The Peneplain. — Davis. 219 
may see a gradual decrease of altitude in the palisades, a 
ridge formed on a monoclinal sheet of dense intrusive trap, 
from a hight of about 600 feet in the north, to sea level in the 
south. There is no corresponding variation in the thickness 
of the trap sheet. The uplands of schists and gneisses on the 
east of the Hudson have a similar descent from the Highlands 
to Long Island sound. In Connecticut, the view from East 
Rock, Xew Haven, discloses the extraordinarily even crest 
line of Totoket mountain, the edge of a strongly warped sheet 
of extrusive trap. The crest line slowly descends southward 
and is continued by the somewhat lower crest line of Pond 
mountain, of similar structure. Furthermore, the descent of 
these crest lines agrees very Avell with the descent of the 
cr\'stalline uplands next on the east. The systematic rela- 
tion of these and many other crest lines and uplands suggests 
a peneplain, and the peneplain thus inferred is strikinglv in- 
different to the structures that it truncates. It might be urged 
that the observed discordance of form and structure is of some 
other origin than peneplanation ; but the discordance does not 
seem open to question. 
A 7. The rocks of monadnocks arc not proved to be more 
resistant than those of the adjoi?ii/ig pe/ieplain. It is urged 
that there is no other proof of the durability of the rocks of 
monadnocks other "than that which comes from the necessity 
of such an explanation, made necessary by first accepting the 
existence of the peneplain" (p. 358). As far as my own work 
is concerned, there is some ground for this objection. I have 
as a rule given no particular attention to the composition of 
the monadnock rocks: indeed, it has generally seemed to me 
reasonable to infer their greater resistance on account of their 
form. But so far as attention has been given directly to this 
phase of the problem, the inference based on the peneplain the- 
ory is borne out b\- petrographic study. The buttes and mesas 
that surmount the plains of the upper Missouri are maintained 
by dense igneous rocks. The monadnocks of the Virginia 
piedmont belt "are ribbed with siliceous schists or (]uartzites 
or other rocks that resist well the work of the weather . . . 
while the rocks underlying the fertile fields of the plain are 
softer schists, easily weathered and worn away" (McGee. 1. c. 
262, 263). Xcar .\tlanta, Ga., the P iedmont area is a well 
