70 Transactions of the South African Philosophical Society. 



The Saltpetre Kop Group. 



Saltpetre Kop, or Old Sneeuw Kop as it is called on the maps, is 

 a prominent peak about 12 miles south-east of Sutherland. It 

 stands some 5,500 feet above the sea and 1,000 feet above the 

 country at its base. The peak is formed of a hard siliceous and 

 ferruginous agglomerate, part of a large vent which is 1,000 yards 

 long. Other portions of the agglomerate in the vent are soft and do 

 not stand out from the surface. This large vent is the chief one of 

 a group of at least twenty distinct necks and over forty dyke-like 

 outcrops of agglomerate and tuff. The arrangement of the necks 

 and dykes is shown in the accompanying plan (Fig. 2), and the 

 section (Fig. 3) gives an idea of the relation of the necks to the 

 surrounding sedimentary beds. The latter have been bent upwards 

 into a dome, so that they dip away from the central vent on all 

 sides. The angle of dip is greatest near the centre and decreases 

 gradually ; at a distance of from 1^ to 2 miles from the highest peak 

 the dip due to the local up-thrust is no longer determinable. In 

 most volcanic centres a slight dip towards the vent is seen in its 

 immediate neighbourhood, but it only effects the rock within a few 

 yards of the wall of the neck ; the outward dip noticed at Saltpetre 

 Kop and other pipes of the type we are here dealing with seems to 

 be quite abnormal. The Saltpetre Kop dome remains one of the old 

 illustrations of the " craters of elevation," though it must be remem- 

 bered that the strata with the outward dip in the latter are of 

 volcanic origin, and not ordinary sandstones and shales as in the 

 case of Saltpetre Kop. It has long since been recognised that the 

 dip of the volcanic beds round vents is entirely due to their manner 

 of deposition, and that the strata through which the necks were 

 blown are very slightly if at all arched upwards by the explosive 

 agency that gave birth to the volcanoes. The Saltpetre Kop dome 

 is all the more striking in that it is situated well within the great 

 interior region of the Colony, where the rocks lie nearly horizontally 

 It is like a solid blister raised in these horizontal strata. There can 

 be no doubt whatever that the dome is due to the force exerted 

 during the formation of the great vent. 



The other nineteen are of much smaller size than the great central 

 one, and are not arranged quite symmetrically about it. There seem 

 to be no necks south-east or east-south-east of the large one, though 

 it is possible that one or more vents filled with a soft agglomerate or 

 tuff may have escaped notice, or may be buried under some alluvial 

 deposits that lie in that direction. The small vents do not appre- 



