280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 



Local surface evidences indicate that heat, in some places at least, 

 is being absorbed at the base of permafrost faster than it is being 

 dissipated at the surface (Hopkins, 1949; Young, 1918). Hence the 

 cold reserve is being lessened, and the thickness of permafrost is de- 

 creasing from the base upward. 



The mean annual air temperature required to produce permafrost 

 undoubtedly varies many degrees because of local conditions. Gener- 

 ally it is given as 30° to 24° F. ; theoretically permafrost can form 

 above 32° F, (Theis, unpublished manuscript), and apparently it is 

 doing so locally in parts of southwest Alaska where poor drainage, 

 abundant vegetation, cloudy summers, and low insolation are found 

 (S. Abrahamson, oral communication, and Ernest H. MuUer, written 

 communication) . 



The relative effects of past climates have been inferred qualitatively 

 through a study of present temperature profiles and indirectly through 

 a study of past deposits, pollen analysis, vegetal changes, structural 

 soils, and blockfields. 



The origin of large, clear ice masses in the permafrost is a special 

 problem in itself. Numerous theories are extant, and one or more may 

 apply to a particular mass of ice (Taber, 1943a; Leffingwell, 1919; 

 and others) . 



GEOLOGIC RAMIFICATIONS 



Throughout the Arctic and sub-Arctic the role of permafrost is ex- 

 tremely important. As an impervious layer in continuous perma- 

 frost zones, it exerts a drastic influence on surface waters, completely 

 prevents precipitation from entering the natural ground-water reser- 

 voirs, and commonly causes a concentration of organic acids and of 

 mineral salts in suprapermaf rost water. In discontinuous permafrost 

 zones, and less so in areas of sporadic permafrost, ground-water 

 movements are interrupted or channelized. Quality of water, too, can 

 be materially affected by the storage for centuries and subsequent re- 

 lease by thawing of organic and inorganic materials (Kaliaev, 1947). 

 In fact, our present conceptions of ground-water reservoirs, ground- 

 and surface-water movements, infiltration, quality of water, and so on 

 must be reevaluated in considering permafrost as a new geologic 

 formation, generally not uniform in composition or distribution, that 

 transcends all rock and soil formations. Furthermore, it must be 

 considered as much in the light of past as of present conditions. 



It is well known that in cold climates physical disintegration (frost- 

 splitting, congelifraction) plays a more important role than chemical 

 weathering. The repeated freezing of water-saturated materials and 

 the growth of ice crystals in numerous small pores, cracks, joints, 

 cleavage planes, or partings is by far the most effective destructive 

 process. Taber (1943a) has shown that, without water, disintegration 



