TJic Peneplain. — Davis. 225 
tion. Any sort of movement that does not cause a distinct dis- 
section of the surface below the peneplain level is admissible. 
Well-preserved peneplains, now dissected only by young, nar- 
row valleys, give assurance that no significant valley-cutting 
below the peneplain level was permitted before the uplift by 
which the erosion of the existing valleys was initiated. Even 
in so uneven a region as southern N^ew England, the gradual 
decrease of relief on approaching the coast makes it extremely 
probable that the deep valleys of the interior were not cut 
till after the peneplain was essentially finished. Any other sup- 
position involves special conditions of oscillation and tilting 
that I believe are less probable than those involved in pene- 
planation, as may be seen by drawing a series of diagrams to 
represent the successive attitudes assumed by the land under 
different hypotheses. 
It is sometimes suggested that before peneplanation, but 
after valleys like those of southern Xew England had been ex- 
cavated, economy of w-ork and time would be served by postu- 
lating a depression and a truncation of so much of the moun- 
tains as then remained above baselevel. The truncated surface 
thus produced w'ould, under this supposition, correspond to 
the New England uplands. This truly effects an economy 
of work, measured by area of baselevelling, but it effects no 
important economy of time; for it will require essentially as 
long a time to truncate or baselevel a large cone as a small 
cone, structure and slope being equal. Moreover, unless very 
special suppositions were made as to the attitude of the land 
before, during, and after such a truncation of its mountains, 
the existing forms of southern New England could not be ex- 
plained. During a submergence long enough to truncate the 
mountains remaining above baselevel, many shallow valleys 
would be filled with marine deposits; and after elevation, the 
streams might frequently abandon their former valleys for new, 
superposed courses. The narrow new valleys excavated on 
such courses, and the former valleys in which remnants of 
marine deposits might long linger, are not represented in 
southern New England. 
Yet any supposition or process that will aid in the destruc- 
tion of a land mass must be welcomed by those who believe 
that land masses have been destroyed, close down to baselevel. 
