368 THE ZUURBERG VOLCANIC FISSURE 



The tuffs are fine-grained rocks of very various colours, 

 white, brown, green, yellow and red. The white, 

 brown and yellow varieties are chiefly composed of 

 quartz grains in a very fine ground mass ; a green 

 chlorite, probably derived from another mineral by 

 alteration, gives the colour to the green tuffs, which are 

 otherwise like those just described. The red tuffs con- 

 tain much comminuted lava of a kind similar to the 

 more glassy varieties seen in this volcanic belt. These 

 lava fragments are very small, too small to be distin- 

 guished without the aid of a lens. 



The lava frequently encloses pieces of tuff, and small 

 vein-like dykes of lava penetrate the tuff, which is dis- 

 tinctly hardened near the contact. 



In the course of the known part of the volcanic belt 

 there are three rather marked enlargements occupied 

 by masses of breccia and tuff, which probably mark 

 centres of explosive eruption. 



From an inspection of a map of this volcanic zone it 

 might be thought that the rocks composing it rest un- 

 conformably upon the Dwyka and Witteberg series of 

 the Zuurberg Kange and are either unconformably over- 

 lain by the Uitenhage beds or are separated from them 

 by a fault. If the Uitenhage beds lay unconformably 

 on the volcanic rocks one would expect to find fragments 

 of the latter in the Enon conglomerate, but they have 

 not been found. The most significant fact, however, is 

 that the Witteberg quartzites are much shattered where 

 they are seen immediately on the northern or upthrow 

 side of the fault in contact with the Enon conglomerate 

 east of the end of the volcanic belt and also where they 



