230 THE VOLCANIC NECKS 



resembles the Cave sandstone, not only from a distance 

 but even in the hand specimen. Such necks are well 

 represented in Wodehouse and Herschel, and are all 

 the more remarkable considering the extremely poor 

 development of siliceous tuffs interbedded with the 

 lavas, a state of things which would hardly obtain were 

 the Drakensberg vents strictly comparable with modern 

 lava and ash volcanoes, or those of Devonian and Car- 

 boniferous age in the British Isles. In the east of 

 Herschel there is a peculiar dyke several miles in length 

 formed of this siliceous tuff cutting through a plateau of 

 Red beds and Cave sandstone. 



The bigger pipes are filled partly by agglomerate and 

 partly by lava, and in a few cases there is a connection 

 still existing between the igneous rock in the vent and 

 the lava-flows which have obviously issued from the vol- 

 cano. Generally, however, denudation has proceeded 

 so far that the original connections have long since been 

 destroyed. 



The Belmore volcano in Barkly East is about three- 

 quarters of a mile in diameter and is entirely surrounded 

 by Cave sandstone, which dips inwards towards the 

 centre of the pipe. The agglomerate is cut by sills 

 of dolerite and glassy enstatite-andesite, while the cen- 

 tral plug is formed by a great mass of the latter rock 

 with columnar structure. On the north and east there 

 are alternations of tuffs and basaltic and andesitic lavas 

 dipping at high angles, together with masses of coarse 

 agglomerate. The more acid lavas cut the basaltic 

 varieties and are later in age. The lavas forming the 

 hills round about are split up by intercalations of sand- 



