80 Transactions of the South African Philosophical Society. 



and biotite ; these are the only minerals and rocks yet observed in 

 the Balmoral breccia. This vent is evidently similar to the Silver 

 Dam pipe, but up to the present it has yielded a smaller variety of 

 rocks and minerals than that one. 



On the farm Spiegel River in the Division of Riversdale there is 

 an outcrop of melilite-basalt in the Uitenhage beds.* It is more or 

 less oval in outline and about 300 feet in its longer diameter. The 

 rock stands out from the softer Uitenhage beds, but there are no 

 sections showing the junction, and as the surrounding rocks are 

 sandy conglomerates with feebly-developed bedding planes the 

 change of dip, if it exist, near the igneous rock is not observable. 

 The only reasonable explanation of this singular occurrence is that 

 the basalt fills a pipe or neck. It is the only igneous rock of post- 

 Uitenhage age in the Colony whose date is proved. 



The rock is composed of porphyritic crystals of olivine lying in a 

 base of melilite, augite, perofskite, magnetite, and glass. It has a 

 very strong family likeness to the melilite-bearing rocks of Sutherland 

 Commonage and Matjes Fontein, but is not identical with any one 

 of them. Its chief points of difference from those are its freshness : 

 hardly any sign of alteration can be seen ; the only obvious change 

 is the formation of serpentine along some of the cleavage cracks in 

 the olivine ; the augite is very pale green in colour and is an 

 abundant constituent in the form of grains and crystallites ; this 

 mineral is not abundant in any of the Sutherland specimens yet 

 examined, and is entirely absent from some of them ; calcite is not 

 seen in the Spiegel Eiver rock, though it is abundant in those from 

 Sutherland; and biotite is entirely absent.! In spite of these 



* See Ann. Kep. Geol. Comm. for 1898, p. 62. 



f Mr. J. Lewis, of the Government Analytical Laboratory, very kindly made a 

 complete chemical analysis of the Spiegel Eiver rock, with the following results : — 



Si0 2 36-15 



Ti0 2 2-30 



A1 2 3 15-18 



Fe 2 3 4-87 



Cr 2 3 -10 



FeO 9-11 



MnO -33 



CaO 11-40 



MgO 13-63 



BaO -06 



Na 2 2-42 



K 2 1-81 



P 2 5 -26 



SO s -49 



0H 2 on ignition 1*95 



0H 2 driven off below 110° -37 



' JOO-43 



