434 ME. W. GIBSON ON THE GEOLOGY OF THE GOLD-BEAKTNG 



4. The Igneous Roclcs of tlie Southern Transvaal. 



The description of the igneous rocks of the Transvaal generally is 

 outside the object of the present paper. Those intimately connected 

 with the Main Eeef Series have already been discussed. The 

 amount of volcanic activity displayed is enormous • the whole of the 

 stratified country seems to be jQloating in igneous rocks, the chief 

 varieties of which are dolerites, diorites, quartz-amygdaloid lavas, 

 and basalts. 



In Dunn's map of South Africa (2nd ed. 1887) these igneous 

 rocks are represented as one vast sheet. Mr. Penning, in Quart. 

 Journ. Geol. Soc. vol. xlvii. (1891) p. 451, represents the dolerite 

 as contemporaneous and interstratified with the Witwatersrandt 

 beds. Now the types of igneous rock in the Transvaal are similar to 

 those that pierce the Karoo Beds of the Orange Free State. Moreover, 

 the oldest sedimentary beds of the Transvaal are greatly altered, 

 while the igneous rocks, where not weathered, are as fresh-looking 

 as if they had cooled but yesterday. The volcanic rocks of the Gats 

 liandt are not, in the writer's opinion, interstratified, but appear to 

 have come up an east-and-west fissure, and overflowed the Wit- 

 watersrandt beds at the time when the main movement and faulting 

 of the country took place. 



So far as the writer's observations enable him to judge, the 

 igneous rocks of the Southern Transvaal are much posterior in date 

 to the formation of the Witwatersrandt beds. Eut whether the 

 eruption of igneous material took place before or after the depo- 

 sition of the Karoo Beds, or was contemporaneous with the laying 

 down of the Upper Karoo Series, is uncertain. If similarity of 

 type is of value in correlation, then, as previously mentioned, these 

 igneous rocks are very probably of the same age as those of the Orange 

 Free State, which certainly appear to be of Upper Karoo (Stormberg) 

 age. 



5, Summary of Conclusions respecting the delation of the Strata of 

 the Neighbouring Areas to those of the Witwatersrandt Series. 



(a) Our examination of the beds in the districts lying more or less 

 immediately outside the Witwatersrandt area thus shows them to be 

 thrown into numerous folds, which vary greatly in intensity. 

 Sometimes the beds form shallow basins (as in the Nigel Mine 

 district), but more often the strata are bent into sharp anticlines 

 and synclines, the' axes of which run either north and south or 

 east and west. On the whole the beds are of a similar lithological 

 character throughout the Southern Transvaal, being composed of 

 sandstones, shales, and thin conglomerates, variously arranged, and 

 more or less locally altered into quartzites and schists. 



(h) The direct local superposition of the conglomerates on shales 

 shows that the beds were either laid down by currents varying 

 greatly in strength, or that they were deposited in areas undergoing 

 rapid changes of level. 



