66 Transactions of the South African Philosophical Society. 



great holes being blown through the strata by the explosive action 

 of highly-heated gases. In some cases no lava issued from the 

 vent, and the fragments of the strata fell back into the funnel and 

 blocked it up, so that what we now find exposed is a mass of 

 agglomerate built up almost entirely of masses of sandstone and 

 shale, set in a groundmass of finely pulverised gritty material. 



In other cases the explosions were followed by ejections of 

 volcanic ash, bombs, and molten material, which spread for miles 

 around each vent. These explosive eruptions produced the bed of 

 ash, often of considerable thickness, which is met with over such a 

 large area in Barkly East, and which usually rests directly upon the 

 Cave sandstone. 



Lavas poured forth from the volcanoes, and many of the smaller 

 craters were overwhelmed by the material and choked after a brief 

 existence. The more active volcanoes continued their outpourings, 

 though beds of ashy material show that there were occasional lulls 

 during which finely divided material only was being thrown out. 



As we get higher up in the midst of the lavas we find that these 

 intercalations of sandstone and ash become less frequent, and 

 finally disappear, and the only outbursts were of molten rock. 

 Probably, too, the supply of sediment was cut off from this area 

 either by the drawing off of the waters of the lake, by the barrier 

 formed through the accumulation of the lavas, or by the sinking of 

 the old land surface to the south. One after another the volcanoes 

 exhausted their energy, and were buried beneath the ejections of 

 their more active neighbours until even the eruptions of these in 

 turn became feeble, and the volcanoes finally became extinct. The 

 amount of volcanic material must have been enormous ; in many 

 places there are over 3,000 feet of rock exposed. At Ongeluk's 

 Nek, in Griqualand East, Mr. Schwarz * has noted a thickness 

 of 4,000 feet, while, according to Mr. Churchill,t this amount is 

 much exceeded in Natal. 



The lofty cliffs of lavas which occur right on the present escarp- 

 ment, together with the regularity of the lava flows, indicate that 

 the volcanic rocks must have originally extended far to the south- 

 east beyond the present known south-eastern limit of the volcanic 

 necks. 



We have no idea of the interval of time during which the eruptions 

 continued, but it may have been of long duration, perhaps extending 

 well into Jurassic times. 



* Ann. Kept. Geol. Commission for 1902, p. 45. 



f " Notes on the Geology of the Drakensberg, Natal." Trans. South African 

 Philosophical Society, vol. x., part 3, 1898. 



