DALY: PHYSIOGRAPHY OF ACADIA, 79 
The third hypothesis would not necessarily posit more than one cycle 
for the development of the existing land forms; one cycle advanced 
to maturity, in addition to the episode of recent drowning, would be 
required. 
It must be confessed that we have not as yet for plains of marine 
erosion, criteria either so numerous or so well worked out as those for 
the peneplain. It has been often, and with justice, emphasized that the 
theory of the peneplain must recognize the fact that there is no living 
representative of these ancient plains of denudation. Among the many 
peneplains so far described, no one of them has the attitude which it 
should possess during and at the time of its completion ; that is, no one of 
them stands so near the sea-level that the relief can no longer be seriously 
diminished by its sluggish streams. It may be equally well advanced 
against the theory of marine abrasion that no extensive submarine plain, 
proved to be of this origin, can now be found on the flanks of the con- 
tinents. In either case, explanation may be sought in the sugges- 
tion that there has been, in late Tertiary time, a special amount of 
epeirogenic movement. Absolute uniformity of crustal uplift and de- 
pression throughout geological time can now no more be believed in 
than absolute uniformity of climate or of the advance of organic life in a 
given region. The geological record shows that long periods of quiet 
have been preceded and followed by others of uneasiness and of rela- 
tively paroxysmal changes in elevation of continental masses. If the 
brilliant generalizations of Suess regarding wholesale Tertiary subsidence 
of parts of the ocean-floor are shown to be correct, particularly those 
referring to the North Atlantic, it is to be expected that plains of denu- 
dation of either origin would be seriously disturbed from their original 
position. This expectation would remain, although we might not feel 
complete agreement with Suess’s fundamental position as regards the 
cause of the negative movements of sea-level. The future adjustment 
of geological opinion with respect to these matters may some day remove 
this real difficulty which adheres to either theory of the greater facets of 
denudation. 
Both theories demand of necessity a special behavior of the earth’s 
erust. The cycle of subaerial erosion can only be completed and a 
peneplain produced if there has been average crustal stability through 
an enormous period of time. The cycle of submarine erosion lead- 
ing to the fashioning of a plain of marine denudation requires during 
a period at least as long a more or less steady subsidence which will 
permit the waves to cut inside the belt of aggraded sea-floor over 
