590 W. M. DAVIS PENEPLAINS AND THE GEOGRAPHICAL CYCLE 



of the cycle of erosion. The addition of three other classes of streams- — 

 insequent, obsequent, and resequent — .thus enlarging the original trio 

 to a septet, is a refinement of much less importance than the recognition 

 of the class of subsequents. 



The geographical Cycle and its Complications 



Two of my European correspondents have lately written me that they 

 must give up the cycle of erosion because it does not include certain 

 facts to which they call attention. One group of facts is the well-opened 

 valleys in areas of relatively weak rocks; and it is urged that these 

 valleys were never "young' 5 in the sense of being narrow; they were 

 widened as well as deepened while the region was raised. I believe it is 

 true that my own writings do not contain any explicit statement of this 

 rather evident idea; the nearest I have come to it is to say that if a 

 region of resistant rocks be slowly uplifted its valleys will be widened as 

 they are deepened. "In such a case there would have been no early 

 stage of dissection in which the streams were enclosed in narrow valleys 

 with steep and rocky walls; the stage of youth would have been elided 

 and that of maturity would have prevailed from the beginning, but with 

 constantly increasing relief as long as uplift lasted." 2 Immediately 

 mature valleys of this kind would evidently be much more common in 

 areas of weak than of resistant rocks;* it may indeed be a rare exception 

 to find "young" valleys, in the sense of narrow, steep-sided valleys, in 

 weak-rock areas. In such cases the valleys are probably, Minerva-like, 

 born mature. It is certainly a puzzling and regrettable omission not to 

 have explicitly stated a matter as manifest as that; but what is still 

 more puzzling is why the omission of a subordinate matter of this kind 

 should be regarded as a reason for discarding the whole scheme of the 

 cycle of erosion. The omission clearly gives reason for amending and 

 improving the scheme; but, in view of the abundance of regions in 

 which typical "young" valleys are found in resistant rocks and in which 

 a relatively rapid uplift is thereby proved, the scheme of the cycle seems 

 to me still worth preserving, as well as worth improving as far as possible. 



Even if the general presentation of the scheme sets out from a region 

 of relatively resistant rocks uplifted with comparative rapidity, in which 

 the first-cut valleys of the larger streams are typically "young," the 



2 Complications of the geographical cycle. Proc. 8th Internat. Geogr. Congr., 1904. 

 Washington, 1905, pp. 150-163 ; see p. 153. 



* Since writing this article I find that explicit, though hrief, mention of the more 

 probable occurrence of immediately mature valleys in weak than in hard rocks is made 

 in my "Erkldrende Beschreibung der Landformen/' 1912, p. 147. 



