168 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Carboniferous strata. It would be difficult to determine by ordinary 
tests which one of these two formations is the more resistant, so strong 
does each appear; but avery good natural test of their relative strength 
is found in a comparison of the upper part of Marble canyon, cut in the 
upper Carboniferous, with Glen canyon, cut in the Triassic rocks them- 
selves, where they descend to the level of the lower plateaus east of the 
Paria-Echo monocline. The two canyons are of similar date, for the 
monoclinal flexure that separates them is much older than either canyon; 
they are of similar width (Dutton, ¢, Atlas, sheet XXII.) ; and hence 
the resistance of their walls must be similar. It is true that the widen- 
ing of Glen canyon has been retarded by the failure of the river as yet 
to cut down to the weak blue clays that underlie the heavy red sand- 
stones of the Trias; while the stripping of the Trias from the great area 
of the plateaus south of the Grand canyon, and the recession of the 
Vermilion cliffs for fifty miles or more north of the canyon have been 
greatly aided by the sapping of the underlying clays; but even with 
this aid it does not seem possible to explain the great denudation of the 
plateaus in contrast to the narrowness of the canyon bya single cycle of 
erosion. 
Stage of Development of the Canyon.— The day has passed when it 
was necessary to ask whether the canyon is the work of the river; but 
in the renewed attention lately given to the relation of trunk and 
branch valleys, certain features of the canyon serve as important wit- 
nesses. In spite of the youth of the canyon, its branch streams gen- 
erally enter at accordant grade with the main river, and thus testify to 
the promptness with which side valleys are cut down to the depth of 
the main valley at their points of junction. This feature must be 
considered in some detail. 
Rapids in the Canyon. — The canyon is so young that the great river 
at its bottom has not yet established a completely graded channel. 
True, there are no leaping waterfalls now remaining: “ Throughout the 
cafions there are no cataracts ; that is to say, at no place does the river 
fall from a ledge of rock into the pool below” (Gilbert, a, p. 75). But 
there are still many rapids, especially in those parts of the canyon 
where the fundamental crystallines are trenched. When these resistant 
ledges are rasped away, the upper canyon will be significantly deeper 
than it is now. On the other hand, the corrasion of the canyon must at 
present be proceeding at a slower rate than at some earlier time, before 
the development of the graded stretches that now constitute the greatest 
part of the river. It was the existence of these graded stretches, where 
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