HENDERSONI 



KOBBDis J CLIMATE AND EVIDENCE OF CLIMATIC CHANGES 67 



of the southern Rockies are nearly balanced, the latter being slightly 

 in excess. An increase of a very small percentage in the snowfall 

 would check the waste and a further very slight increase without the 

 least change in mean temperature would cause the glaciers to extend 

 down the canyons. Even an infinitesimal excess of snowfall is suf- 

 ficient, if time enough be allowed for the accumulation of annual 

 excesses, to produce results. The mean temperature of some of our 

 higher peaks which bear no glaciers is said to be below that of some 

 regions which are supplied with glaciers, a condition which indicates 

 that the thing lacking is precipitation, not low temperature. 



It is important to observe that the glaciation of the Rockies was 

 recent as compared with the great Continental glaciers; that the 

 evidence shows very recent retreat, apparently still continuing; 

 that at least the latter part of the Bonne\TJle and Lahontan lake 

 period coincided with the existence of mountain glaciers, as shown 

 by Russell and Gilbert ; that both the lakes and the ghiciers had two 

 distinct periods of great extension, whose curves coincide in character, 

 these curves having been platted by Russell and Gilbert. 



The suggestion long ago made that the desiccation of the region 

 may have been caused by the cutting of the canyons, which estab- 

 lished more complete drainage, can not be considered in the present 

 connection, because the canyons were cut almost to their present 

 depth, and thus complete drainage was established, before the con- 

 struction of the buildings whose ruins form the chief evidence of 

 human occupancy of the region. 



The extinction of great mammals, as the mammoth, camel, etc., 

 and the change in the molluscan fauna in the Southwest have also 

 been urged as evidence of desiccation, but these faunal changes have 

 not been very recent compared with the ruins of human habitations ; 

 moreover, in case of the mammoth it has befcome extinct, also in 

 regions where there is no paiticular reason for supposing any marked 

 • desiccation has taken place recently. 



The distribution of terrestrial mollusks m the Southwest indicates 

 that the intermontane spaces have not been moist enough for migra- 

 tion of land snails during a period of sufficient length for each iso- 

 lated mountam area to develop its own peculiar forms, especially of 

 such genera as Sonorella, Ashmunella, and Oreohelix} This practical 

 isolation of snails could have resulted, however, even wnth the pre- 

 cipitation enough greater to permit of raising hardy, drought-resisting 

 varieties of corn \\dth a very small percentage of failures, in many 

 favorable localities where it is not practicable now. 



1 Pilsbry, Henry A., and Ferriss, J. H., Mollusca of the Southwestern States, in Proc. Acad. Kat. 

 Sci. Phila., Lvii, 211-90, 1905. See also same publication, Lvm, 123-75, 190s; l.xi, 495-518, 1909; especially, 

 LXIl, 44-147, 1910 (pp. 46-47). 



