76 Transactions of the South African Philosophical Society. 



(3) Necks of agglomerate or tuff, including all the Saltpetre Kop 

 vents and that of Silver Dam. 



While there are very great differences in the materials filling these 

 vents they have one feature in common ; they differ very greatly 

 from the contents of the volcanic pipes of Stormberg age in the east 

 of the Colony. The Stormberg volcanoes are occasionally penetrated 

 by dolerite dykes, and if this dolerite be regarded as belonging to 

 the same great system of intrusions of similar nature stretching 

 from Calvinia to Natal, then the fact that the Sutherland volcanoes 

 are of later age than the dolerite of that district proves them to be 

 younger than the Stormberg volcanoes. There is no direct evidence 

 of their relative ages ; so far as we know at present the vents of 

 Stormberg age do not occur west of the Molteno Division, and the 

 volcanoes of the type we are now dealing with have not been found 

 so far to the east. 



Comparison with the Stormberg Volcanic Eocks. 



The contents of the Stormberg vents and the lavas and tuff that 

 accompany them are of a different nature from those connected with 

 the Sutherland pipes. In the first place the Stormberg lavas are 

 thoroughly doleritic or basaltic in composition ; they belong to the 

 great group of basic igneous rocks, of which labradorite or an allied 

 species of felspar and augite are very important constituents.* 

 Fragments of these basaltic lavas are abundant in the Stormberg 

 vents, and they differ from the blocks of dolerite in the Sutherland 

 vents in being glassy and often vesicular instead of holocrystalline 

 dolerites ; they are also much more abundant than the dolerite frag- 

 ments occasionally found in the western agglomerates. The only 

 igneous rocks in the Sutherland vents, other than those forming 

 boulders, are melilite-basaHs or very basic glasses, rocks which are 

 much more basic than the Stormberg lavas. The felspathic rock 

 forming one of the Saltpetre Kop dykes is an exception to the 

 general rule that felspar is absent or very rare in the western rocks, 

 but this dyke has at least no obvious resemblance to the Stormberg 

 lavas or the dolerites. 



Many of the Stormberg vents are filled with agglomerate 

 or tuff largely composed of debris of sedimentary rocks or of 

 deep-seated igneous ones such as granite, but hitherto no trace 

 of the peculiar mica, diopside, hornblende, ilmenite, garnet, and 

 perofskite, which are such conspicuous constituents of the Suther- 



* See Cohen, " Neues Jahrbuch fur Min. Geol. and Pal.," 1875, 113-127 

 Schwarz, Geol. Comm. (1903), Report on Matatiele. 



