﻿part 3] VOLCANIC ROCKS OF MOZAMBIQUE. 243 



weathers it changes in the direction of laterite, not of palagonite. 

 The latter substance appears generally to be associated with 

 amygdaloidal basalts, not with vesicular types. The formation 

 of palagonite, like that of amygdaloidal infillings, is therefore to 

 be ascribed to a concentration of water (and possibly of other 

 volatile fluxes) in the residual liquors left in the crystal network 

 of the nearly consolidated rock. In the present case, the occur- 

 rence of thomsonite with palagonite is significant, on account of 

 the light that it throws on the origin of the alkali rocks which 

 succeeded the basalt. Thomsonite may contain as much as 7 or 

 8 per cent, of soda, and the low refractive, index of the present 

 example suggests that it is a soda-rich varietj r . Moreover, 

 palagonite often contains 4 per cent, of soda or more. In contrast 

 with these figures, the rock itself (including a certain amount 

 of interstititial palagonite) contains only 3*29 per cent, of soda, 

 showing that the progress of crystallization led to an enrichment 

 of the residual liquors in soda. The same conclusion would be 

 deduced from the characters of the two generations of plagioclase. 

 Thus, even within the limits of a lava-flow that has rapidly 

 cooled, it is possible to discern a process of differentiation which, 

 on a larger scale and with slower cooling, should result in the 

 production of a magma relatively rich in soda. It is, surely, 

 more than a mere coincidence that the magma erupted next in 

 succession was that which crystallized as the phonolite already 

 described. 



Chemical Composition. 



Portions of specimen 156, visibly free from amygdales and 

 weathered crust, were crushed and analysed. The results obtained 

 are as follows : — ■ 



Molecular Mineral Composition 



Percentages. Proportions. (Norm). 



SiO., 48-82 -814 Orthoclase 8-34 i 



Al.,0., 15-93 -156 . Albite 27-77 [ 



Fe",0 3 7-18 -045 Anorthite 24-46 J 



FeO .' 6-63 -092 



MgO 6-02 -151 Diopside 13-79"| 



CaO 8-67 -155 Hypersthene ... 7-84 



Na o 0 3-29 -053 Olivine 4-79 f> 



K 2 0 1-41 -015 Magnetite 10-44 



H 2 0+ 0-98 ... Ilmenite 1-52 J 



H 0 0- 0-54 ... 



Tib., 0-83 -010 98-95 



MnO 0-32 -004 Water 1-52 



Salic 

 = 60-57. 



Femic 

 = 38-38. 



Total 100-62 



Total 100-47 



Ra = l - 38 X 10~ 12 grms. per grm. 

 Specific gravity =2-78. 



Class III . . . Dosalane. 



Order 5 G-allare. 



Rang 3 Camptonase. 



Subrang' Camptonose. 



