396 W. M. DAVIS OBSERVATIONS IN SOUTH AFRICA 



opposite to the course that a consequent river would take. The ridge on 

 the western limb of the syncline was seen to decline and end some 20 or 

 more miles north of the gorge, and we were told that its ending was due 

 to a north-pitching anticlinal roll in the determining sandstones. All 

 this adds new features of resemblance to the Allegheny ridges, where 

 streams frequently flow outward from a synclinal valley near the apex 

 of the inclosing synclinal mountain and where zigzag mountain crests are 

 of frequent occurrence. 



The contrast of the anticlinal and the synclinal mountains just referred 

 to is of value as an aid in dispelling the myth, so long current in geolog- 

 ical literature, that anticlines are weak structures and that synclines are 

 strong structures; that anticlines, naturally prevalent in young moun- 

 tains, are unusual or abnormal in mountains of great age, where syn- 

 clinal ridges should prevail. The Cape Colony ranges agree with the 

 Alleghenies of Pennsylvania in proving that whether the structure be anti- 

 clinal or synclinal is of secondary importance, in so far as resistance to 

 erosion is concerned ; the prime factors which determine relief are in the 

 first place the relative resistance of the formations involved, and in the 

 second place their attitude with respect to the controlling baselevel. It 

 is of course true in general that if a region of folded structure be worn 

 down below the level of the deepest folds, so as to expose the unconforin- 

 ably underlying formations, the synclines will, other things being equal, 

 survive the longest ; but such a condition is seldom met with. Moreover, 

 when the general statement is made that anticlinal ridges are exceptional 

 and synclinal ridges are prevalent in mountains of great antiquity, no 

 mention is made and no reference is intended to the complete obliteration 

 of the folded structures, as in the ideal case just conisdered. The gen- 

 eral statement is based on an essential misconception, for the correction 

 of which the Cape Colony ridges and the ridges of Pennsylvania may be 

 confidently appealed to. Anticlines are not inherently weak structures, 

 nor are s}mclines essentially strong structures. In old mountains syn- 

 clines of relatively weak rocks are worn down, while synclines of hard 

 rocks stand up; likewise anticlines of weak rocks are worn down, while 

 anticlines of strong rocks stand up; hence it is the nature of the rock 

 and its attitude with respect to baselevel, and not the anticlinal and the 

 synclinal structure, that are dominant in determining these residual 

 reliefs. 



PLANATION SURFACES JZV THE KARROO 



During our excursions from Laingsburg we saw an excellent example 

 of a broad and smooth planation surface or terrace, truncating the 

 strongly folded Witteberg, Dwyka, and Ecca beds and strewn over with 



