404 W. M. DAVIS OBSERVATIONS IN SOUTH AFRICA 



The lowest or basal member consists of shales, distinctly stratified, and 

 contains small and scattered fragments, up to a foot or so in diameter, 

 of various rocks. The fragments are rarely more than 3 or 4 inches in 

 diameter near the base of this member; they are larger, tip to 10 or 15 

 inches in diameter, and more frequently striated, 50 or more feet above 

 the base. It is only by the presence of these fragments that the basal 

 member is to be distinguished from the underlying Witteberg shales, for 

 the passage from one to the other is not marked by an unconformity or 

 distinct change of composition, except for a discontinuous layer of quartz 

 rock that sometimes separates the two. As none of the formations in the 

 Karroo series contain marine fossils and as freshwater or land fossils 

 occur at several horizons, it was inferred that the basal member of the 

 Dwyka was deposited either in a sheet of water or by a river marginal to 

 an ice-sheet; preference was given to the former, as far as our observa- 

 tions went, because of the presence of the scattered scratched stones, 

 which are thought not to characterize river deposits in general, and be- 

 cause of the absence of gravelly layers and cross-beclded structure, which 

 should be expected in river deposits not far from an ice-sheet. Thus in- 

 terpreted, the basal member of the Dwyka differs from the next under- 

 lying layers of the Witteberg in marking a time when an ice-sheet had 

 advanced near enough to add stones, carried by floating ice, to the de- 

 posits of a body of standing water. In the absence of marine fossils, the 

 water seems to have been that of a lake. The side from which the ice 

 advanced is not indicated by anything that we saw in this district, but it 

 is well proved to have come from the north by observations elsewhere, of 

 which more is said below. 



The second member of the Dwyka is a characteristic sheet of tillite — 

 compact, dark bluish when fresh, yellowish brown when weathered, en- 

 tirely unstratined except for a single vertical layer of conglomerate a few 

 feet thick — which is exposed in a fine notch, or "poort," where Witteberg 

 river has cut through the ridge. A view of the eastern wall of the poort is 

 given in plate 50, figure 2; a similar view is given by Bogers (plate on 

 page 167). The dominant joints, many of which are here shown, divide 

 the mass into prisms that stand about square to the tillite sheet — that is, 

 they dip gently to the south. The bedding is nearly vertical, as is shown 

 by the layer of conglomerate near the left margin of the view. On the top 

 of the ridge (plate 52, figure 1) the prisms are weathered into "bales" 

 and "pillows" by the loss of their corners, and the pillows are reduced to 

 curious spike-like fragments by flaking and splintering on surfaces of 

 what seemed to be incipient schistosity. The points of the pillows and 

 spikes often stand out on the slopes of the ridge, giving it a very peculiar 



