74 Transactions of the South African Philosophical Society. 



approaches an ordinary igneous rock in character. One of the other 

 dykes is shown by a microscopical examination to be of precisely 

 the same nature as one of the gritty calcareous tuffs of the vents. 



On the farm De Vrede, at a distance of 2J miles south-south-east 

 of the central vent of Saltpetre Kop, there are two necks of the 

 Saltpetre Kop type containing a yellow gritty rock with many frag- 

 ments of quartzite, sandstone, and shale. One block of sandstone in 

 this agglomerate is over 10 feet in length. From one of the vents a 

 dyke runs northwards through the Beaufort beds; it is a yellow, 

 earthy-looking rock containing both sulphate of barium (barytes), 

 and carbonate of lime, and also numerous flakes of black mica. The 

 carbonate of lime forms long lath-shaped crystals, probably pseudo- 

 morphs after another constituent. 



On Portugal's River, 3 miles east of the De Vrede necks, there is 

 a long curved dyke of coarse agglomerate, which was traced over 

 500 yards through the veld ; at one place it is 80 yards wide. It is 

 •composed of a ferruginous agglomerate of sandstone and shale frag- 

 ments ; calcite and gypsum are found in cavities and cracks in this 

 rock. The sandstone fragments are often of large size, blocks 5 feet 

 in length are not uncommon. 



On the farm Blaauw Blommetjes Keep, 2^ miles north-west from 

 Saltpetre Kop, there is a wide area of agglomerate, which may be a 

 neck ; from it a sheet-like extension of fine-grained tuff passes across 

 the nearly horizontal sandstone and shales of the Beaufort series 

 and also through a dolerite sheet, a point of great interest, as it con- 

 firms the evidence given by the dolerite fragments in the Matjes 

 Fontein, Silver Dam, and Saltpetre Kop breccias, that the necks 

 were posterior in date to the dolerite intrusions (see Fig. 4). The 

 tuff is very different in appearance at different spots ; the bulk of it 

 is a dull, rather decomposed-looking rock with numerous flakes of 

 black mica, and it is highly calcareous. Other parts are more 

 compact, and are heavily charged with iron oxides and calcite : 

 numerous angular pieces of sandstone and shale are seen in the 

 ferruginous rock. 



Summary of the Sutherland Pipes. 



The above descriptions of the various pipes in the Sutherland 

 Division shows that they may be roughly divided into three 

 classes : — 



(1) Those filled with purely igneous material, either melilite- 

 basalt or a basic glassy lava. These are confined to the Sutherland 

 Commonage. 



