CONSlbElRATION OP HYPOTHESES 279 



changes. This fact demands for its explanation a cause sufficiently 

 powerful to start such a rapid wave. 



HYPOTHESIS OF CLIMATIC CAUSES 



In seeking an explanation for the advance of a glacier, one naturally, 

 first of all, considers climatic causes. In the attempt to apply this ex- 

 planation to the case in hand, two serious difficulties have arisen. In the 

 first place, no siich profound result from climatic variation has ever been 

 noticed before. In the second place, it seems impossible that the amount 

 of additional snowfall required to start such a tremendous wave of ad- 

 vance could be supplied by seasonal variations, for it would mean a sud- 

 den and great increase in the snowfall, interrupting a half century of 

 fairly uniform conditions. There is no direct evidence against this 

 hypothesis, for we have no rainfall records in this region ; but nevertheless 

 we seem warranted in dismissing it merely on the basis of improbability, 

 for such a profound, sudden change in climate would of itself be more 

 remarkable than the phenomenon which we are called on to explain. 



HYPOTHESIS OF POSSIBLE UPLIFT 



A second hypothesis that has been considered is that of uplift. Con- 

 cerning this, also, we have no definite facts tp advance; but against it 

 may be argued the improbability of so decided a change in so short a 

 time as to profoundly disturb the glaciers radiating from a mountain 

 center. To produce such an effect, an uplift would need to amount to at 

 least hundreds of feet and to take place in a brief interval of time. In fact, 

 since the glaciers all head in lofty, snow-covered mountains, up whose 

 slopes damp ocean winds rise and on which the snow-cover descends to 

 within two or three thousand feet of sealevel, it is very doubtful if the 

 amount of snowfall on the mountains would be greatly increased, even if 

 there had been a sudden uplift of hundreds of feet. This hypothesis is so 

 utterly improbable that it also may be dismissed without further con- 

 sideration. 



HYPOTHESIS OF CHANGE OF GRADE 



We gave consideration to the hypothesis of change of grade in the valley 

 glaciers as a result of alterations of level accompanying the earthquake of 

 1899. This explanation also lacks probability, for a change in grade 

 sufficient to transform a nearly stagnant valley glacier to one as badly 

 crevassed as an ice-fall is wholly unlikely. Moreover, in the case of 

 Atrevida and Variegated glaciers at least, the grade was essentially the 



