78 Transactions of the South African Philosophical Society. 



Matjes Fontein pipe identical with that of Silver Dam, but the 

 proximity of the two pipes and their general resemblance leave no 

 doubt as to their close relationship. Amongst some specimens sent 

 to the South African Museum from Kimberley there is one labelled 

 by the donor " aphanite in blue-ground," which seems to occur as a 

 fine-grained rock forming a narrow dyke in the blue. This rock is 

 composed of small but well-shaped crystals of olivine, very slightly 

 altered to serpentine along cleavage cracks, lying in a ground mass 

 of calcite, magnetite, and perhaps ilmenite, perofskite, and a few 

 irregularly shaped fragments of a pale biotite. The calcite is very 

 abundant and conceals other constituents than those mentioned, but 

 when it is removed from a slice by means of dilute acid a consider- 

 able quantity of indeterminate substance remains, which yields 

 gelatinous silica on treatment with hydrochloric acid. The rock is 

 evidently an igneous material injected whilst in a molten condition, 

 for it has none of the characteristics of the breccias, broken crystals 

 of olivine, garnet, diopside, and fragments of shale being absent from 

 it. It resembles in general appearance the melilite-basalt of Matjes 

 Fontein, but melilite is not visible in the specimen we have 

 examined. So far as we know it is the first example of an ultra- 

 basic igneous rock recorded as probably forming an intrusion in the 

 blue-ground. Further information concerning intrusions of this 

 type will be most interesting. The name " aphanite" merely 

 indicates that the constituents are not large enough to be visible to 

 the naked eye. The term has been especially used in connection 

 with fine-grained augite-labradorite rocks, with which the Kimberley 

 specimen has no relation. 



The so-called " snake-rock " of the De Beer's mine is merely a 

 hard variety of blue-ground apparently forming a dyke in the softer 

 blue and passing outside the limits of the pipe into the surrounding 

 rock (a quartz porphyry) at the 1,700 feet level." The chief differ- 

 ence between thin sections of the " snake " and the blue-ground is 

 that the olivine is less altered in the former than in the latter. 



Amongst some specimens of the inclusions from the blue-ground 

 of the Monastery Mine, about 30 miles south of Winburg, O. E. C, in 

 the South African Museum there are rocks with similar characters 

 to the eclogites and related rocks of Silver Dam. One of these has 

 already been mentioned ; another consists of bright green augite 

 containing chromium, colourless augite, biotite, olivine, garnet, and 

 ilmenite ; and a third one of olivine, pale green augite, colourless 

 augite, and pale biotite. Olivine is not abundant in the rock con- 



* This fact was communicated in a letter to one of the writers by Mr. T. E. 

 Bobertson, for a time lecturer in the School of Mines, Kimberley. 



