278 Report S.A.A. Advancement of Science. 



interpenetrating twins. The mineral, when fresh, is colourless in 

 thin slices, and its double refraction is low, giving interference colours 

 not higher than yellow or orange of the first order in a slice of the 

 normal thickness. The extinction is oblique, ranging up to 45°, so 

 that the mineral belongs to the monoclinic system. It is therefore to 

 be identified as one of the colourless members of the augite group, 

 diopside or fassaite. Partly intergrown with the augite in parallel 

 position, and partly occurring in independent prismatic crystals is a 

 pale green chloritic mineral, possessing the characters of bastite 

 pseudomorphs after a rhombic pyroxene. This is probably the 

 mineral described by Bonney *, and by him referred rather hesitat- 

 ingly to enstntite. Feldspar crystals of the first generation are 

 entirely absent, but large crystals of ilmenite, showing the character- 

 istic bar-structure, probably belonging to the intratelluric stage. 



The ground mass consists chiefly of an aggregate of small pris- 

 matic crystals of the same pyroxenes, with a certain amount of 

 interstitial feldspar, and, as before mentioned, a very small amount 

 of quartz. The feldspar is much decomposed, and it is difficult to 

 determine its original character. However, a careful examination, 

 both by daylight and in artificial light, has failed to reveal any albite- 

 twinning, except in one or two cases, and it is clear that the most of 

 the feldspar must be referred to orthoclase. 



It is somewhat difficult to assign this rock to its type. The very 

 small proportion of feldspar, and the fact that this is orthoclase and 

 not plagioclase, serve to distinguish it from the porphyrites and 

 normal dolerites. Perhaps it can be most satisfactorily classi- 

 fied as a member of the lamprophyre group. According to Rosen- 

 busch's definition t, it must then be described as vogesite. 



The other two examples of dyke-rocks (222c, 222d) differ from 

 the preceding in showing; a higher proportion of feldspar, especially 

 plagioclase and quartz. They approach much more nearly to the 

 normal dolerite type, and, as is common in this group, the last con 

 stituent to crystallise in the ground mass is a eutectic of quartz and 

 feldspar. They may be quite safely described as quartz-dolerites, 

 .and their macroscopic appearance confirms this view. 



We have no evidence concerning the age-relations of these basic 

 rocks to the acid series in which they occur, but they are probably 

 subsequent to, and intrusive in the latter, and this supposition is 

 confirmed by their greater degree of freshness. 



C. LOWER SEDIMENTARY SERIES. 



Below the acid volcanic series comes another small development 

 of sedimentary rocks, which in the Kimberley Mine appears to be 

 about 30 feet thick. It is represented in our collection by one 

 specimen only, from a depth of 2470 feet (220). This is a heavy, 

 black rock of crystalline appearance, which shows conspicuous and 

 rather irregular bedding. 



* Geol. Mag., 1897, p. 499. 



t Rosenbusch, Elemente der Gesteinslehrc, p. 239. 



