ALKALI ROCKS IN THE TRANSVAAL 769 



Tuffs and volcanic breccias are found all over the areas where 

 the effusive rocks are developed. The main rock types of the 

 Pilandsberg are disposed in concentric circles, of which the outer- 

 most consists of syenites and nepheline syenites and is followed 

 toward the center by a ring of effusive rocks. The latter dip, with 

 a few local exceptions, from the center outward, and the highest 

 hills formed by intrusive rocks in the central area of the complex 

 still carry a capping of volcanic rocks which have resisted denuda- 

 tion. 



If the Pilandsberg is considered as the remnant of what was 

 once a volcano and its subsidiary peripheral vents, this must have 

 been of stupendous dimensions, since the intrusive rocks cover a 

 surface whose diameter varies from 15 to 18 miles, the lavas having 

 extended far beyond the periphery of the intrusives. 



It is peculiar that in the territory of the Pilandsberg effusive 

 rocks are found in large quantity between the granular rocks, 

 whereas they do not occur in the surrounding granites and norites. 

 The lavas, which must have extended far beyond its periphery, 

 have entirely disappeared and do not even cap the hills of Magalies- 

 berg quartzite, though at some places the quartzite is found in the 

 immediate neighborhood of the Pilandsberg. It is very likely 

 that in connection with the intrusion of the alkali rocks the roof 

 has locally sunk down, and, while it has disappeared everywhere 

 else in the neighborhood by erosion, we see the remains preserved 

 just on those spots where the roof has given way. 



Subsidences in ancient volcanic regions are by no means rare. 

 Judd, 1 for instance, mentions the comparatively perfect state of 

 preservation exhibited by the great volcano of Mull, if compared 

 with that of the other great Tertiary volcanoes in the Hebrides. 

 It can be shown that this difference is due to a central subsidence 

 which took place in the Mull volcano. From the sections along the 

 shores of the deep fiords it is evident that the basaltic lava sheets 

 dip toward the central mass of eruptive rocks, the inclination 

 increasing as we approach the volcano. Further, there is clear 



1 J- W. Judd, "On the Ancient Volcanoes of the Highlands and the Relations of 

 Their Products to the Mesozoic Strata," Quart. Journal of the Geol. Soc, XXX (1874), 

 256. 



