VOLCANIC PIPES 349 



in diameter marked on the surface by a shallow pan. 

 No outcrops of the breccia are visible, but two prospect- 

 ing shafts allow one to obtain good specimens of the 

 rocks. The breccia is softer in one part of the pipe than 

 elsewhere, and consists of a serpentinous matrix con- 

 taining fragments and boulders of quartzite, sandstone, 

 shale, dolerite of the Karroo type, and peculiar rocks 

 with a granulitic structure ; the last-mentioned rocks 

 are composed of three varieties of monoclinic pyroxene, 

 brown hornblende, brown mica, ilmenite, garnet and 

 some felspar or the alteration products of a basic fel- 

 spar. The felspar is only present in some varieties of 

 the granulites, which are evidently related, in the sense 

 of forming a series of increasing basicity. It is worthy 

 of remark that olivine and rhombic pyroxenes are absent 

 from these rocks, though the former, altered to serpentine, 

 is an abundant constituent in the matrix of the breccia. 

 Similar granulitic rocks of variable composition have 

 been found to be abundantly represented in the blue- 

 ground of many of the pipes in Northern Cape Colony. 

 The minerals which occur in these heavy basic rocks are 

 also the most conspicuous fragments in the breccia, and 

 there is no doubt that they were derived from the same 

 source that the boulders came from. The less conspi- 

 cuous constituents of the breccia, only determinable 

 under the microscope, are perofskite, serpentine pseu- 

 domorphs after olivine, grains of quartz and argillaceous 

 matter derived from sedimentary rocks and calcite. The 

 harder variety of breccia contains less serpentine and 

 more sand and clay than the softer, but all the minerals 

 mentioned above occur in both kinds. 



