384 W. M. DAVIS OBSERVATIONS IN SOUTH AFRICA 



ferred that the tangential thrust by which the compression and over- 

 turning were produced was in both cases directed from the region that is 

 now ocean toward the region that is now land — from southeast to north- 

 west in the eastern United States, from south to north in South Africa. 

 This analogy suggests another, namely, that as the chief source of the 

 sediments was on the ocean side of the present ranges in the Appa- 

 lachians, where Paleozoic land is recognizable in the older crystalline belt, 

 a similar relation may have obtained in South Africa; but this specula- 

 tion is to a certain extent contradicted by the fact that the texture of 

 some of the members of the folded series there increases in coarseness 

 toward the west and northwest, as if the sediments had been derived 

 from the ancient rocks north of the lower Orange river, 300 or 400 miles 

 away from the ranges in question. However, there are in general so 

 many examples of mountain-making deformation having invaded areas 

 of heavy deposition — piedmont to the source of the deposited sediments — 

 that it is tempting to inquire whether such, may not have been the case 

 with the South African ranges also; and in spite of the absence of any 

 visible area of older rocks along the south coast from which the heavy 

 Paleozoic sediments might have been derived, it remains possible that 

 such a source may have lain somewhat farther south, and that it has 

 since then disappeared by submergence. The suggested source of the 

 strata of the Cape system north of the Orange river might in that case 

 be compared to the partial source of the Appalachian sediments in the 

 oldlands of northern Wisconsin, which is not at all inconsistent with their 

 main source having been in the old Appalachian belt. 



A vast amount of erosion has occurred in these mountain systems since 

 they were folded, so that the existing longitudinal valleys and lowland 

 belts are entirely due to erosion, and the linear ranges and ridges of today 

 are merely the residual reliefs of the more resistant formations, without 

 essential relation to belts of uplift. Thus the Little and the Great 

 Karroo are nothing more than narrower and wider lowland belts that have 

 been eroded along the strike of the less resistant formations between and 

 north of the east and west ranges. They correspond closely in origin, 

 though not at all in climate, to the similar lowland belts or valleys in the 

 Appalachians. As a result of this heavy erosion, the formations of the 

 Karroo system have been almost completely removed from the district 

 of the Cape Colony ranges, but several significant patches of Dwyka and 

 Ecca beds remain in synclines, and from this their former general south- 

 ward extension must be inferred. 



As to the occurrence of two or more cycles of erosion during the wear- 

 ing down of the South African ranges, I did not see any striking exam- 



