E. T. Mellor — Glacial Conglomerate of South Africa. 107 



Art. XI Y. — The Glacial (Dwyka) Conglomerate of South 

 Africa ; by Edward T. Mellor. 



[Communicated by permission of the Director of the Geological Survey of 



the Transvaal.] 



Introductory. — Few rocks have aroused so widespread and 

 so sustained an interest as the Glacial Conglomerate occurring 

 at the base of the Karroo System of South Africa, generally 

 known as the Dwyka Conglomerate. From the time when 

 attention was first directed to it by Bain in 1856, down to the 

 present, the Dwyka Conglomerate has continued to be a source 

 of almost continual discussion. 



In the first instance, this interest was in a great measure due 

 to the very different views held by various geologists as to the 

 nature of the conglomerate, and especially to the opposition 

 offered by many to the theory of its glacial origin — a question 

 which one may venture to regard as finally settled by the 

 accumulation of evidence in recent years. This establishment 

 of the glacial character of the deposits included under the 

 term Dwyka Conglomerate, which occur over thousands of 

 square miles in South Africa, and which correspond closely 

 with similar formations of corresponding age in India, Aus- 

 tralia, and South America, lends a newer and perhaps more 

 widely spread interest to the study of this series, and of the 

 conditions under which it was formed. To the South African 

 geologist the rock derives additional interest from the fact that 

 it affords the only geological horizon common to the various 

 colonies yet established with any degree of certainty. 



Nomenclature. — The term " Glacial Conglomerate " was used 

 by E. J. Dunn on his map published in 1873* for the northern 

 outcrops of the conglomerate, while he still retained for the 

 more southerly occurrences an old name, " Trap Conglomerate," 

 used by Wyley. In the second edition of his map,f two years 

 later, while retaining the term Glacial Conglomerate for the 

 northern outcrops, Dunn applied the term " Dwyka Conglom- 

 erate" to those of the southern parts of Cape Colony and 

 Natal. The term Dwyka is derived from a river of that name 

 in Cape Colony in the neighborhood of which the conglom- 

 erate is typically developed. The name is now frequently 

 applied to the glacial conglomerate at the base of the Karroo 

 System generally throughout South Africa. It might perhaps 

 be more appropriately restricted to the southern type, which, 

 as will be pointed out, differs in some important respects from 

 the more northerly occurrences, especially as the intermediate 



*E. J. Dunn, Geological Sketch Map of Cape Colony, London, 1873. 

 f E. J. Dunn, Geological Sketch Map of South Africa, London, 1875. 



