226 The America?! Geologist. April, i899 
The lateral swinging of large rivers, occasional incursions of 
the sea, changes of climate, anything that will contribute to 
the end is a pertinent part of the theory of peneplanation. 
Still my.own opinion is that, of all processes, subaerial denuda- 
tion is the most important. This is not simply an opinion of 
preference; it is an opinion based on the arrangement of 
rivers in uplifted and dissected peneplains (Bull. Geol. Soc. 
Amer., vii, 1896, 377-398). Such rivers frequently exhibit 
adjustments that tliey could not have gained from a disordered 
arrangement during the present cycle of denudation alone; 
adjustments that could have been gained for the most part 
only during the cycle of peneplanation, and that would have 
been lost if the rivers had wandered far, or if the sea had 
abraded much of the land during the later stages of that cycle. 
It is of course perfectly possible that a peneplain should be 
smoothed ofif by the sea after it had been worn down under 
the air: such appears to have been the case with the Cambro- 
Silurian plain of northwest England (see B3). But it is not 
reasonable to suppose that every uplifted and dissected pene- 
plain was thus smoothed before it was uplifted, although this 
supposition finds much favor Avith certain English geologists. 
As to the arguments based on the slow progress of denuda- 
tion during a brief period of observation (p. 355)or during post- 
glacial time (p. 361), I can only reply that a geographical cycle 
must be so enormously longer than either of these intervals 
that their evidence is not of value. Truly denudation is re- 
tarded when a capping of waste protects the rocks from the 
attack of the weather, but rather than side with De Luc, who 
concluded that waste-covered mountains are practically pro- 
tected from further change, I should prefer to side with Hut- 
ton, who maintained that even the slow denudation of waste- 
covered slopes could produce great changes of form. As to 
the time that has elapsed during the denudation or dissection 
of peneplains, there is apparentl)' no way of measuring it but 
by the work done. Hence the Cjuestion returns to the verity of 
the peneplains; whether nuich or little time is needed to pro- 
duce them is a secondarv matter. Above all, a preconception 
as to the insufficience of geological time should not in this day 
be urged (p. 361) as a reason for not believing in the possibil- 
ity of peneplanation. One sometimes hears a student say: 
