124 bulletin: museum of comparative zoology. 



their appearance, huge landslides become abundant. Just the reverse 

 relations should obtain if the' erosion of the region had been accom- 

 plished in a single cycle. 



Imposing as these landslides are to one who follows the road along 

 the base of the cliffs, they must be considered only as subordinate 

 details among the colossal cliffs and platforms of this large-featured 

 11. for no mention of them is found in the reports of Powell, Gilbert, 

 and Button, although all these observers describe the Triassic escarp- 

 ment from which the slides have slipped down. Some further account 

 of them may therefore be given here. 



In the Vermilion cliffs of the Paria plateau bordering the northern 

 or middle part of House-rock valley, the upper members of the Triassic 

 sandstone retreat somewhat, leaving the lower members to stand forth 

 in a bench, and thus making two steps of mature expression in the 

 descent from the plateau to the valley. Here the blue clays of the 

 lower Trias and the underlying weak Permian layers are all con- 

 cealed beneath the washed alluvium of the graded valley floor. If one 

 looks down on the valley from the heights of the Kaibab on the west, 

 an axial arroyo is seen to separate the wash of red Triassic waste on 

 the east from that of gray Carboniferous waste on the west. In the 

 southern part of the valley near House-rock spring, the blue clays be- 

 gin to appear, and at once the lower bench of the cliffs breaks down 

 here and there in normal landslide form; that is, the gentle dip of the 

 cliff-making strata in the Paria plateau is steepened in the slide, so 

 that the fallen mass takes the form of a ridge, more or less shattered 

 and disordered, yet with a monoclinal structure apparent enough ; the 

 back slope of the ridge descends to an uneven depression along the base 

 of the refreshed cliffs, while the outcropping face of the ridge descends 

 to a confused mass of jumbled mounds in which the weaker basal strata 

 are greatly disordered. The monoclinal slides become more nearly 

 continuous and gain a formidable volume near the southern corner of 

 the Paria about Jacob's pool; the harder strata that once formed the 

 lower bench of the cliffs have here dropped down several hundred 

 feet ; but their forward movement is much greater, often being the 

 better part of a mile. Passing around to the southeastern face of 

 the Paria. the lower part of the cliffs consists of Permian strata, and 

 the Shinarump sandstone (base of Trias) caps a conspicuous basal bench 

 and cliff, splendidly carved. Here the slides are of two types. If their 

 forward movement does not carry them over the edge of the Shinarump 

 i. they preserve the relatively orderly monoclinal form already 



