ANALOGY OF CAPE COLONY RANGES WITH ALLEGHENIES 385 



pies of formerly baseleveled monoclinal ridge crests in South Africa com- 

 parable to the even crested monoclinal ridges of Pennsylvania; but the 

 relation of the southern ranges to the peneplain of the interior highland 

 made it seem very probable that in South Africa, as in Pennsylvania 

 and Virginia, the erosion of the mountains has not been a continuous 

 process with respect to a single baselevel, and that the Cape Colony 

 ranges may well have been, like the Appalachians, once at least reduced to 

 a much smaller relief than that of today, only to be brought into strength 

 again as a result of revived dissection and etching out of the weaker 

 beds, following renewed elevation. The drainage in both systems in- 

 cludes numerous subsequent rivers and valleys* which exhibit marked 

 examples of adjustment of streams to structures rather than persistence 

 in originally consequent courses. Whether the amount of adjustment is 

 no greater than could have taken place in the continuous processes of a 

 single cycle of erosion, or whether it is so great as to demand for the 

 Cape Colony ranges the aid of a second cycle in which to supplement the 

 work of a first cycle, as seems so clearly to have been the case in the Alle- 

 ghanies, I can not venture to assert from the facts in hand; but the data 

 presented by Schwarz (a) regarding the high-standing, gravel-covered 

 planation surfaces or terraces among the ranges and the strong evidence 

 of peneplanation in the Veld make more than one cycle of erosion of the 

 mountain belt extremely probable. 



DRAINAGE PROBLEMS 



The numerous narrow and deep-cut water gaps in the ridges, through 

 which the open longitudinal valleys are drained, afford remarkably fine 

 instances of the manner in which a belt of resistant rocks may long 

 maintain the narrow form of a young valley, while the belts of weaker 

 rocks, upstream and downstream from the gaps, permit their valleys to 

 be carried forward to the stage of maturity or even to that of old age. 

 The suggestion that the gaps are due to convulsions of nature, with the 

 tacit postulate that the ridge in which the gap is opened and the open 



*The term "subsequent*' is here used consistently with tbe definition given to it in 

 1889 — see "The rivers and valleys of Pennsylvania," National Geographic Magazine, 

 vol. i, 1889, p. 207 — to designate rivers and valleys that have been developed by head- 

 ward erosion along a belt of weak strata, and not to include all valleys of erosion, in 

 contradistinction to original or tectonic valleys, as later suggested by J. Geikie in his 

 book "Earth Sculpture," 1898, pp. 277-279. While the rule of priority is not recognized 

 as binding in physiography, it seems regrettable tbat a term like subsequent, the use of 

 which in a certain limited sense has been clearly set forth, should afterward be used in 

 a much more general sense, particularly when no indication of the change of meaning 

 is given to the reader. 



