68 Transactions of the South African Philosophical Society. 



The Silver Dam pipe lies a little more than a mile to the north- 

 east of the vent just described. A shallow pan marks the position 

 of the pipe, but it is not conterminous with its boundaries. The 

 agglomerate of the pipe does not crop out at the surface, and it was 

 only from the material piled round the top of two deep shafts made 

 by a prospector that we could obtain specimens of the rock. As the 

 materials from the two pits are rather different they will be described 

 separately. The boundaries of the vent are ill-defined, but it is about 

 180 feet in diameter. 



The rock from the south-western shaft, the one sunk within the 

 shallow pan, is a soft blue-grey material crowded with fragments of 

 rocks and minerals. Amongst the rocks are shales and sandstones 

 evidently derived from the Karroo formation, quartzites, dolerite of 

 the Karroo type, granulite, and peculiar rocks allied to the eclogites ; 

 with these there are numerous fragments of minerals that are the 

 most conspicuous constituents, black mica, hornblende, augite, and 

 ilmenite. Under the microscope the matrix is seen to consist largely 

 of calcite in which there are crystals of perofskite, iron ores, fibrous 

 serpentine, and dusty matter that cannot be determined ; in this 

 matrix lie fragments of the minerals mentioned above. The mono- 

 clinic pyroxene is of four varieties : (1) Almost colourless with very 

 well-developed prismatic cleavage ; it occurs occasionally in well- 

 formed crystals and is identical in character with the augite in one 

 of the granulitic rocks present in the form of fragments in the pipe. 

 (2) Colourless with many diallagic inclusions ; the prismatic cleavage 

 is less well developed than in the first variety. It encloses felspar 

 laths ophitically, and is evidently derived from olivine dolerite of the 

 Karroo type. (3) A very pale green pyroxene with very slight 

 pleochroism derived from one of the eclogites. (4) A bright green 

 distinctly pleochroic mineral, a chrome diopside, derived from one of 

 the granulitic rocks ; it has the prism cleavages very well developed. 

 The hornblende is in large fragments ; it has a brown colour and 

 moderately strong pleochroism. It also occurs in conjunction with 

 biotite and ilmenite, and is certainly derived from one of the eclogite- 

 like rocks. Mica is present in large and small flakes ; a biotite with 

 rather weak pleochroism is abundant, but another variety with 

 strong pleochroism is also there. All these minerals are rather 

 altered on the outside ; the coloured ones are bleached, and contain 

 dusty inclusions. In addition to these minerals there are also 

 apatite, garnet, and a plagioclase felspar considerably altered. The 

 sandstone and shale fragments are altered to a depth of about 

 -i- inch ; they are lighter in colour in that zone than within it. 

 There are three varieties of heavy basic rocks found as fragments 



