770 H. A. BROUWER 



evidence of the existence of faults, the downthrow of which is in 

 all cases toward the great central mass. A similar subsidence 

 took place after the period of the eruption of acid lavas and before 

 that of the basaltic lavas. 



The state of preservation of the Pilandsberg complex and sur- 

 rounding area is not very favorable to a study of the amount of 

 subsidence in the sunken area. The lavas, which must have 

 extended far beyond the mountain proper, have entirely disap- 

 peared ; the junction of the intrusive rocks of the Pilandsberg with 

 those of the Bushveld is not well exposed; and the amount of 

 denudation in the area surrounding the complex cannot be esti- 

 mated. This probable subsidence and the large dimensions of 

 the plutonic body lead us to mention another hypothesis to explain 

 the mechanism of intrusion of many batholites, which has been 

 set forth by Daly. 1 He termed this process "overhead stoping"; 

 it consists of a continued breaking free of roof blocks and a sinking 

 down of the detached blocks into the magma, which consequently 

 rises and occupies the place of the sunken fragments. 



The cover of the intrusive rocks of the Pilandsberg entirely 

 consists of lavas, the effusive equivalents of the intrusive rocks, 

 and this is very common in batholitic intrusions from other parts 

 of the world. How these facts are explained by overhead stoping 

 has been elaborately discussed by Ussing 2 in a recent treatise on the 

 geology of the country around Julianehaab. If a batholitic magma 

 on one or more occasions during its intrusion has penetrated its 

 cover, this will presumably lead to a volcanic outburst of cata- 

 strophic character, accompanied by the outpouring of lava flows 

 and followed by a period of quiescence. After a time, when hot 

 magma from below is brought into contact with the newly formed 

 roof, the stoping process will continue, interrupted by few volcanic 

 outbursts until the magma has cooled to its point of solidification. 



In several batholites with a permanent cover of sedimentary 

 rocks the stoping process came to an end and the magma was 



1 R. A. Daly, "Geology of the Ascutney Mountain," Un. St. Geol. Surv. Bull. 

 No. 20Q (1903), p. 93; "The Mechanics of Igneous Intrusion," Amer. Jour, of 

 Science, 4th series, XV (1903), and XXVI (1908). 



2 N. V. Ussing, "Geology of the Country around Julianehaab, Greenland," Medd. 

 om Gronland, XXXVIII (191 1), p. 302. 



