208 MB,. H. H. THOMAS ON THE [May I907, 



The felspars, which are practically all of one generation, build 

 lath-shaped crystals ranging up to 1*2 mm. in length. Generally 

 they are twinned according to the Carlsbad law alone, but a few 

 show albite-lamellation. The low and sometimes almost straight 

 extinction of these crystals indicates an acid plagioclase, approxi- 

 mating to if not actually reaching oligoclase. The augite, which 

 is a pale variety, but not quite colourless in section, forms plates 

 ophitically enclosing the plagioclase-laths. The orthoclase occurs as 

 small untwinned patches, filling the spaces between the earlier- 

 formed crystals, and was evidently the last product of consolidation. 

 A few small needles of apatite may be seen, but the mineral is rare ; 

 and chlorite occurs as a decomposition-product of the ferromagnesian 

 minerals. Small chloritic patches might possibly represent residual 

 glass, but proof is wanting. Both these rocks were completely 

 devoid of flow-structure. 



In the apparent high percentage of alkalies these dolerites 

 approximate in composition to the mugearites of Harker, but differ 

 strongly from them in a greater percentage of the bisilicates and 

 in the paucity of apatite. 



The Flows. 



These rocks may be divided into two types : — 



(a) Those in which the augite is mostly granulitic, but which also contains 



larger subophitic plates. 



(b) Those in which all the augite is granulitic in character. 



(a) The Subophitic Dolerites. 



F 1019. Locality : Creek at Mamba Camp, north of the Zambezi. 



1028. Locality : Bed ofLukunguli(Jambezi), near Dambi's; (bedded). 



1029. Locality: First crossing of the Lukunguli from Tsheza's. 



1030. Locality : Matetsi River, south of Matetsi Station. 



In the hand-specimen the above rocks are of a dark-grey colour, 

 one alone (1028) having a dull-red tinge. They are heavy and 

 compact, and show bright felspar-crystals. They are typically non- 

 amygdaloidal. Specimens 1019 & 1029 show a platy structure, 

 apparently parallel to the surface of the mass ; while 1030, on a 

 weathered surface, shows light- coloured circular patches about a 

 quarter to three-eighths of an inch in diameter, the origin of which 

 is obscure unless it be a greater percentage of felspar in that area. 



The rocks consist of magnetite, augite, and plagioclase-felspar, 

 with olivine and apatite as accessories. The texture is fine-grained 

 and there is no flow-structure. 



The magnetite builds octahedra, but occurs also in irregular 

 patches and strings. The augite, which is pale in colour, occurs in 

 two forms, either as plates wrapping round but seldom enclosing 

 the felspar, or as knots or small crystals without good outline in 

 the groundmass. 



These rocks may be said to be rich in augite, but this is compen- 

 sated in a measure by the almost complete absence of olivine ; only 



