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 one 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. 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.?_ 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;* the result 
thereof being a series of divides declining to the sea at about the same 
Amer. Geol., 1898, Vol. 21, p. 351. 
1 
2 Bull. Depart. Geol. Univ. Cal., 1899, Vol. 2, p. 155. 
8 Shaler, Bull. Geol. Soc. Amer., Vol. 10, p. 268. 
