170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
accounting for lateral valleys standing five hundred or more feet over 
main valleys whose flood plains are ten or twenty times as broad as their 
rivers. On the other hand, the observers above named conclude that 
the discordance is due to glacial erosion. If the former view were cor- 
rect, then surely the discordance of side and main valleys ought to be 
very strong in the Grand canyon, where there is no question of glacial 
erosion, where the disparity of volume between trunk river and side 
streams is notoriously great, and where the main valley is still so young 
that no significant widening of its floor has been yet accomplished. Yet 
singularly enough, the side canyons of the Colorado join the main canyon 
at accordant levels in nearly all cases. The views from the southern rim 
of the canyon at various points near Hance’s and Cameron and Berry’s, 
and still better from the great Red-wall spur that advances northward 
from the ‘‘copper mine” on the Grand view trail, show repeated in- 
stances of dry or nearly dry lateral canyons, only five or six miles long, 
which are nevertheless cut down at their mouths as deep as the main 
river, if their lower course lies on the stratified rocks. It is only where 
the main canyon narrows on entering the resistant crystallines that the 
side canyons are held up at a discordant level ; and even there the large 
lateral canyons seem to enter closely at grade (see detailed map, by 
Bodfish, Dutton, ¢, p. 258, Plate XLII.). However discordant the side 
and the main canyons may have been during a still earlier stage of the 
present cycle of erosion, Playfair’s law is already exemplified to-day ; 
and if, under conditions of peculiar difficulty, some of the smaller 
streams have not yet reached a normal relation to the master stream, 
they only prove the verity of the law by the need of their being 
excepted from it. 
As my own views of the canyon were only in the middle of the Kaibab 
section and from Vulcan’s throne in the Toroweap, the following refer- 
ences to records made by others may prove pertinent in this connection. 
Powell describes his ascent of a side canyon west of the Kaibab, and 
mentions waterfalls in its course, one of which was one hundred and 
fifty feet high ; but as no fall is mentioned at the mouth of the side 
canyon, it probably unites with its master in accordant fashion (a, p. 92). 
A little further down the main canyon, a stream from the north leaps 
into the river “by a direct fall of more than a hundred feet” over a 
“bed of very hard rock, . . . thirty or forty feet in thickness ” (a, p. 93). 
Before reaching the crystallines of the Shivwits canyon, “‘a little stream, 
with a narrow flood plain, comes down through a side cation” from 
the north. An Indian settlement was found there, with fields of corn 
