

1907] 



YO UNG—F021EST 



329 



group of plants of definite composition as to species, occupying a 

 definite habitat as to the physical characteristics of the latter, or a 

 definite area as to geographical position. A "society" is a definite 

 group of plants as to component species occupying a secondary 



position within a formation. The "characteristic species" of a 

 formation are those which dominate the area occupied by that for- 

 mation. Thus the "characteristic species" of the "cottonwood- 



i 



Fig. 4. — The Picea-Abies formation with a glacial pond in middle foreground. 



willow formation" of the lower canons and plains are Popuhis 

 occidentalis and Salix fluviatilis. Within th's ''formation" we find 

 the Crataegus and Corylus ^'societies."^ 



Composition of the forests 



The forests of Boulder County may be classed under the fol- 

 lowing formations: (i) Populus occidentalis-Salix fluviatilis forma- 

 tion; (2) Populus angustifolia-Salix Nuttallii formation; (3) Pinus 



1 1 have given these definitions for the sake of clearness, but with no claim of 

 originality. The nomenclature of the paper is that used by Rvdberg (fip. cit.). 



