358 F. P. Mennell — South African Petrography. 



at Taba s'Induna, a conspicuous hill about 12 miles north of 

 Bulawayo. The under part of the flow is highly amygdaloidal, but 

 the scoriaceous upper portion was evidently denuded off before 

 the deposition of the overlying rock. Sections from the compact 

 parts of the mass show it to be made up of lath-shaped felspar with 

 much less abundant granular augite. Glomero-porphyritic aggregates 

 of larger felspars are distributed through the mass, and magnetite 

 occurs as an accessory. 



We now come to the basalts, and here our only difficulty is one 

 of selection. Undoubted lava-flows are not particularly common 

 in Cape Colony, but further north, and especially in the Zambesi 

 Valley, they are developed on an extensive scale. A rock from 

 Livingstone Island at the Victoria Falls may be taken as typical. 

 It shows a flow structure which is by no means so clear under the 

 microscope as on a polished surface, but is indicated by the dis- 

 tribution of coarse-grained glomero-porphyritic aggregates of augite 

 and labradorite. The bulk of the rock is a granular mixture of 

 augite, plagioclase, and magnetite, with some interstitial matter, 

 the ophitic structure not being developed in any of the specimens 

 I have examined from this extensive area. Another example from 



Fig. 1. 



the north bank of the river at the Falls is not so fresh, but otherwise 

 quite similar. Some varieties are highly amygdaloidal, the cavities 

 being filled with pectolite and other zeolites as well as calcite, 

 agates, etc. The individual flows can frequently be distinguished 

 by the occurrence of bands of these amygdules at intervals in the 

 great masses of lava. A specimen from the Dekka Eiver on the Falls 

 Koad presents some interesting features (Fig. 1), Eesinous-looking 

 crystals of felspar are obvious to the naked eye ; indeed, they 

 occasionally attain a length of ^ inch, and under the microscope 

 these are seen to form part of glomero-porphyritic aggregates which 

 also comprise good-sized crystals of colourless augite and a few 



