﻿part 3] 



VOLCANIC ROCKS OP MOZAMBIQUE. 



275 



Assimilation not being admitted, there are two ways in which 

 the requisite magma may have been formed. There have been in 

 Mozambique at least three successive periods of granitic intrusion, 

 the first being by far the most extensive, the second and third 

 being almost entirely restricted to small intrusions and pegmatite 

 dykes respectively. It may be that the deep-seated differentiation 

 that gave rise to the early granitic magmas was never quite 

 clean, the result being that a considerable proportion of water 

 accompanied by silica would be retained by the deeper basaltic 

 magmas. 1 



On the other hand, even if there had been no free silica in the 

 basaltic magma that formed in Tertiary times, a certain amount 

 may have been liberated by the early crystallization of olivine, 

 the olivine being afterwards separated from the residual liquor by 

 sinking, 3 the liberated silica being maintained in solution by the 

 activity of water and the crystallization of lime as silicate being 

 deferred by the same agency. No olivine-rich lavas are associated 

 with the basalts, but this may be due to the eruption of the 

 lighter magmas in preference to the heavier crystal-charged 

 differentiates. 



XXI. The Eadioactivity of the Lavas, and its Bearing 

 on their Origin. 



The radium-content of most of the lavas was estimated by 

 Strutfs solution method. 3 In the case of the andesites from the 

 Monapo River this could not be done, on account of lack of 

 material. The results obtained, together with those for some 

 other Mozambique rocks, are tabulated below : — 



Radium in grms. 



Rock. per grin, of rock. 



f Solvsbergite 2'26 X 10 " 12 



| iEgirine-trachyte 2"83 „ 



c . . I Phonolite 4-10 



Series A.< ^ , ... . „,,„ 



] Tephritic pumice 2 - 59 „ 



| Basalt (Sanhuti River) "... 1'38 



l^Picrite-basalt 047 



fBasalt sill (Sokoto Hill) 0-98 '„ 



Series B ■< Basalt d y ke (Mochelia) 0-85 „ 



" | Microvesicular basalt (Sokoto Hill) 0*94 ,, 



(^Amygdaloid (mean of four specimens) 0"76 ,, 



Tertiary. Sandstone (Mochelia Hill) 071 



(" Limestone (Sokoto Hill) 0'43 



Cretaceous s Felspathic sandstone (Mount Mesa) 1*59 ,, 



L Ferruginous sandstone (Mosuril) 1'18 ,, 



1 The differentiation of granitic magma from basaltic magma has been 

 fully dealt with by N. L. Bowen, Journ. Geol. Chicago, Supplem. to vol. xxiii 

 (1915) p. 46. 



2 Ibid. p. 39. 



a R. J. Strutt, Proc. Roy. Soc. ser. A, vol. lxxvii (1906) p. 472 & vol. lxxviii 

 (1907) p. 150 ; A. Holmes, Sci. Progress, no. 33 (1914) p. 13 ; and ' The Age 

 of the Earth ' London, 1913, pp. 104-107. 



