588 W. M. DAVIS PEXEPLAIXS AXD THE GEOGRAPHICAL CYCLE 



growth of the many ideas that are compactly embodied in the scheme 

 of the cycle of erosion or the geographical cycle. 



A peneplain developed on a large landmass of homogeneous structure 

 slionld be margined along its retrograding ocean shore by sealevel delta- 

 free valley plains, alternating with low, wave-cut bluffs on faint inter- 

 valley swells; and should very gradually ascend to greater altitude and 

 greater relief at an interior divide, beyond which similar features would 

 be repeated in reverse order to a farther ocean. The survival of large 

 mountain-like hills along the interior divide where the initial upheaval 

 was greatest is not inconsistent with the occurrence of a well developed 

 peneplain — broad swells of gentle convexity between wide valley floors — 

 over the greater part of the area between the divide and the ocean. 

 Penck has proposed the term, mosore, for the residual hills that survive 

 along the divide, not by reason of greater resistance, but by reason of 

 representing a greater original mass to be consumed. From the mosores 

 along the main divide, gradually dwindling trains of hills would follow 

 the secondary divides. There is no break in the long sequence of slow 

 changes by which the smaller hills of the secondary divides and the 

 larger hills of the main divides are gradually reduced to so small a relief 

 that they, too, may be regarded as part of a peneplain. The term pene- 

 plain should therefore be taken as especially applicable to certain 

 advanced phases of land sculpture not sharply separated from the phases 

 that precede and follow. 



If a peneplain have a breadth of 1,000 miles or more, or if a peneplain 

 be developed in the interior of a large continent, its interior part — not 

 merely the residual hills of fairly strong relief, but also the gently undu- 

 lating swells that rise but little over the broad valley floors — may have 

 altitudes of from 1,000 to 3,000 feet or more above sealevel. Hence an 

 evenly uplifted peneplain, now undergoing dissection, should not be ex- 

 pected to stand everywhere at the same altitude, and the present altitude 

 of even its best developed parts should not be taken as necessarily giving 

 a measure of its uplift, as if it had previously stood at sealevel. The 

 uplift may have been several thousand feet less than the altitude. Cvijic 

 has emphasized this point in his discussion of the physiography of the 

 Balkan region. Similarly, if the coastal two-thirds of a broad peneplain 

 be flexed or faulted down near or beneath sealevel, while the inner third 

 remain at its former altitude, the rivers there will at once proceed to in- 

 cise new valleys beneath their former valley floors ; hence the mere occur- 

 rence of valleys incised in a peneplain should not be taken as evidence of 

 uplift. Philippson has urged the importance of this interesting principle 

 in an account of the Slate Mountains of the middle Rhine. 



