46 G Geological Society : — 



February 22nd. -Prof. W. W. Watts, Sc.D., M.Sc, F.R.S., 

 President, in the Chair. 



The following communication was read : — 



' The Geology of the Districts of Worcester, Robertson, and 

 Ashton (Cape Colony).' By R. H. Rastall, M.A., F.G.S. 



After a brief description of the physiography of the district and 

 the general sequence of the rocks composing it, in which the 

 incompleteness of the stratigraphical record is especially noted, 

 a detailed account is given of the structure and characters of the 

 Malmesbury rocks of Worcester and the region near that town. 

 These are shown to include a lower and an upper sedimentary 

 series, predominantly gritty and slaty respectively, and evidently 

 of great thickness, probably over 20,000 feet. The upper division 

 is pierced by granitic dykes, which have been subsequently crushed 

 and foliated, forming ' phyllite gneiss.' Certain bands of limestone 

 are metamorphosed by them to pure white marble. A remarkable 

 isolated mass of igneous rock in Rrewels Kloof appears to be 

 intrusive, but the rock is andesitic in character. A thin though 

 conspicuous band of ottrelite- schist has been found in Waai Kloof 

 and in the Hex River Pass. All these rock-types are described in 

 detail. Similar, but somewhat less detailed treatment is accorded 

 to the Malmesbury rocks of Robertson and Ashton, and a petro- 

 graphical description is given of the large granite intrusion of 

 Wolve Kloof, Robertson. 



The distribution and characters of the rocks of the Cape and 

 Karroo Systems are only dealt with, in so far as they throw light 

 on the principal subject of the paper ; but a fairly full description 

 is given of the occurrences of Enon Conglomerate, which is shown 

 to occupy a series of isolated basins, arranged along an east-and- 

 west line, and to lie with a strong discordance upon all the older 

 rocks. After a careful examination of the ground, it is concluded 

 that the Enon Conglomerate does not overlap the Worcester- 

 Swellendam Fault, as indicated in the official maps ; and that 

 conglomerate does not appear to contain any fragments of the 

 Malmesbury rocks, which cannot therefore have been exposed 

 when it was formed. 



After a careful discussion of all the available evidence, it is con- 

 cluded that the Worcester-Swellendam Fault, which has a maximum 

 throw of probably 10,000 feet, is in great part of post-Cretaceous 

 a^e, although there are indications of earlier movement along the 

 same line of fracture. From a study of the dominant trend-lines 

 of South- Western Cape Colony it is concluded that the district in 

 question is situated at or near the central line of the syntaxis of 

 two great sets of folds at right angles, which have assumed a fan- 

 shaped arrangement in plan, and that the great fault is a line of 

 fracture and subsidence running transversely across these lines of 

 folding. The folding and the faulting are clearly phases of one 

 general series of events, and the faulting probably resulted from a 

 diminution or even reversal of the pressure which had previously 

 given rise to the folding. 



