DAVIS: THE GRAND CANYON OF THE COLORADO. 167 



rising mountains, for there are some well-attested examples of such a 

 process, in which its successive steps are more or less fully traced ; but 

 rather that this theory makes a single stride from the beginning to the 

 end of a long and complicated series of movements and erosions, over- 

 looking all the opportunities for drainage modifications on the way. 

 The simplicity of the theory is certainly attractive in comparison with 

 the rather tedious length of the considerations that are involved in the 

 attempt to analyze the processes of spontaneous river adjustments; but 

 it should now be generally recognized that nothing less than a deliberate 

 analysis of all movements (of which the uplift of the present plateaus is 

 the last) and erosions (of which the cutting of the Grand canyon is the 

 least) will suffice to discover the actual origin of the Colorado. The 

 preceding paragraphs are offered as the beginnings of such an analysis. 



The Erosion of the Grand Canyon. 



The Canyon Cycle. — The general uplift that introduced the canyon 

 cycle in the Grand canyon district seems to have been accompanied by 

 a strong renewal of movement on the faults that divide the Basin range 

 province from the plateaus. The northward increase in the heave of 

 the bounding faults between the Basin range province and the plateaus 

 suggests that the uplift of the High plateaus at this time was greater 

 than that of the Grand canyon district. Dutton says : " Until near 

 the close of the Pliocene the High plateaus were not only the theatre of 

 an extended vulcanism, but those portions which never were sheeted 

 over by lavas were low-lying areas, where alluvial strata tended to 

 accumulate. They remained, in fact, base levels of erosion during the 

 greater part of Tertiary time" (a, p. 23). It is possible that some 

 movement was renewed on other than the bounding fault-lines at the 

 time of the general uplift, but it has been pointed out above that there 

 is good reason for thinking such movement to have been insignificant on 

 the fault line near Pipe spring. Still, in the south Toroweap valley 

 (Dutton, c, p. 94), in the Great Basin (Gilbert, d, p. 341), and in the 

 High plateaus (Dutton, a, pp. 250, 277), there are various indica- 

 tions of faulting at a much later date than the beginning of the canyon 

 cycle. 



Comparison of Glen and Marble Canyons. — Chief among the fore- 

 going considerations that point to the division of the Tertiary history 

 of the Grand canyon district into at least two cycles of erosion is the 

 similar resistance to weathering manifested by the Triassic and the 



