VOLCANIC PIPES 367 



belt is not more than 100 yards wide where the 

 road from Coerney to the Zuurberg crosses it, but it 

 swells to nearly a mile on the east, where tuffs and 

 breccias play an important part in its constitution, and 

 it also widens out to the west of the Zuurberg Road. 

 The belt terminates bluntly towards the east on the 

 farm Duncairn, but its western end is not yet known. 



The most important rock in it is a lava of a basaltic 

 type, red in colour, and made up of a once glassy base 

 in which lie crystals of labradorite, augite and olivine, 

 but the last-named mineral is present in a quite small 

 proportion. The glassy base is usually opaque in thin 

 sections under the microscope owing to the amount of 

 hydrated oxide of iron in it. Much of the rock was 

 vesicular, but the steam-holes have been filled with 

 various minerals, of which calcite, heulandite and chal- 

 cedony are the most striking. This basalt is not quite 

 like the known volcanic rocks in Cape Colony, but it re- 

 sembles some of the Stormberg lavas more closely than 

 any other. 



The breccias contain fragments of quartzite which 

 may have been derived from the Witteberg beds ; these 

 fragments are set in a matrix of sharp-edged quartz 

 grains and minutely divided indeterminate matter ; fel- 

 spars of kinds that occur in granites are also present, 

 but they are a much less important constituent than 

 quartz. The breccia from several spots is a buff- 

 coloured rock, very similar to the matrix just described, 

 but with fewer grains of quartz and somewhat darker 

 in colour. Fragments of the lava do not occur in these 

 breccias. 



