294 On the Physical Geography of the Alps. 



of the Alpine springs and the isogeo thermal conditions of 

 these mountains. 



3. On the Erosion and Weathering of the Surface of the Earth. 



After considering the hydrography of the Alps; in chapter 

 xii., our authors discuss two important topics, namely, the 

 erosion and the weathering of the surface of the earth, by 

 natural agencies. The present eroding effects of rivers, and 

 their relation to the formation of valleys in general, are 

 considered. The sudden emptying of large reservoirs of 

 vi^ater; notices of the course of the great flood (from the 

 bursting of the Vernagt Lake) in the year 1848, with a table, 

 showing the remarkable diff'erences between the time re- 

 quired for the passage through the valley of the great bulk 

 of the flood- water from place to place, and the usual rate of 

 the river-water passing the same places ; the influence of 

 valley-basins on the course of great bodies of water (as seen 

 in the table above referred to) ; the transport of blocks and 

 shingles ; and the collecting of the water of the Alpine lakes. 



The subject of weathering, or the mechanical disintegration 

 and chemical decomposition of rocks by means of the atmo- 

 sphere, and its precipitations, succeeds, and demandsnotices of 

 the physical properties of the earth and its composition, of 

 the nature and properties of humus, and of the influence of ve- 

 getation on the formation of the earth ; of the influence of 

 glaciers on the destruction of rocky materials, the formation 

 of sand, the transport of shingle, land and mountain slips, 

 and the movement of great masses of debris. 



The results arrived at from the study of these and other 

 numerous, allied, and subordinate subjects, connected with 

 aqueous and atmospheric erosive agencies, appear to be : — 

 1. The influence of the masses of " firn" and glaciers on the 

 Alpine streams is not confined to the increase, but extends 

 also to the various distribution of the water. 2. At a certain 

 depth all of the larger lakes have a nearly constant tempera- 

 tures, connected with the maximum density of the water ; the 

 vertical distance of this stratum from the surface varies ac- 

 cording to the mass of the water, the form of the lake basin, 

 and the season of the year. 3. The velocity of the mountain 

 streams, in comparison with the rivers of the plains, is not in 



