THE GLACIAL THEORY. 



5 



preceding, extends to 3,200 feet ; the large glacier of Aletsch, the longest 

 of all those in Valais, descends 4,000 feet." Agassiz. 



Large patches of ground, covered with verdure, are often found en- 

 circled by glaciers, at an elevation of 7,000 feet.'* Agassiz. 



*' Glaciers occur in vallies of the temperature of +5°." (Reamur.) 

 Agassiz. 



" Atmospheric temperature is an essential agent in the formation of 

 glaciers, their extension and movement." Agassiz. 



" The temperature of the ice constituting the lower glacier of the Aar 

 is constantly below zero." Agassiz. 



" The result of all examinations which I have made as to the tem- 

 perature of the small streams of water occurring in the glaciers of 

 Chamounix, Trient, Aar, Aletsch, Zermatt, St. Theodule and Zmutt has 

 established it at 0'', so long as they flowed over the glaciers ; but as soon 

 as their courses were through the gravel beds, the temperature varied 

 from +0.1. to +0.7." Agassiz. 



We are desirous of placing the observations of careful 

 naturalists before our readers, in this and all other instances, 

 where such a course can be adopted, in order that they may 

 form their own ideas apart from the opinions expressed by 

 other geologists, or by ourselves., and we consider this pre- 

 liminary remark necessary on that account. 



By glaciers then^ we are to understand those sheets of 

 ice which occur in the valleys, and on the slopes of higbly 

 elevated districts and mountains, at variable altitudes, and 

 under different circumstances. Their substance differs very 

 considerably, as we may learn from the observations alluded 

 to, as also that they are formed of snow in various 

 stages of congelation, produced by the vicissitudes of climate ; 

 this is proved by the bands of snow and ice in the glaciers of 

 Mount Rosa, Mont Blanc, and St. Theodule the an- 

 gular fragments mentioned by Agassiz, and the gradual 

 transformation is traced not only in the occurrence of these 

 bands, but in the very clear and well-defined example of 

 the glacier of the Gruenhorn. 



Their origin is a separate question \ from the above facts, 



