CH. XI. ABSENCE OF PERPETUAL SNOW. 291 



perpetual snow on this part of the Great Atlas. From its 

 position between the Grreat Desert to the south, the At- 

 lantic Ocean to the west, and the low country to the 

 north, it is obvious that a range of mountains from 11,000 

 to 1 3,000 feet in height must frequently be the seat of 

 violent atmospheric distmrbances. Whenever these draw 

 from the ocean currents of heated air, nearly saturated 

 with moisture, into the upper region, the cooling effect 

 consequent on rapid expansion must produce copious pre- 

 cipitation, and it is most probable that on the higher part 

 of the range this, even in the hottest season, takes the 

 form of snow. But, as we had seen, the snow melts with 

 extreme rapidity under the almost vertical sun during the 

 summer months ; and hence one traveller may have seen 

 the range thickly snowed even in the hottest season, 

 while another, with equal truth, may describe it as almost 

 completely bare. The state of things is such that a very 

 moderate change in the physical conditions might easily 

 lead to the accumulation of an annual surplus of unmelted 

 snow, which is the first condition for the formation of 

 glaciers. A mere increase in the amount of precipitation, 

 with little change in the general conditions of temperature 

 of this region, might produce glaciers reaching as low 

 down as that whose moraine we saw at the head of the Ait 

 Mesan valley. 



Many early-flowering plants were already withered, 

 but we collected on the mountain several interesting 

 species. Of two tall and very distinct Resedas found 

 here, one is also a native of Spain ; the other, R. elata, of 

 Cosson, was first gathered by M. Balansa, and seems to be 

 confined to this district. Of another curious plant dis- 

 covered by the same active naturalist we now first saw 

 satisfactory specimens. It is at first sight scarcely to be 

 distinguished from a species characteristic of the hot and 

 dry region of North Africa — the Cynara acmdis of Lin- 

 naeus. The latter was discovered by Tilli, a Florentine 

 physician, afterwards professor of botany at Pisa, who was 



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