Deflation in Colorado Canyon. . ,175 



find a hold; with dull rumbling the blocks loosened by our feet 

 from the edge of the abyss tumble down into the vast depth. 

 The upper edge is cut sharp as with a knife ; nowhere do we see 

 a rivulet descending ; nay, the plane of the plateau slopes so 

 steeply away from the edge that even in heavy rainfall no tor- 

 rent could tumble over it — the whole region is drained toward 

 the land, away from the gorge. 



Like ail enigma these amphitheaters now appear to us ; all the 

 more so when we learn from ^Nlajor Powell, the famous explorer 

 of the canyon, how rarely it rains here. Major Powell had not 

 thought it necessary to take along tents for us, and when in one 

 bad, stormy night we lay shivering around the smok}^ fire, un- 

 protected against the hail and the streaming rain, not a few of 

 us must have received the impression that we were in a region 

 of large precipitation; and yet that storm was an unforeseen 

 exception. But even if rain were more frequent than it is, still 

 on account of the topographic conditions of drainage the amphi- 

 theaters would be affected only by the erosion of that water 

 which falls on their burface. Bursts of rain from thunder-storms 

 generated within the canyon, such as were graphically described 

 to us by Major Powell, may indeed wash down all the debris 

 which has become loosened in the course of the rainless period, 

 but nevertheless the denuding eff"ect of water can there be only 

 secondary. 



But where is the force that carved such amphitheaters ? 

 Where is the cause of so singular forms of denudation? Again 

 we stand face to face with the question whether in these kettles 

 denudation is at rest during the 300 rainless days. If we ob- 

 serve with unbiased minds what are the forces that act during 

 these 300 rainless days, we see dry air and (\xy heat exerting their 

 destructive influence on the rocks. The eye trained for such 

 processes recognizes that the heat of the day, alternating with 

 .the cold of the night, may produce the same effect as fissure- 

 frost in our latitudes. We ourselves experienced variations of 

 temperature of 30° C. Avithin the canyon. Thus insolation and 

 weathering penetrate into the rocks and break them, and the 

 wind carries off the light powder of the weathering. The harder 

 ledges of rock are undermined without the aid of eroding water, 

 hang over, and await the first rain-storm. Now comes one of 

 those rare cloud-bursts. The water everywhere finds loose debris 

 and shaky blocks. Thus the relatively small amount of water 



