FORMER EXTENSION OF SOUTH AFRICA 441 



under such conditions the peneplanation of the highlands by the processes 

 of arid erosion might well have taken place. Since then the depression of 

 the outer area would have brought the sea in to the present coastline and 

 would have at the same time given both a more humid climate to the re- 

 maining highlands and an effectively lower baselevel to their rivers, and 

 thus the dissection of the highland, now in progress, would have been 

 initiated. The evidence of the former greater extension of South Africa 

 now to be presented, as the first step in this series, may not be regarded 

 as absolutely conclusive, but it is certainly highly suggestive and deserv- 

 ing of further study. 



First, in regard to the heavy sandstones of the Cape system: The 

 source of these sediments is not surely known, but the curious analogy 

 already pointed out between the folded structure of the Cape Colony 

 ranges and the folded structure of the Alleghenies in the eastern United 

 States tempts one to think that the sediments of the Cape system may 

 have come, in part at least, from the south or beyond the border of the 

 present continent. In the Alleghenies, as in the ranges of southernmost 

 Africa, a heavy series of stratified rocks has been pushed into parallel 

 folds over part of their area, while the remaining area of the series still 

 preserves its original horizontality with insignificant modification in an 

 adjoining plateau, overturned folds and overthrust faults in both of the 

 mountain belts being directed toward the plateau area. In the Alle- 

 ghenies the source of the sediments was demonstrably on that side of the 

 folded belt which lies away from the plateau area, and if this analogy 

 has any value the source of the sediments in the South African ranges 

 should also be on the side away from the plateau — that is, in a former 

 land, now submerged in the ocean to the south of the present continent. 



A second point to be considered concerns the geographical conditions 

 prevailing during the deposition of the heavy series of strata of the Kar- 

 roo system, certainly of great thickness in its area of greatest accu- 

 mulation. The entire series is free from marine sediments; it begins 

 with the remarkable Dwyka glacial formation, and then continues as a 

 great series of continental deposits, containing fossils of land plants and 

 land animals. The stratified beds of the Karroo system, like so many 

 other continental deposits, have been explained as of lacustrine origin, 

 but they are much more plausibly regarded as of mixed lacustrine and 

 fluviatile origin. Their sediments imply a vast erosion from the con- 

 temporaneous higher peripheral areas and an accompanying slow depres- 

 sion of the central area or basin of deposition. There is little probability 

 that highlands then existed next north of the present area of the Karroo 

 system, for in that district the older Waterberg sandstones (presumably 



XXXVIII— Boll. Gbol. Soc. Am., Vol. 17, 1905 



