396 DAVIS — PLAINS OF MARINE AND SUBAERIAL DENUDATION. 



valleys a significant rearrangement of many drainage lines will result 

 from the processes of spontaneous adjustment of streams to structures. 

 This involves the adjustment of many subsequent streams to the weaker 

 structures and the shifting of many divides to the stronger structures. 

 Adjustment begins in the early stages of dissection, advances greatly 

 in the mature stages, and continues very slowly toward old age, while 

 the relief is fading away. Indeed, when the region is well worn down 

 some of the adjustments of maturity may be lost in the wanderings of 

 decrepitude, but this will seldom cause significant loss of adjustment 

 except in the larger rivers. Now, if a region thus baseleveled or nearly 

 baseleveled is raised by broad and even elevation into a new C3'cle of 

 geographical life, the rivers will carry the adjustments acquired in the 

 first cycle over to the second cycle. Still further adjustment may then 

 be accomplished. The master streams will increase their drainage area 

 in such a way that the minor streams will seldom head behind a hard 

 stratum. In a word, the drainage will become more and more longi- 

 tudinal and fewer and fewer small streams will persist in transverse 

 courses. All this is so systematic that I believe it safe to assert that the 

 advanced adjustments of a second cycle may in many cases be distin- 

 guished from the partial adjustments of a first cycle. It should l)e noted 

 further tliat in the early stages of the second cycle the residual reliefs of 

 the first will still be preserved on the U])lands. and that they will be sys- 

 tematically related to the streams by which the dissection of the upland 

 is in progress, as noted in the examples described l)y Darton and llershey. 

 It is manifestly impossible to ajiply wliat may be called the river test 

 to plains of denudation U[)on wliich a cover of unconformable sediments 

 is spread ; Ijut, before assuming that such buried [)lains are of marine 

 origin, their uppermost i)ortion next beneath the cover should be exam- 

 ined to see if it presents indications of secular decay l)efore l)urial ; and, 

 if so, a subaerial origin for the [)lain may l)e argued. Certain aspects of 

 this division of the subject have been discussd by Pumpelly.^^ Another 

 matter of imi)ortance is the character of the undermost layers of the 

 cover. If these are fresli-water beds a subaerial origin for the plain on 

 which they rest may be inferred. The 'Potomac formation offers an 

 example of this kind, f 



Consequences of marine Denudation. 



Now suppose that a region of disordered structure is jxirtly worn down 

 by rain and rivers and is smoothly planed across by the sea during a time 

 of still-stand or of gradual depression. The land waste gained in the 



* Bull. Geol. Soc. Am., vol. 2, 1891, p. 211. 



t McGee : Am. Jour. Sci., vol. xxxv, 1888, p. 137 ; Fontaine : Monogr. xv, U. S. Geol. Survey, 1889, 

 p. 61. ^ 



