Dr. F. H. Hatch — Geological History of South Africa. 163 



The period during which the muds and sands now forming the 

 Pretoria Series were deposited was probably one of slow oscillation 

 between rising and sinking, the conditions representing on the 

 whole a shallow-water phase. The shore-line of a bay-like portion 

 of the sea in which the sediments of the Potchefstroom formation 

 were deposited is well marked in the Transvaal by the outcrop 

 of the Black Eeef Quartzite, as may be seen by consulting a 

 geological map of the Transvaal, but some allowance must of course 

 be made for subsequent denudation, especially in the Lydenburg 

 district. The main sea, however, extended over the southern 

 portion of Bechuanaland and Griqualand, between the Vaal and 

 Orange Eivers. 



The final emergence of the rocks of the Potchefstroom System 

 produced, by denudation, another great break in the succession, 

 namely, the unconformity (No. IV) which separates the Waterberg 

 from the Potchefstroom System. During the interval represented 

 by this break, the beds of the latter were tilted, flexured, and 

 dislocated. The resultant land surface furnished the material for 

 the building up of the Waterberg formation. The basal con- 

 glomerates and breccias of the latter were first formed, namely, 

 during the subsiding period. From the fact, to which attention 

 has been drawn by Mr. Holmes, that pebbles of red felsite are 

 found in these basal conglomerates,^ it is probable that an eruption 

 of acid lavas began while the Potchefstroom strata were being 

 upraiseji ; this igneous activity was probably long continued, for 

 quite recently Mr. Mellor - has described in the lowest portion of 

 the Waterberg Series at Ehenosterkop the occurrence of fragmental 

 beds consisting for the most part of igneous material, from which 

 he draws the conclusion that this period was marked by vigorous 

 contemporaneous volcanic action, and he makes an interesting 

 comparison with the conditions that obtained at the commencement 

 of the Old Eed Sandstone period in Scotland. 



The character of the Waterberg rocks, consisting as they do 

 largely of conglomerates, grits, and coarse sandstones, indicates 

 shallow- water conditions in a slowly subsiding area. The constant 

 occurrence of false bedding points to rapid sedimentation in 

 shallow waters affected by strong and variable currents.^ The 

 pebbles of the basal conglomerate consist largely of quartzite 

 derived from the Pretoria Beds, and there is evidence that the 

 Waterberg sediments were deposited against an old land surface 

 of the Pretoria rocks, which underwent denudation to supply the 

 necessary material. The present distribution of the Pretoria Beds 

 along the southern margin of the Waterberg Sandstone probably 

 marks the shore-line of the sea or lake in which the latter was laid 

 down. This sea extended over the greater part of the Northern 



1 G. G. Holmes, " Some Notes on the Geology of the Northern Transvaal " : 

 Trans. Geol. Soc. S. Afr., vol. vii (190-1), pp. no-56. "The Geology of a part of 

 the Eustenburg District" : Trans. Geol. Soc. S. Afr., vol. viii (1905), p. 6. 



^ E. T. Mellor, "Volcanic Action in the "Waterberg Formation" : Trans. Geol. 

 Soc. S. Afr., vol. viii (1905), p. 38. 



3 E. T. MeUor : Transvaal Geol. Surv. Eep., 1903, p. 17. 



