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 a very 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, c, 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 by a 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 

 canons 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 



