﻿Vol. 6 1.] CONGLOMERATE IN THE TRANSVAAL. 683 



averages not more than 400 or 500 feet, and it is not possible at 

 present to recognize in these portions of the Transvaal the many 

 divisions which the Karroo System presents in its fuller development 

 in Cape Colony and Natal. 



So far as our present knowledge goes, the Karroo rocks in 

 the Central and Northern Transvaal may be divided into an Upper 

 Division, consisting of sedimentary rocks including conglomerates, 

 grits, sandstones, and shales, associated with coal-seams, referred 

 to by Dr. Molengraaff as the 'High-Veld Series'; and a Lower 

 Division, consisting of glacial deposits including the Glacial Con- 

 glomerate (Dwyka) and the associated shales and sandstones. The 

 general succession of the beds and their relationship to the underlying- 

 formations in most localities is usually similar to that shown in the 

 section (fig. 2, p. 682). In the district here referred to, the Glacial 

 Conglomerate varies considerably in thickness, averaging about 

 50 feet. In places, where it has been deposited in depressions in 

 the old land-surfaces, it may attain a thickness of 200 feet or more ; 

 while, on the other hand, it may be locally absent altogether, the 

 Upper Karroo rocks then resting directly upon the older formations. 



The following description of the Conglomerate is based upon that 

 previously given by me in the Annual Report of the Geological Survey 

 of the Transvaal for 1903. 



When exposed at the surface, the Glacial Conglomerate is light- 

 yellow in colour, while specimens from shafts and boreholes are 

 frequently greenish or grey. Owing to the rapid disintegration of 

 the rock at the surface into abundant sandy material, outcrops are 

 rarely met with. When they occur, they have very generally the 

 huramocky appearance shown in fig. 3 (p. 684), which represents a 

 very characteristic example. 



The matrix of the conglomerate is a sandy-looking material, 

 consisting for the most part of sharply-angular fragments and grains 

 of quartz, and of various rocks similar to those of which the 

 boulders in the conglomerate are composed, including quartzites, 

 hard shales, felsites, and granophyres occurring in the district. 

 The fragments range in size from minute grains to pieces several 

 inches in diameter. The proportion of quartzose material is usually 

 great, and the rock weathers to sandy debris ; but in some cases 

 felspathic material is abundant, and the weathering of the matrix 

 produces a whitish sandy clay. 



Scattered through the matrix occur abundant boulders and 

 subangular rock-fragments. These show no definite arrangement 

 or orientation, large and small fragments lying confusedly together. 

 The boulders and fragments measure generally from 1 to 3 feet 

 in diameter, but may attain as much as 8 or 10 feet ; they are 

 usually facetted, and especially when composed of material not 

 excessively hard and somewhat fine in grain, such as hard shales and 

 weathered felsitic rocks, frequently show striations. The smaller 

 pebbles are often traversed by a network of cracks, dividing them 

 into a number of pieces which have been re-cemented into a whole. 



Q. J. G. S. No. 244. 3 c 



