The Sutherland Volcanic Pipes. 83 



Dwyka conglomerate cannot have supplied the boulders. In the 

 first place that conglomerate is thinner in the north of the Colony 

 than elsewhere — at Kimberley, for instance, it is only a few feet in 

 thickness ; secondly, the conglomerate and its contents are fairly 

 well known in Prieska and between Loeries Fontein and Willowmore, 

 but eclogite is not known to occur in it, and it is unlikely that rocks 

 of this nature should be so abundant at certain localities where vents 

 happen to have been established ; thirdly, rounded boulders of the 

 ordinary coarse-grained ophitic olivine-dolerite of the Karroo type 

 are abundant in the Balmoral pipe ; these dolerites were evidently 

 derived from the intrusive sheets and dykes that traverse the Beaufort 

 beds of the Nieuweveld, and are not known to occur in the form of 

 boulders in the Dwyka conglomerate, which is a much older rock 

 than the intrusions. In the case of certain boulders noticed at 

 Saltpetre Kop, the quartzites and granites, it is quite possible that 

 their origin was similar to that advocated by Professor Bonney for 

 the eclogite boulders, for quartzites and granites are extremely 

 abundant in the Dwyka conglomerate throughout its extent, so far 

 as it has been surveyed in Cape Colony ; and in the Tanqua Karroo, 

 due west of Saltpetre Kop, the conglomerate is probably some 800 

 feet thick, while directly south of the Kop, along the south of the 

 Karroo, its thickness is still greater. The Dwyka conglomerate 

 almost certainly underlies the Sutherland area, at a depth of 

 perhaps 3,000 feet. The quartzite, however, is distinctly like 

 that in the Table Mountain series, so the boulders may have 

 been derived directly from that series which perhaps underlies 

 the Saltpetre Kop area at a depth of at least 5,000 feet. 



The great area of crystalline schists, gneiss, granite, and granulites 

 of the north and north-west of the Colony is as yet very imperfectly 

 known, but so far as the rocks have been examined they do not 

 include granulitic masses of nearly such basic composition as the 

 eclogites of the blue-ground ; it is unlikely that the latter formed 

 part of the old crystalline schists now exposed at the surface, for 

 had they done so some out-crops would probably have been found 

 of this remarkable type of rock, evidently of wide distribution deep 

 under the surface. 



