VOLCANIC PIPES 345 



diamonds was carried on in the alluvial deposits or 

 "River diggings " on the Orange and Yaal Rivers; the 

 later or "Dry diggings" in the volcanic pipes, which 

 have been the source of so great an industry in South 

 Africa, followed upon the discoveries mentioned above. 



Since then scores of these pipes and many fissures 

 have been discovered, and their number is continually 

 being added to. Their wide distribution throughout 

 South Africa is proved by their discovery in East 

 Griqualand, the Central Transvaal, Southern Rhodesia, 

 Gordonia, German South- West Africa, Van Rhyn's Dorp 

 and Eiversdale. They appear, however, to be most 

 numerous in the Orange River Colony and the adjoining 

 portion of Cape Colony. 



The neck on the farm Spiegel River in Riversdale is 

 of very great interest, as it affords more evidence of the 

 later origin of this class of vents than is obtainable 

 farther north, and it is at one end of the group in a 

 petrological sense, as it is filled with the well-known 

 but scarce igneous rock melilite-basalt (alnoite). 



The Saltpetre Kop (Sutherland) vents stand at the 

 other end of the petrological series in being almost en- 

 tirely filled with fragments of sedimentary rocks. 



There are many intermediate conditions between the 

 two extreme types and nearly every gradation has so 

 far been recognised. 



We shall commence the description of the pipes with 

 an account of those filled with rocks of the purely 

 igneous type, and proceed in the order of their departure 

 from this type without regard to their geographical 

 positions. At the end of the description the reasons 



