80 bulletin: museum of comparative zoology. 



which they run. This subsidence must be so slow and so long continued 

 as to allow of the removal of alpine or subalpine relief over the region 

 attacked by the horizontal wave-saw. The oue hypothesis premises the 

 enduring constancy of a crust at rest or with but faint oscillations of 

 level; the other, the enduring constancy of a crust affected by tolerably 

 steady motion in one direction. 



Partly on account of this difficulty in believing in the requisite 

 constancy of the base level during the cycle of subaerial erosion, the 

 inquiry has been recently made by Tarr as to whether the peneplain 

 explanation is correct when applied to the New England and New Jersey 

 uplands; he has proposed a third theory in its stead. 1 The main idea 

 of his thesis has been enforced in a later paper by W. S. Tangier 

 Smith, who, however, seems to adhere to the peneplain explanation in 

 certain cases. 2 A second objection to that explanation was founded on 

 Tarr's criticism of the method used to account for the disturbance 

 of the New Jersey and New England peneplains since completion. 

 These facets are supposed, on the hypothesis of peneplanation, to have 

 been tilted seaward, and more complex movements are believed by 

 Hayes and Campbell to have affected the peneplained surfaces of the 

 Southern Appalachians. In order to obviate the necessity of invoking 

 such warpings (albeit among the simplest and commonest of crustal 

 deformations), and to make more credible the denudational theory of the 

 uplands in these old-mountain areas, Tarr has suggested that essentially 

 the whole work of erosion has been accomplished in one cycle. In the 

 mature stage of that cycle, not only are the larger streams well graded, 

 but the slopes on the interstream spaces will be graded in sympathy 

 with the axial profiles of the streams. The line of divide belonging to 

 any interstream space will gently slope seaward, because the moun- 

 tainous terrane will have lost more material near the sea than farther 

 inland. On account of the greater volume of each stream near its 

 mouth, the lower part of the valley . carved by that stream will be 

 deepened nearly to baselevel before the upstream part. Hence the 

 valley-sides will waste faster near the mouth than toward the head- 

 waters. Smith has strengthened this hypothesis of "beveling" by 

 referring to the tendency at maturity toward a roughly equal spacing 

 of master-rivers in a region even of diverse structures; 8 the result 

 thereof being a series of divides declining to the sea at about the same 



i Amer. Geol., 1898, Vol. 21, p. 351. 



- Bull. Depart. Geol. Univ. Cal., 1899, Vol. 2, p. 155. 



8 Shaler, Bull. Geol. Soc. Amer., Vol. 10, p. 263. 



