126 bulletin: museum of comparative zoology. 



They are not so large or so numerous as those by Jacob's pool in the 

 cliffs of the Paria, but they are of perfectly similar development when 

 they occur. At first only forming monoclinal ridges above the Shina- 

 rump bench, they soon sprawl forward and downward on the broad plat- 

 form beneath the bench. Among many others, one sprawling slide that 

 advances at least a mile is a very striking feature to the traveller who 

 follows the monoclinal valley road northward towards Lee's Ferry. When 

 first seen at a distance of eight or ten miles, the slide seems to be a vari- 

 colored ridge or spur, stretching forward in abnormal position from the 

 base of the Shiuarump bench ; on a nearer view, its origin is evident 

 enough, and when seen from in front, so that its full breadth is recog- 

 nized, its lower part takes the form of a huge paw, below a wrist that is 

 narrowed at the fall over the bench. We did not recognize any wrinkles 

 ploughed up in front of the slides, such as Russell has described in asso- 

 ciation with certain great landslides in Washington (b. 47). 



All this seems inconsistent with the scheme of one cycle of erosion, 

 but perfectly consistent with the scheme of two cycles : the first having 

 witnessed the development of a floor of weak Permian and lower Trias 

 over the northeastern part of the Marble platform, with a border of 

 maturely stable cliffs of Triassic sandstones on the north and east • the 

 second having witnessed the incision of the Marble canyon, along with 

 which came a rapid removal of the weak Permian strata down to the 

 more resistant. Carboniferous members, and with this an accelerated 

 recession of the Triassic cliffs in the Lee's Ferry angle. It may be well 

 to say again that the " scheme of one cycle" is an essentially single 

 and continuous uplift, during and after which the denudation of the 

 a has been accomplished in a single and persistent effort of the 

 destructive forces; while the "scheme of the two cycles" involves two 

 uplifts separated by a pause of relative quiescence, during which the 

 great denudation of the plateaus was well accomplished ; the incision of 

 the canyons, the removal of weak strata from the denuded plateaus, 

 and the revival of certain activities, such as have just been described, 

 being the result of the second uplift. 



The Migration of Certain* Divides. — Among the minor evidences 

 of a modern revival of erosion, mention may be made of two examples 

 of migrating divides in localities where a stable water-parting might 

 have been expected if the erosion of the region had been accomplished 

 in a single cycle. 



Divide near Pipe Spring. — One example of this kind lies in the weak 

 lower Triassic area west of Pipe spring and south of the Vermilion 



