192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the anticlinal valleys, and the monoclinal valley seem to be indifferent 

 witnesses as to changes of climate in the past. 



The Pluvial Equivalent op the Glacial Period. — There is much 

 probability that the rainfall over the Grand canyon district was in- 

 creased at those times when ice sheets and glaciers were formed over 

 other parts of the continent ; yet it seems to me difficult to find in- 

 dependent proof by purely local evidence that such has been the case. 

 Button, however, suggests that evidence of this kind is to be found 

 in the ravines of the Kaibab plateau, concerning which he finds " no 

 conjecture so satisfactory as that which supposes that during the glacial 

 period the rainfall was sufficient to sustain living streams in these 

 ravines and that they were then carved by running water" (c, p. 196). 

 The same conjecture is thought to hold true for the ravines of the Paria 

 plateau (c, pp. 202, 228). Nevertheless, it is difficult to find valid 

 reason for ascribing the erosion of these ravines only to the glacial 

 period, for it seems admissible to conclude that they can have been in 

 large part eroded by the intermittent streams of a climate as arid as 

 that of to-day. The maturity of many of these ravines in resistant 

 strata indicates that, besides glacial time, some of preglacial and all of 

 interglacial and of postglacial times must have been needed for their 

 erosion; hence instead of ascribing theui to the glacial period alone, 

 their beginning may be carried back even into the later part of the 

 cycle of the great denudation, as has already been suggested on page 

 141. Moreover, existing processes do not seem to be inoperative. 

 The stream beds in the lower ravines of the Kaibab bear every mark of 

 being strongly flushed by occasional wet-weather floods. Well-defined 

 fans of coarse and fine waste extend forward from the mouth of the 

 ravines at the eastern base of the Kaibab into House-rock valley, 

 as has been already stated ; and a torrent fan was seen at the western 

 base where the road from Jacob's lake descends toward Fredonia. 

 However slow the erosion of the ravines may now be, it is still going 

 on ; indeed, the scantiness of vegetation makes the removal of waste so 

 easy that it may be questioned whether rain and stream work is not 

 advancing here at a comparatively rapid rate, — a rate that very likely 

 far exceeds that which prevails in the peat-covered uplands of much 

 moister regions. 



Volcanic Phenomena. 



Our interest was frequently attracted by magnificent displays of vol- 

 canic phenomena in various parts of the Grand canyon district, but we 

 had no time to turn from our route to study them. A few notes on 



