i9o8] COOPER— ALPINE VEGETATION IN COLORADO 327 



from that agent afforded by the canon walls. The difference in alti- 

 tude between the two habitats, averaging about 610"^, also favors the 

 canons, since atmospheric rarity tends to increase evaporation. 



Ground water.— The smooth polished floor of the canons has 

 already been described. In some parts the surface slopes gently and 

 evenly; in others there are low rounded prominences {rocJies moidon- 

 nees) with depressions between. The greater part of this region is 

 dry during all of the growing season, since the surface drainage from 

 the snow banks above is the only source of moisture that need be taken 

 into account. The small amount of rainfall during the growmg 

 season may be neglected altogether, since the smooth rock surfaces 

 cannot retain it, and it very soon evaporates or joins the dramage 

 channels from the snows. Snow accumulates at the bases of the 

 upper cliffs in large quantities, and many of the drifts remain through 

 the summer, affording by their melting a constant supply of water 

 to a restricted portion of the area below. The drainage channels from 

 the various permanent drifts form a network over the canon floor m 

 following the principal depressions among the roches moutofinees 

 Some cases were observed where the water flowed in a broad sheet 

 for long distances over a smooth gently sloping rock surface. 



Sources of soil maler ids. -The soil materials which render possible 

 the establishment of the pioneers of the higher plants are dem ed 

 principallv from two sources : rock decay and wind-blown dust. Kock 

 decay has made verv little progress in the canon heads since glacial 

 times. Since the process is conditioned by the presence of moisture, 

 the larger part of that which has been done and is now being clone 

 is within the sphere of influence of the drainage channels. As na 

 been said in describing the physiography of the region, decomposmo 

 is going on principally along joint planes, and the tine pr«duc s ^^ 

 this process are either deposited in the cracks near to where int v 

 produced or are carried down by flowing water and deposited in 



situations. ' 1 iv „ f^ tlip 



I believe that wind-blown dust is an important addition ^ the 

 scanty supplv of soil material available to the pioneers. Its so ^_^^ 

 doubtless is largelv the dr^• meadows above the canons, ^v J^^.^^^^^^=' 

 the summer large areas of sparsely covered ^^^^^^^^^^ ^^ ,^ried 

 Some of the dust from this and other sources is doubtless 



