282 
THE GEOLOGIST. 
blisli that ou 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 considerai)le 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 Mis 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, 184:2, by MM. Bravais 
and Peltiei-, 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 18.39, by the meteorological 
commission attached to the expedition of the ship La Recherche. 
This i-elative elevation of the temperature of the soil exercises a 
powerful 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 (SoldaneUa Alpina, L., and S. Cludii, 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, 1 0,000 feet above the sea, 
1 9 species were observed : — Draba Fladnizensis, WulfF. ; Cardamine 
