The Volcanic Fissure under Zmirberg. 197 



rock than the solid quartzite owing to the want of a uniform 

 cementation. 



The total known length of the volcanic band is 19 miles, but it is 

 probably continued further west than Enon. 



The general habit of the rocks in this band proves that they 

 occupy a nearly or quite vertical fissure, the only exception being 

 the southerly inclined layers of different texture in the Mimosa 

 valley, but this is a small outcrop in the middle of a band of 800 

 yards width. I searched every exposure of the Enon conglomerate 

 for fragments of the lava and tuffs, but could find none, and the 

 general form of the whole band is not consistent with the view that 

 the volcanic rocks are contemporaneous with the early stages of the 

 Uitenhage period ; moreover, no volcanic intercalations have ever 

 been found in those strata. It is also evident that the volcanic 

 rocks in their present form have not been subjected to the forces 

 that bent and twisted the Witteberg and Dwyka series to the north, 

 for they show none of the incipient cleavage that characterises the 

 latter rocks in the Zuurberg range. There remains no other view 

 than that assumed early in this paper, that the volcanic rocks 

 reached their present position by eruption along the line of fault 

 between the Uitenhage beds and the Zuurberg. 



The question as to whether the eruption accompanied the faulting, 

 or followed it, is not an easy one to settle, but from the presence of 

 rather broad bands of shattered rock both on the boundary of the 

 volcanic fissure on Duncairn, and their presence further east where 

 the fault alone is observable without the volcanic rocks, I am inclined 

 to believe that the eruption accompanied the faulting. The similarity 

 of the breccia seen near the Waggwa road to those of Uniondale and 

 Willowmore, and the occurrence of the quartzite breccia at Duncairn 

 in connection with the volcanic rock, lead to the suspicion that the 

 brecciation of the rocks along the faults is due to some sort of explo- 

 sive action, and that the breccias differ in this respect from the usual 

 type of broken rock which is developed on a comparatively small scale 

 along many faults. 



Hitherto volcanic rocks have not been found to accompany faults 

 in this Colony. The only possible exception yet described is the 

 pipe of melilite-basalt on Spiegel Eiver, which is situated very near 

 the northern boundary of the Heidelberg outlier, and this boundary 

 is probably a fault. The distribution of the scattered pipes of the 

 Kimberley and Saltpetre Kop type has not been connected with any 

 structural lines of the country, nor have the Stormberg volcania 

 necks any definite arrangement, 1 '' though their south-eastern limit in 

 * Du Toit, " Ann. Rep. Geol. Com. for 1904," map. 



