ig 19] VESTAL— PHYTOGEOGRAPHY OF COLORADO 171 



flowing streams and their tributaries. On it rests the highland of 

 the Spanish Peaks, and it is ribbed by resistant dikes of igneous 

 material from two outflows, one set radiating downward from the 

 peaks themselves. With the plateaus should be classed the high 

 lava-capped mesas of the mountain-front and plains in the area 

 near the Colorado-New Mexico boundary. 



As in the hogback ridges, vegetation distribution in the plateau 

 and butte areas is largely determined by soil texture and topog- 

 raphy. Atmospheric conditions vary with exposure to wind and 

 sun. The tops of the plateaus are covered with short-grass and 



4 -\ " li •*•'-<•"„'' 







Fig. 12. — Unbroken short-grass ground cover in plains 



mixed grassland over the level upland stretches of comparatively 

 fine-textured soil. Exposed cliffs and crests, and rocky debris- 

 slopes, afford lodging places for woody xerophytes (Cercocarpus, 

 rock pines, pinyons, and cedars), with primitive grassland as the 

 general ground-cover. The deeper and shaded parts of canyons 

 and ravines approach a mesophytic condition, with mixed shrub 

 and woodland vegetation. 



PLAINS 



Plains topography is typically flat or gently rolling country, 

 with fine clay soil from a soft-shale substratum. Short-grass is 

 the characteristic vegetation (fig. 12). Where the substratum is 



