﻿part 3] 



VOLCANIC ROCKS OF MOZAMBIQUE. 



2o5 



Chemical Composition of the Sokoto Vesicular Basalt 

 (No. 132). 







tit 7 7 _ 



MolecuLar 





Mineral Composition 





Percentages. 



Proportions. 





(Norm). 



SiO, 



56-42 



•943 



Quartz 



8-97"] 



Al.,63 



17-33 



•170 



Orthoclase 



9-45 [ Salic 



Fe 0 3 



2-78 



•018 



Albite 



24-89 f - 73-19, 



FeO 



3-23 



•044 



Anorthite 



29-88 J 



MgO 



4-67 



•117 







CaO 



8-61 



•154 



Diopside 



10-301 



Na,0 



2-95 



•047 



Hypersthene ... 10 - 09 1 Femic 



K.,6 



1-58 



•017 



Magnetite 



4-18 f -25-03. 



K~,0 + 



1-32 





Ilmenite . , , 



0-46 J 



H.^O- 



0-44 









TiO., 



0-25 



•003 



Total 



98-22 



MnO 



0-17 



•002 





1-76 



Total 



99-75 





Total 



99-98 









Class II 



Persalane. 



Ba = 094 X 10 -1 '- arms, per arm. 



Order 4 



Austrare. 



Specific gravity = 2-43. 





Rang 3-4 ... 



Tonalase-Bandase. 









Subrange 4 . . . 



Tonalose-Bandose. 



Comparing this analysis with that of the Sokoto sill, the most 

 striking difference lies in the excess of silica in the vesicular lava. 

 This is clearly referable to the presence of chalcedony in the 

 microscopic vesicles of the latter. However, even if free silica be 

 subtracted from the analysis, and the remaining figures recalcu- 

 lated to 100, there still remain noteworthy differences. The lava 

 contains a smaller relative proportion of iron-oxides and magnesia 

 than the sill, and the compositions of the normative plagioclase in 

 the two rocks are : — 



Sokoto sill An 3s 



Sokoto vesicular laya -A- 1 *,.-, 



It thus appeal's that the lava is deficient in the constituents of 

 chlorite and of lime-zeolites. The absence of these amygdale 

 minerals from the rock analysed and their abundance in the 

 neighbouring lavas confirm this result. If we regard the Sokoto 

 sill as representing an approximation to the parent magma of the 

 surrounding flows of vesicular and amygdaloidal basalt, then we 

 may follow the progress of differentiation (controlled b_y the 

 presence of abundant water and perhaps of other volatile fluxes) 

 and reconstruction through various stages that may be represented 

 as follows : — 



