PAPERS ON GEOLOGY AND GEOGRAPHY 213 



slopes of the LaPlatas to the northeast is marked even more 

 strongly. These slopes less than twenty miles away have suffi- 

 cient rainfall to produce a luxuriant growth of vegetation. The 

 mesa lies, therefore, on the borderline of arid and humid cli- 

 mates, the line separating the two, shifting according to the 

 amount of precipitation for each year, which is extremely var- 

 iable. In 1911 the precipitation for this region was more than 

 25 inches, as heavy as anywhere in the state, while in 1912, 

 it was mapped as having less than ten inches, as light as any- 

 where in the state". 



This great variation in the annual precipitation has a ten- 

 dency to increase the amount of erosion over what it would 

 be, did the rainfall each year more nearly approach the mean 

 of 16.88 inches. The type of vegetation is determined by the 

 driest years and not by the mean of precipitation for a series of 

 years. With a rainfall of less than 10 inches in certain years, 

 only vegetation with special provision for resisting drouths can 

 survive. Such a vegetation has little value as a protective cover- 

 ing, neither is there, under such conditions, an accumulation 

 of humus to prevent rapid run off or to aid in the disintegra- 

 tion of the rock. 



The occasional heavy annual precipitation in this type of re- 

 gion does not increase the value of vegetation as a protective 

 covering, but produces a degree of erosion which is far in ex- 

 cess of the normal for either a humid or an arid desert. Desert 

 conditions for plant life are still further enhanced by the major 

 part of the rain falling during the latter part of the growing 

 season on a hot, parched earth when evaporation is extremely 

 rapid. Again, the major part of the rainfall of the growing 

 season may be concentrated in a single shower near the close 

 of the growing season. These infrequent and violent down- 

 pours lead to a minimum of plant protection and give a maxi- 

 mum efficiency of the running water. The possibilities for 

 erosion under such conditions are many times what the mean 

 rainfall would seem to indicate. It seems hardly necessary, as 

 is frequently done, to postulate more humid conditions to 

 account for the unusual amount of erosion everywhere in evi- 

 dence through the mesa. T 



•U. S. Dept. of Ag. Weather Bur. Col. Sec. Ann. Sum. for 1911 and 1912, p. 11. 

 T Atwood, W. W. : Annals of the Assoc, of Am. Geographers, Vol. I, p. 100. 



