Chap. IX] 



DESERTS 



645 



the horizontal and vertical directions, and it may with certainty be 

 assumed that adequate meteorological data are still wanting in regard to 

 the driest uninhabited districts. Thus Hann wrote regarding a district 

 in which extensive deserts occur : ' In the neighbourhood of the Peak 

 of Orizaba, the north-east trade-wind prevails in summer, the land for 

 eighty kilometers to the south-west is dry and dusty, a shower of rain 

 falls only occasionally ; on the eastern side of the Peak, however, it rains 

 every afternoon. . . . During winter hardly any rain falls, during summer 

 it is plentiful, above 500 meters, except on the south-west side of the 



Fig. 385. North American desert. Yucca glauca on a high rocky plateau in West Nebraska. 

 P"rom a photograph by the Geological Department of the University of Nebraska. 



mountain. From the coast up to 500 meters the land is a steppe and the 

 vegetation poor.' 



To the varied climate of Mexico there corresponds just as great variety 

 in the composition of the vegetation. Those slopes of the mountains which 

 enjoy a rich rainfall bear high-forest of varied type ; the dry slopes and 

 high plateau, on the other hand, bear chiefly xerophilous thorn-woodland 

 with numerous succulent plants ; with increasing dryness of climate, these 

 formations pass over into desert. Naturally a sharp boundary line cannot 

 be drawn, and transitional formations of semi-desert occur in all its stages. 

 Edaphic influences, in any case, play an important part in the differentiation. 



