ioio] 



VESTAL— PHYTOGEOGRAPHY OF COLORADO 



167 



levels. They are described in the accounts of Lee (6) , Johnson (5) , 

 Fenneman (2), Shantz (15), Ramaley, Robbins, and Dodds (12), 

 and Vestal (17). The topographic parts of a mesa are: (1) the 



':, Hi 





Figs. 6, 7. — Climatic transition at mountain-front: fig. 6, outer mountains just 

 south of Boulder, seen from university campus; clouds beginning to form at summit 

 of Green Mountain, while much of South Boulder Peak, at extreme left, is already 

 obscured; snow covers the mountain slopes and fades out toward base of high mesas; 

 roofs of distant buildings also white; fig. 7, practically same view, a little later, with 

 upper slopes obscured; at one time it began to snow on mountains and upper mesas, 

 and a few minutes later to rain in town; shortly afterward it changed to snow in the 

 upper edge of town, so that the roof of the building with the short steeple at the right 

 in midground, and of nearer houses, were well whitened, while rain still fell on the 

 campus, less than half a mile away, and not more than 50 ft. lower; difference in ele- 

 vation at mountain-front is critical as regards climatic change. 



mesa-top, with flat surface covered with mixed rock-debris; (2) the 

 edge or mesa-crest; (3) the side-slope; and (4) valleys or draws in 

 the side-slope. The soil conditions and their effects on plant dis- 

 tribution have been discussed in the three articles last cited. 



