Vol. 67,.'] PETROGRAPHY OF THE ZAMBEZI BASIN. 209 



in two specimens (1028 & 1030) were doubtful pseudomorphs 

 after this mineral detected. The granules of augite are often 

 twinned, and elongated parallel to the twin-plane. 



The felspars occur in two generations, but are seemingly very 

 similar in composition, both being just on the acid side of typical 

 labradorite. The earlier generation builds porphyritic slightly- 

 zoned crystals, usually twinned according to the Carlsbad and 

 albite-laws, but occasionally showing pericline-lamellse. The sym- 

 metrical extinctions on the twin-plane range up to 36°. The micro- 

 liths are extremely slender, usually twinned on the Carlsbad plan 

 only. Apatite occurs sparingly, and there seems to have been a 

 small residue of interstitial glassy material. 



It is quite probable that some of these rocks may be sills ; but, 

 apart from field-evidence, there is no reason for removing them 

 from those considered as true flows. 



(b) The Granulitic Dolerites. 



F 1016. Locality: Railway-cutting at west side of bridge, Victoria Falls. 

 1017. Locality : Bottom of the Zambezi Gorge at the entrance to the 

 Songwi. 



1020. Locality: Lower cataract at the Tshimamba. 



1021. Locality : Upper cataract at the Tshimamba. 



1022. Locality : South side of the Zambezi, floor of the gorge near 



the Namaruba confluence. 



1024. Locality : Bwani River, north of the Zambezi. 



1025. Locality: Bwani River, north of Makwa (platy beds). 



1026. Locality : Bwani River, north of the Zambezi (platy beds). 

 1033. Locality: Kopje near the railway-bridge, 6 miles west of 



Wankie. 

 1037. Locality: Rock in the fault, Deka River, kopje near the 

 railway-bridge. 



The fresh rocks, such as 1016, 1020, 1031, 1033, are compact, 

 and dark-grey to grey-brown in colour, weathering brownish or 

 greenish according to the extent to which oxidation or chloritiza- 

 tion has proceeded. They are feebly vesicular : in P 1024 cavities 

 reach half an inch in diameter ; in other specimens they are smaller, 

 while in some they are absent. Alineralogically they are identical 

 with the rocks containing subophitic augite, and the two groups 

 undoubtedly pass one into the other. F 1017 represents an inter- 

 mediate stage, in which there are occasional bigger crystals or knots 

 of augite. 



The felspars occur in two generations, as before ; but the micro- 

 liths often tend to take up a parallel arrangement indicative of flow, 

 although flow- structures are generally ill-defined or absent. In 

 composition and dimensions the felspars agree with those described 

 above. 



Magnetite is comparatively abundant in octahedra, but by no 

 means constant in its percentage. 



In texture these rocks are all fine-grained, some more so than 

 others. Olivine is rare and in many cases absent, while the augite, 

 where it exists as large crystals, has undergone partial re-absorption. 



