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THE - GEOLOGIST. 



blish that on mountain ranges the soil must be heated by the solar 

 rays to a greater extent than the air ; whereas in flat countries or 

 plains the contrary must take place. The theory indicates, and 

 experiment proves, that our atmosphere absorbs a considerable por- 

 tion of the heat which comes from the sun to the earth. M. Pouillet 

 estimates this quantity at four-tenths of the entire heat arriving at 

 the earth from the sun at any given moment. 



A sunbeam falling upon an elevated mountain-top traverses a much 

 thinner layer of atmosphere than one which falls upon a soil level 

 with the sea ; the former must therefore distribute more heat to the 

 summit of the mountain than the ray which continues downwards 

 until it reaches the level of the sea can bestow upon the soil of the 

 plain. 



But the rarefied atmosphere of the mountain-top is less heated than 

 the more condensed air of the plain. It remains evident then, that 

 the soil of a mountainous elevation, at its surface, and at some inches 

 below the surface, must each day be heated by the sun to a higher 

 degree than the air which reposes upon it ; whilst precisely the 

 reverse must take place on plains which are only a little above the 

 level of the sea. 



The correctness of this theory is demonstrated by certain observa- 

 tions made on the Faulhorn (Alps) in August, 1842, by MM. Bravais 

 and Peltier, and in September, 1844, by MM. Bravais and Ch. 

 Martins, when compared with corresponding data collected at Brus- 

 sels by M. Quetelet, and at Spitzbergen, in 1839, by the meteorological 

 commission attached to the expedition of the ship La Recherche. 



This relative elevation of the temperature of the soil exercises a 

 powerfal influence upon the physical geography of the Hautes-Alpes. 

 To it alone must be attributed the rise of the snow-line. Any 

 traveller who has visited these elevated regions knows that the snow 

 is melted underneath by the heat of the ground. Often he must have 

 remarked that when he placed his foot upon the border of a field of 

 snow, the weight of his body caused the superficial crust to break, and 

 observed that this crust does not repose upon the ground. Some- 

 times, under such icy vaults he will have seen with astonishment 

 flowering soldanella {Soldanella Alpina, L., and aS'. Gludii, Thom.) 

 and rosettes of dandelions ! It is this melting of the snow which is 

 in contact with the warmer soil that causes those immense fields of 

 frozen water to slide down the verdant slopes and form terrible 

 avalanches in the spring. Finally, to the warmth of the soil in these 

 high regions must be attributed the presence of so great a variety of 

 vegetable species, and such numbers of plants, which cover the soil at 

 the very limits of perpetual snow. On the conical summit of the 

 Faulhorn, at a height of 8,800 feet above the level of the sea, M. 

 Charles Martins collected 131 species of phanerogamic plants. At 

 the Grands-Mulets, on peaks of schistous protogine which rise from 

 the centre of the glaciers of Mont-Blanc, 10,000 feet above the sea, 

 19 species were observed : — Draha Fladnizensis, Wulff". ; Cardamine 



