﻿part 3] 



VOLCANIC ROCKS OF MOZAMBIQUE. 



241 



percentage of iron, and especially of ferric oxide. This character- 

 istic distinguishes the rock from the trachyandesites and trachy- 

 dolerites, and suggests that the name tephrite would be somewhat 

 more appropriate. According to Dr. H. ,S. Washington, 1 the name 

 trachydolerite should be restricted to rocks containing labra- 

 dorite or a more basic plagioclase, as well as alkali-felspars ; and the 

 name trachyandesite to rocks containing andesine in addition 

 to alkali-felspars : both types of rock, moreover, should be rather 

 rich in potash. But for the high iron-content, the composition of 

 the rock now described would fulfil moderately well the conditions 

 for a trachyandesite. Its close similarity to the trachyandesite 

 from Reunion (analysis D) bears out this opinion, but the chemical 

 characters of the rock are probably expressed more accurately if 

 it be named a tephritic pumice. 



XII. Basalt. (PI. XX, fig. 4.) 



The lava-flow represented by specimen 156 was found by 

 Mr. R. L. Reid at the base of the volcanic succession in the Sanhuti- 

 River district. It is exposed on the north side, of the river near 

 the boundary-fault, where it lies on a basement of gneiss, and 

 probably extends to the Cretaceous sediments on the east. The 

 basalt is a dense dark-grey rock holding rusty amygdales that 

 average ^ inch in diameter. The amygdales are lined and some- 

 times entirely filled by palagonite, which in thin section varies 

 in colour from yellowish green to reddish brown, though in the 

 hand-specimen, beneath the rusty shell of iron-oxides, it is nearly 

 black. The specific gravity is about 2 - 42, thus agreeing closely 

 with that of the palagonite from Cape Flora (Franz Josef Land) 

 examined by Sir Jethro Teall. 2 The same material occurs in the 

 main body of the rock, wedged in the crystalline network of the 

 ground-mass. 



The only other minerals found in the amygdales are thomsonite,. 

 easily recognizable by its moderately-high double refraction, and 

 calcite. Thomsonite occurs in radiating aggregates, each acicular 

 crystal giving straight extinction. The refractive index, about 

 T52, is low for thomsonite, and points to a high content of soda. 



Excluding the amygdales, the mineral composition of the rock, 

 determined by the Rosiwal method, is approximately as follows : — 



Estimated percentage 



Minerals. by weight. 



Labradorite , : 56 



Augite 24 



Palagonite 15 



Magnetite, ilmenite, 3 and decomposition-products 5 



Total 100 



1 Journ. Geol. Chicago, vol. v (1897) pp. 366-68. 



2 Q. J. G. S. vol. liii (1897) p. 486. 



3 The percentage of these minerals is greater than 5, for only a few of the 

 inclusions in felspar and augite could be measured. 



