258 H. L. FAIRCHlLl)— GEOLOGY UNDER PLANETESIMAL HYPOTHESIS 



salt deposits, are unreconcilable under the old hypothesis. The new 

 hypothesis, on the other hand, gives a satisfactory explanation for both 

 classes of phenomena. 



Glaciation 



It is beyond the plan of this paper to discuss at length the phenomena 

 of glaciation under the new hypothesis, since it has already been trav- 

 ersed by our leader in this field. Briefly it may be said that glaciation 

 is not the remarkable and unusual phenomena it was once thought to 

 be, but may have occurred even in early geologic time. The conditions, 

 geologic and meteoric, requisite for glaciation do not seem to be extraor- 

 dinary. Glaciation is essentially a local phenomenon — a fact which 

 was not formerly appreciated. Cyclonic circulation of the atmosphere, 

 which localizes and concentrates the precipitation, with just sufficient 

 cold to produce snow instead of rain, may initiate a snow field, the per- 

 petuation and extension of which is an ice body. A cause of general low 

 temperature would seem to be the reduction in amount of atmospheric 

 carbon dioxide. The extent and altitude of the land masses probably 

 have an indirect influence by determining the great barometric areas and 

 the paths of cyclonic storms. It is possible that the attitude of the earth 

 toward the sun may have some effect, and that the precession of the 

 equinoxes might, at a critical time, help to produce the alternation of 

 glacial with interglacial epochs. It is probable that glaciation is not a 

 simple effect, but the product of the interaction of several factors, atmos- 

 pheric, geologic, geographic, and astronomic, and all these so delicately 

 balanced that any slight change may cause great effects. 



We are living in a glacial period. The waning glaciers of the Pleisto- 

 cene are still found in every quarter of the globe ; but within a genera- 

 tion the northern glaciers have shrunk in conspicuous degree, although 

 the meteorologic data have given no certain hint of climatic change. It 

 may be that decrease in snowfall is a greater factor in the shrinkage of 

 the Alpine and Alaskan glaciers than increase of temperature, and it is 

 probable that climatic changes which would be competent to produce a 

 rapid growth of the existing glaciers might occur and yet be so imper- 

 ceptible as to be undetermined by less than a century of accurate obser- 

 vation. 



Diastrophic Movements 



That mountain systems have been formed by lateral or tangential press- 

 ure and consequent crumpling of the strata is a fact of observation. 

 The apparent cause of the mountain-making compression is shrinkage of 



