Vol. 67."] GEOLOGY OF C3BRTA.IN DISTRICTS OF CAPE COLONY. 701 



20. The Geology of the Districts of Worcester, Robertson, and 

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

 Fellow and Lecturer of Christ's College, Cambridge. (Read 

 February 22nd, 1911.) 



[Plate LI— Geological Map.] 



Contents. 



Page 



I. Introduction and Bibliography 701 



II. General Geological Structure 702 



III. Physiography of the Area 703 



IV. The Worcester District 704 



V. The Petrography of the Malmesbury Rocks of the Worcester 



District 708 



(1) The Sedimentary Rocks. 



(2) The Granitic Intrusions. 



(3) The Igneous Rocks of Brewels Kloof. 



VI. The Ottrelite-Rocks of Waai Kloof 712 



VII. The Robertson District 714 



(1) The Malmesbury Rocks. 



(2) The Granitic Intrusion of Wolve Kloof. 



VIII. The Cape and Karroo Rocks of Worcester and Robertson ... 718 



IX. The Enon Conglomerate 719 



X. The Ashton District 721 



XI. General Conclusions ' 725 



I. Introduction and Bibliography. 



During- the Long Vacation of 1910 I was enabled to pay a visit to 

 South Africa, partly for the purpose of making collections for the 

 Sedgwick Museum at Cambridge, and partly for original research in 

 South African geology. Acting chiefly on the advice of Dr. A. W. 

 Rogers, the head of the Geological Survey of Cape Colony, I undertook 

 the investigation of the neighbourhood of the towns of Worcester, 

 Robertson, and Ashton, in the Western Province of Cape Colony. 

 Owing to special conditions, the geology of this district comprises 

 some features of exceptional interest, and the elucidation of the 

 problems there presented would cast valuable light on several 

 obscure features of the geological history of the country as a whole. 

 The most important of these problems refers to the origin and age 

 of the Worcester-Swellendam Fault, one of the greatest dislocations 

 of its kind with which we are acquainted. This forms the prin- 

 cipal subject of the following paper ; but several other questions 

 are dealt with in considerable detail, especially the lithological 

 character and structure of the Malmesbury Series, the oldest rocks 

 of this part of the Colony — a subject on which hitherto little work 

 has been done. Incidental reference is made to other points 

 bearing on the principal objects of the investigation. 



The district is comprised in Sheets 1, 2, and 4 of the geological 

 map, on the scale of about 3f miles to the inch, issued by the 

 Geological Commission of the Cape of Good Hope, and many 



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