 7 



2-65 

 2-68 



2- 70 



3- 10 

 3-20 



Sank Magnetite and ilmenite with glass 6 



Labradorite 40 



Enstatite- augite 18 



Total 100 



Taking the specific gravity of the material that sank as 5, the specific 

 gravity of the rock should be 2 - 74. The actual value, 2 - 70, corresponds with 

 the slightly vesicular character of the rock. 



t2 



