﻿part 3] 



VOLCANIC ROCKS Or MOZAMBIQUE. 



245 



on the contrary, occurs only in tiny laths distributed sparsely 

 through the ground-mass. Purple-brown augite is the most 

 abundant mineral present, and bears a close resemblance to the 

 augite described by Dr. A. Scott 1 from the Arenig augite-andesites 

 of Bail Hill (Dumfries-shire). The crystals have a tendency to 

 group together in a roughly radial fashion about a central nucleus 

 of olivine. In the ground-mass small grains of deep-purple augite 

 are very abundant. Whether the purple coloration of certain 

 augites is due to the presence of titanium or manganese cannot 

 yet be definitely stated.. The usual belief that it is due to the 

 former element is supported by the analyses tabulated in this paper, 

 but the subject is one that still awaits systematic investigation. 

 The ground-mass is very fine-grained, and dark owing to the 

 abundance of magnetite and ilmenite, which are present in 

 well-shaped crystals and in a dendritic network of branching rods. 

 Iron-ores, titaniferous augite, and a sprinkling of labradorite-laths 

 are crowded together in a residuum of grey-brown glass. In 

 sporadic patches labradorite becomes sufficiently abundant to' 

 control the texture, which is then not unlike that of the inter- 

 sertal basalts described below. 



Chemical Composition. 



Portions of specimen 155, free from phillipsite, were analysed 

 with the following results : — 



Molecular Mineral Composition 



Percentages. Proportions. (Norm). 



Si0 o 44-05 -734 Orthoclase 4-00] _ 



A1 2 0 3 13-39 -131 Albite .'. 16-83 I 



Fe,0, 7-64 -048 Anorthite 25-44 J 



Feb.' 8-42 -117 



MgO 10-51 -263 Diopside 21-06") 



CaO 10-46 -187 Hypersthene ... 3-79 



Na.,0 2-01 -032 olivine 13-46 



K 2 6 0-67 -007 Magnetite 11-14 



H 2 0 + t 0-44 ... Ilmenite 3"65 



H. 2 0- 0-47 ... 



Ti0 o 1-93 -024 99-37 



MnO 0-25 -003 Water 0-91 



46-27. 



Femic 

 = 53-10. 



Total 100-24 Total 100-28 



Class III ... Salfemane. 



Ea = 0 - 47xl0 _ 12 grms. per grm. Order 5 Gallare. 



Specific gravity=2'78. Rang 4 Auvergnase. 



Subrang 4 . . . Auvergnose. 



The rock is similar, both chemically and mineralogically, to the 

 Hillhouse or pic rite type of the Scottish Carboniferous basalts. 2 

 Chemically, it bears some resemblance to the camptonite of Kjose 

 Aklungen (Norway), 3 and this is the more interesting because the 



1 Min. Mag. vol. xvii (1914) p. 100. 



2 G. W. Tyrrell, Trans. Geol. Soc. Glasgow, • vol. xiv (1912) p. 228 

 (Analysis II, p. 250). 



3 W. C. Brogger, ' Eruptivgesteine des Kristianiagebietes : III' 1899, p. 51, 



Q. J. Gr. S. No. 287. t 



