282 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
then, it is concluded that an elevation of the region has caused a 
revival ( Neubelebuny ) of the main river; and the present greater 
depth of the main valley is, according to Heim, merely the natural 
result of this revival, while the smaller side streams have not yet 
been able to deepen their valleys. The height of the Tholstufen 
or remnants of the former valley floor, seen in the benches above 
the basal cliffs of the bottom trough, is taken as a measure of the 
elevation that the mountain mass has suffered. 
Apart from the improbability that the deepening of a bottom 
trough by a revived main river could truncate so many lateral valleys 
with so great nicety as is repeatedly the case, leaving their streams 
to cascade down in clefts but slightly incised in the main valley 
walls, the following considerations lead me to reject the possibility 
of explaining the discordance between side and main streams by a 
normal revival of river action. 
Relation of Trunk and Branch Valleys. — The general accord¬ 
ance of maturely developed main and lateral valleys in non-glaciated 
regions, as recognized by Playfair, is today fully established by 
innumerable observations in many parts of the world. Truly, 
during the attainment of mature development, it is possible that a 
large river may outstrip a small branch stream in the work of deep¬ 
ening its valley, but the discordance thus produced can prevail only 
during early youth ; for as soon as the main river approaches grade 
the further deeping of its valley is retarded, while at the same time 
the steepened descent of the lateral streams at their entrance into 
the main valley accelerates their erosive work. Hence, even if a 
large trunk river has for a time eroded its valley to a significant 
depth beneath the tributary valleys, this discordance cannot endure 
long in the history of the river. Examples of such normal discord¬ 
ance are to be found in non-glaciated regions only in the branch 
streams of rivers that occupy very narrow canyons ; and even rivers 
in canyons sometimes receive their branches at accordant grade, as 
seems to be usually the case with the Colorado, if one may judge by 
photographs. The narrow postglacial gorges cut by active streams, 
habitually receive their branches — when they receive any — from 
hanging side gorges; and an excellent example has long since been 
on record in the gorge of Cattaraugus Creek in western New York, 
where a branch, the Canaserowlie, falls into the main gorge from a 
side gorge of much less depth. Referring to this, Hall wrote : — 
“In the more recently excavated channels we find the streams fall¬ 
ing over the very edge of the cliff, having produced no perceptible 
