1907] 



YOUNG— FOREST FORMATIONS OF COLORADO 



335 



as a secondary form in the preceding formation. The upper limit 



Popuhis occidentalis i 

 Populus angustijolia 



m 



At this altitude it is replaced 



Like the preceding, this formation lacks continuity, the character- 

 istic species only occasionally exercising control, and being interrupted 

 in places by the Pseudotsuga-Picea formation. In scattered associa- 

 tion with Populus angtisti folia and Salix NiittaUii are Alnus tenuijolia^ 



f 



t 



Fig, 6. — A hawthorn society, 



Jontinalis 



tains an occasional Salix Fendleriana. These shrubs form a straggling 



mountain 



bottoms and may be occasionally found 

 (\f^^ in fRvorpd lorRh'ties, Two under la 



one of flowering plants, the other of mosses, livenv^orts, and lichens, 

 are usually present; but these layers, as well as the upper, lack con- 



very 



open. The herbaceous, or middle layer, is very 

 mixed and holds no definite level, owing to the varying height of its 

 constituent species. The frequent presence of a highway in the 



