CLIMATE AND TERRESTRIAL DEPOSITS 173 



surfaces, and therefore typically in rugged mountainous regions, 

 this must result in a more rapid disintegration of the naked rock 

 masses and an increased supply of talus to the streams. Under the 

 small precipitation postulated, however, the streams will be only 

 slightly increased in volume by the decreased evaporation, and 

 presumably not able to carry away the excess of load. The weaken- 

 ing of the vegetative covering over the soil-covered slopes would work 

 to the same end, but in a desert region this factor would be absent. 

 The tendency of the increased disintegration would be to build up 

 piedmont slopes, whose rate of growth would diminish or even cease 

 upon a return to less rigorous winters. The effects of increased 

 cold in many cases may, therefore, disturb the balance of erosion 

 to transportation, in the same way as a change toward more marked 

 aridity without increased cold. 



The Gila conglomerate 0} Arizona.'^ — As an example of a Pleistocene 

 formation which it was thought might be due to some such cause, 

 the writer has examined the literature on the Gila conglomerate of 

 Arizona, a formation now dissected in many places to the depth of a 

 thousand feet and attaining its maximum development in the upper 

 portions of the valleys of New Mexico, Arizona, and southern Cali- 

 fornia. The specific nature of the climatic or tectonic changes which 

 could have resulted in its production does not seem to have been 

 fully discussed, the only definite opinion expressed being that of 

 Lee that in so far as climate was a factor in the accumulation of this 

 upland debris in southern California and Arizona, it was in the 

 nature of a desiccation.^ Others consider that their Pleistocene age 

 and the finding in New Mexico of contemporaneous elephant, horse, 

 and tapir bones are an indication of the accumulation of similar 

 New Mexican deposits during an epoch of moist climate in the early 

 Pleistocene. 3 



An examination of the literature showed that the relations of the 

 two divisions of the Gila conglomerate, the volumes and relative 



1 The writer hopes to publish a fuller discussion of this subject than can be given 

 here. 



2 "Underground Waters of Salt River Valley, Arizona," Water Supply and Irri- 

 gation Paper No. Ij6, U. S. Geological Survey, 1905, p. 115. 



3 George B. Richardson, Science, New Series, Vol. XXV, 1907, p. 32. 



