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



Description of the Dwyha Conglomerate in the Southern 

 Outcrops. — The earlier studies and descriptions of the Dwyka 

 conglomerate were confined to its occurrence in the southern 

 portions of Cape Colony and in Natal. In the southern 

 examples especially the conglomerate has certain characteristics 

 which led to much controversy as to its origin. Its appear- 

 ance in the Dwyka locality was thus described by Mr. E. J. 

 Dunn : * " The conglomerate consists of a bluish grey base so 

 fine that its constituents are not resolvable, except under high 

 magnifying power, and then no crystals are disclosed; it 

 appears to be a very fine indurated mud ; in this base are 

 enclosed bowlders, pebbles, angular fragments, and grains of a 

 great variety of rocks, such as granite, granulite, gneiss, mica, 

 and other schists, quartz rock, hard sandstone, jasper, hornfels, 

 quartz, small pieces of felspar, etc." 



The included fragments, which range in size from mere 

 grains to bowlders several feet in diameter, are distributed in 

 the matrix without definite arrangement. The rock as a whole 

 is very hard and fractures pass indifferently through matrix 

 and bowlders alike. By weathering it frequently produces a 

 yellowish clay, through which the hard rock fragments and 

 bowlders of the original conglomerate are scattered. Besides 

 the conglomerate beds, other shaly beds occur devoid of 

 included fragments. Individual beds persist over long dis- 

 tances, maintaining at the same time their distinctive litho- 

 logical characters. The conglomerate beds vary from a few 

 inches to hundreds of feet in thickness. In the southern parts 

 of Cape Colony the conglomerate often shows a schistose 

 structure resulting from the earth movements which have 

 affected that area — to the effects of which is probably also due 

 in part the extreme hardness of the southern rock as compared 

 with its northern representative. 



Various^ theories concerning the Origin of the Dwyka Con- 

 glomerate. — The dark green color of the conglomerate, its rich- 

 ness in minerals not usually abundant in rocks of sedimentary 

 origin, including much chloritic material, its extreme hardness, 

 its crystalline appearance and the frequent absence of bedding 

 through great thicknesses of rock, disposed almost every 

 observer, including many geologists of wide experience, to 

 attribute to the conglomerate an igneous origin. Expressive 

 of these views are the following names applied to the rock at 

 various times by different workers : " Claystone-porphyry," 

 " Trap-conglomerate," u Melaphyre-breccia," " Volcanic-brec- 

 cia," " Trap-breccia." Many and various were the theories 

 advanced at different times and by different observers to 



* E. J. Dunn, Report on the Camdeboo and Nieuwveldt Coal, p. 7, Cape 

 Town, 1879. 



