THE KARROO SYSTEM 227 



cavities which are filled in with one or more of the 

 above-mentioned minerals. 



The amygdales may be more or less spherical or ir- 

 regular in shape. In certain varieties, known as pipe- 

 amygdaloid, there are pipe-like amygdales from four to 

 ten inches long, usually branching downwards. They 

 are found in layers at the base of certain of the flows, 

 separated from the underlying rock by an inch or two 

 of lava in which the steam-holes are of the usual type. 

 The pipes are more or less perpendicular to the base of 

 the flow, but sometimes they are considerably inclined. 



It is probable that these tubular structures have been 

 produced by the flowing of the lava when in a viscid 

 condition over a moist surface of lava, ash or sandstone. 1 

 Another explanation is that the whole of the vapour 

 which formed the pipes was in the lava at the time of 

 its extrusion, and that it only had time to expand after 

 the lava poured over the surface, or that the bulk of it 

 was only released on the temperature of the rock falling 

 to the certain critical point. 2 On this view it is difficult 

 to understand the localisation of the rows of pipes. 



The Volcanic Necks. 



From areas that have been already mapped, about 100 

 vents filled with lava, agglomerate, or tuff have been de- 

 scribed, and there can be no doubt that many more 

 await discovery. Three of these were observed by Mr. 

 Dunn many years ago ; at least fifteen occur in Wode- 

 house, twenty in Barkly East, seventeen in Elliot, 

 twenty-two in Herschel, nineteen in Matatiele, and 



1 G. M., 1907, p. 13. 2 G. C., vii., p. 57. 



