96 
The American Geologist, 
February, 1896 
than to any increased activity of attack by the waves. The west side 
of the “mainland” is cut into by a flat concave curve; this being a 
slight change from a former straighter shore line, on which the waves 
and currents were working before the building of Provincetown. 
Mr. G.K. Gilbert, in discussion, asked whether there is any evidence 
of general elevation or depression of the cape area; to which the speaker 
replied that there is none. 
Prof. Hitchcock recalled the idea of Louis Agassiz that there was 
once a continuous line of drift from Cape Ann to the forearm of Cape 
Cod; but Prof. Davis said it has been long disproved, and referred also 
to historic records of low sand islands off to the southeast of the High¬ 
land Lighthouse. 
Prof. Shaler stated that the “mainland” of the cape was formed by 
a deposit of drift on a preglacial ridge of Tertiary and Cretaceous strata, 
and that, the former river systems can be traced southward through 
Vineyard sound. The Provincetown people fear lest the cape to the east 
of them will be breached and their harbor be filled with sand; and the 
value of jetties north of the point of attachment between the Province- 
town sand area and the high land of modified drift was emphasized. 
Plains of Marine and Subaerial Denudation. W. M. Davis. In re¬ 
gard to the origin of plains of denudation, there prevail in Europe and 
this country two schools of belief: the European school, attributing 
denuded plains to marine action, follows Ramsay and Richthofen ; the 
American school, looking to subaerial denudation for the same result, 
follows Powell, Dutton, and others. Plains of marine denudation are 
thought to attain their greatest extent in regions of slow depression, 
where the activity of the waves on the coast is continually maintained 
by the deepening of offshore water that would otherwise become very 
shallow. When elevated, perhaps to the attitude of plateaus, the 
plains retain for a time an unconformable cover of marine sediments ; 
but this cover may be wholly stripped off while the surface of the plain 
is still sufficiently preserved for recognition, even though trenched here 
and there by deep valleys. Plains of subaerial denudation attain their 
best development in regions of small vertical oscillations about a long 
maintained average altitude. When more or less perfectly reduced to 
baselevel, these plains or peneplains may be elevated, and thereupon 
introduced into a new cycle of denudation ; or they may be depressed, 
submerged, buried under waste from some neighboring land area, and 
then again elevated with an unconformable cover on their back. In 
the latter case they will simulate the later history of uplifted plains of 
marine denudation. 
It is notable that in the description of plains of denudation, whether 
partly exposed, as at the base of a newer formation lying evenly on the 
denuded surface of an older formation, or completely exposed as in 
uplifted plateaus, geologists generally assume the correctness of the 
belief of the school to which they belong, treating the matter as if it 
had been carried beyond the stage of discussion. Some writers, how¬ 
ever examine the problem more deliberately, and by a, priori argument 
