366 S. S. Buchnan — River Development. 



shown, one large crystal being divided into six equal-sized lamellae. 

 The felspar is allotriomorphic. It forms large grains which exhibit 

 Carlsbad, albite, and pericline lamellation. The extinction angle 

 rises to at least 30°. Small flakes of biotite and granules of ilmenite 

 are present. A little quartz also occurs, chiefly as micropegmatite. 

 A specimen from the Gwelo-Selukwe Koad shows more abundant 

 biotite in small, very pleochroic flakes. The dominant ferro- 

 magnesian mineral is, however, augite, in which the herring-bone 

 structure is almost always shown. A little enstatite and hornblende 

 are present, the latter sometimes bordering the augite. The plagio- 

 clase evidently approaches anorthite in composition, and forms 

 larger grains than the other constituents. Ilmenite sometimes 

 builds good-sized irregular granules, while quartz, chiefly in 

 micrographic intergrowth with felspar, is sparingly distributed. 

 A specimen from another locality is remarkable for the abundance 

 of strikingly pleochroic yellowish biotite, so that it may be described 

 as a biotite gabbro. Hornblende is also present and sometimes 

 borders the augite. In other respects the rock is similar to the 

 two previous varieties. 



Though somewhat rare, as in other parts of the world, ultrabasic 

 rocks are widely distributed in South Africa. They have, indeed, 

 received more attention from European geologists than any of the 

 other divisions. The breccia and eclogite of Kimberley and of 

 Jagersfontein in the Orange Eiver Colony are so well known that 

 it is needless to do more than mention them. The very interesting 

 melilite-bearing rock from the Spiegel Eiver in Cape Colony, 

 described by Professor Cohen,^ provides the only instance of the 

 occurrence of a felspathoid, other than nepheline, in our province, 

 though leucite basalts, etc., have been recorded from Kilimanjaro 

 in Central Africa. Picrites have been described from the Transvaal 

 and elsewhere by various writers, so I will confine myself to a brief 

 mention of a rock from Porselt's Vlei on the road from Inciza to 

 Belingwe in Southern Matabeleland. It is very coarse-grained and 

 of a uniform dark-green colour. Olivine appears to be the sole 

 constituent, as iron-ores are conspicuous by their absence, and there 

 are no indications in thin section which would lead one to suspect 

 the presence of any other ferro-magnesian mineral. The rock is 

 beautifully fresh and quite free from even incipient serpentinization. 



I 



VI. — Eiver Development. 



By S. S. BccKMAN, F.G.S. 



N the Quarterly Journal of the Greological Society, May, 1902, 

 _ vol. Iviii, p. 207, Mr. A. Strahan has a paper on the "Origin 

 of the Eiver-System of South Wales, and its Connection with that 

 of the Severn and Thames." It is with the part of the paper 

 expressed in the latter portion of the title that I am more par- 

 ticularly concerned ; for in that connection Mr. Strahan remarks 



1 T.M.M., 1894, p. 188. See also Ann. Rep. Cape Geol. Coram., 1898. 



