METEOROLOGY. 97 
Walker Filibustering Expedition, where sheltered from the wind, are still 
visible in the deserts in the northern part of the peninsula after half a 
century. 
The seasonal distribution of the rainfall in a region in which the normal 
precipitation bears a low ratio to evaporation may not only exert a 
marked influence on the character of the vegetation, but may also operate 
to produce desert conditions and make possible the support of xerophytic 
vegetation only, in districts with a large amount of rainfall. Thus, if the 
greater part of the precipitation should occur during a season of low tem- 
perature or in the quiescent period of the native species, the resulting 
dryness of the growing season would result in desert conditions. Such 
effects are most marked in regions in which the surface layers of the sub- 
stratum consist of loose material not capable of retaining water in sufficient 
quantity for moisture-loving species. A striking example of this feature 
is offered by the area around Crater Lake, Oregon, as described by 
Coville. The surface layers in this locality consist of powdered pumice 
apparently almost devoid of humus, from which water drains with extreme 
rapidity. Snowfall to a depth of about ro feet occurs in winter, but after 
this melts the soil becomes extremely dry and the plants capable of endur- 
ing the resulting drought show marked protective adaptations, the vege- 
tation consisting principally of such species as Phlox douglasit, Spraguea 
umbellata, and Arenaria pumicola.* 
The above factors must be taken into account in the interpretation 
of alpine districts in many parts of the world. The precipitation on 
mountain summits is very great, but in some instances it is in the form 
of snow, which melts and drains away very rapidly, leaving the humus- 
free soil extremely dry, while the air shows rapid alterations from high 
to extremely low relative humidity. An example of this character is 
offered by the summit of Agassiz Peak in the San Francisco Mountains 
of northern Arizona. A description of the meteorological conditions 
on this mountain will be found on pages 79 and 80. 
SOIL. 
The chief factor in the production of deserts is a lack of water as a 
nutrient substance for vegetation. Deserts may be produced as a result 
of other defective nutritive and mechanical conditions as well. Such 
conditions are to be found in areas in which the soil contains harmful sub- 
stances in injurious concentration in the soil, of which the alkali lands are 
familiar examples. Sterile areas, due to lack of nutritive material and 
water and to the unsuitable mechanical conditions of the soil, are offered 
by stretches of sand-dunes and plains in many parts of the world. In 


*F. V. Coville. The home of Botrychium pumicola. Bulletin Torrey Botanical 
Club, 1901, vol. 28, p. 109. The August vegetation of Mount Mazama, Oregon. 
Mazama, 1896, vol. 1, p. 170. 
