ro1o] ROBBINS—VEGETATION IN COLORADO 257 
country having an elevation of 6000-10,000 feet and inclosed by high 
mountains; hence they are very dry and the vegetation is more 
xerophytic than that growing at the same altitudes in uninclosed 
areas. There are parts of western Colorado, however, where xero- 
phytic conditions exist at high altitudes, the dryness in this case being 
due to the unequal seasonal distribution of precipitation. Western 
Colorado is for the most part high; high, flat tablelands and mesas, 
Fic. 1.—General topographic features of the state; east of the foothill line lie 
the great plains, 
dissected by narrow steep canyons, are characteristic. Many of the 
Minor mountain ranges have an east and west trend. 
An important topographic feature of the eastern part of the state 
is the Arkansas-Platte Divide, extending plainward from the foothills 
in the middle of the state. Foothill forms of vegetation have an 
fastward extension on this divide. It also marks the northern and 
southern limit of some species, such as the pinyon pine (Pinus edulis 
Engelm.), the chandelier cactus (Opuntia arborescens Engelm.), 
and Populus Wislizenii (S. Wats.) Sarg., which do not occur north of 
