﻿part 3] 



VOLCANIC ROCKS OF MOZAMBIQUE. 



233 



determined, the position and symbol of each rock in the quanti- 

 tative (C. I. P. W.) system of classification is added : — 



Rocks of Group A. (Alkali series and associated basalts). 



Solvsbergite Nordmarkase I, 5, 1, 4. 



J3girine-trachyte n. cL 



Trachytoid-phonolite Umptekose II, 5, 1, 4. 



Tephritic pumice Akerose II, 5, 2, 4. 



Basalt Camptonose Ill, 5, 3, 4. 



Picrite-basalt Auvergnose Ill, 5, 4, 4. 



Rocks of Group B. (Calc-alkali series). 



Basalts : sills and dykes ... Hessose II. 5, 4, 4. 



Basalts: lava-flows Tonalose-bandose . 11,4,3-4,4. 



Hornblende-andesite Bandose II, 4, 3, 4. 



Ground-mass Lassenose I, 4, 2, 4. 



Pyroxene-andesite n. d. 



Petrographic Descriptions and Analyses. 

 Rocks of Group A. 



VIII. SOLVSEKKGITE. (PI. XX, fig. 1.) 



Two specimens (153 1 & 154) of solvsbergite were collected by 

 the late Mr. II. L. Reid, between Miali and the Sanhuti River ; 

 but their source, which was possibly a dyke penetrating gneiss, was 

 not discovered. 



The rock has a mottled appearance in the hand-specimen, due to 

 the alternation in ill-defined layers of dark-grey and straw-coloured 

 patches. In this respect it bears a close resemblance to the type- 

 rock of Gran, but it is heavier, darker, and more finely-grained 

 than the latter. Small tabular phenocrysts of clear glassy 

 anorthoclase are sparsely scattered through the rock, reaching 

 1 cm. in their largest dimension. Under the microscope the 

 following minerals can be distinguished. Their approximate 

 proportions by weight, measured by the Posiwal micrometric 

 method, are added : — 





Estimated 



Minerals. 



Percentage by weight. 



A n ortli ncla.se 



77 



Cossyrite (accompanied in small amount by 



katoforite and arfvedsonite) 



13 



Mgivine and tegirine-augite 



8 



Magnetite (and ilmenite P) 



2 





n. d. 





n. d. 



Total 



100 



Anorthoclase determines the texture of the rock, which is 

 trachytic. The crystals are clear and unaltered, and have a 



1 These numbers in parentheses refer to specimens in the collection of 

 Mozambique rocks studied by the author. The collection is at present in the 

 Geological Department of the Imperial College of Science & Technology. 

 When its investigation is completed, it is proposed to present the collection 

 to the Mineralogical Department of the British Museum (Natural History). 



