398 DAVIS PLAINS OF MARINE AND SUBAKRIAL DENUDATION. 



or less dissected, may be found in which no simple test based on the 

 presence of superposed streams will serve to settle the question of marine 

 origin. Indeed, it appears to me a difficult matter to adduce any ex- 

 amples of extensive plains of denudation whose origin is demonstrably 

 marine and to whose planation subaerial agencies have not contrilnited 

 the greater work. A region may be almost reduced to baselevel by sub- 

 aerial denudation when the transgressing sea completes the work, extin- 

 guishing the adjusted valleys and introducing superposed streams in the 

 next cycle of denudation. A region well baseleveled under the air may 

 by quick depression suffer rapid ingression of tlie sea, whose shore waves 

 will during depression nowhere reside long enough to perform a signifi- 

 cant amount of abrasion. When the region is thus submerged and stands 

 again relatively quiet, the waste from a non-submerged area, gained both 

 by marine and subaerial denudation, may be spread over the denuded 

 and depressed plain, and when afterwards elevated with an unconform- 

 able cover that will induce superposed drainage, all trace of former 

 adjustments will be lost; yet here the planation was not marine. A 

 district of superposed drainage in central New Jersey, where the Amboy 

 clays once spread over the red shales and sandstones of the Trias, may 

 probabl}'' be taken as an example of this kind. Superposed rivers cannot, 

 therefore, alwa3's be taken to prove that tlie ui)lands which tliey dissect 

 are uplifted plains whose denudation was chiefly performed by the sea. 

 Regions of essentially horizontal structure normally have wandering 

 streams ; no S3^stematic arrangement of drainage is here to be expected. 

 Discrimination in such regions has seldom been attempted between ex- 

 amples of one cycle of subaerial denudation, now adolescent or mature, 

 and examples of two cycles, the first having reached old age and the 

 second now being in its adolescence or maturity. The sky-line would 

 be smooth and even in examples of either class : in the first, because its 

 original constructional form was a plain; in the second, because it was 

 planed down essentially smooth at the close of the cycle preceding the 

 current cycle. It is, however, sometimes possible in regions of horizontal 

 structure to recognize the records of old age reached in a former cycle by 

 a slight discordance between the general upland surface and the attitude 

 of the strata; or by the association of the region with an adjacent region 

 of tilted structure where indications of an earlier cycle of subaerial denu- 

 dation are manifest, both these tests being applicable in the Allegheny 

 plateau ; or b}^ the arrangement of the faint residual relief of the uj)lands, 

 where not trenched by young or adolescent streams, this test having been 

 applied in the Piedmont district of Virginia, in the Ozark plateau of 

 Missouri, and in the Great plains of eastern Montana. Further study of 

 many other examples is desirable. 



