CLASSIFICATION AND RELATl. 



305 



mixture) is here proposed for the designation of all mixed formations, 

 whether they arise from succession or from juxtaposition. Thus, the Ment- 

 selia-Elymus-tiuctifiin is the transition between the Mcittselia-Pseudocymop- 

 ierus-chalicinm and the Elyinns-Muhleiibcrgia-cJwlicium. Similarly, the 

 Poptihis-Picea-miciiuni and the Pi)ius-Pscudolsiiga-iuictiuin are transition 

 stages in the development of the Picea-hyliuni. On the other hand, the And- 

 Topogon-Bulbil'.s-inictUun is a mixture produced by the mingling of two 

 contiguous prairie formations. In the future development of this subject, it 

 will probably become desirable to name mixed formations on the basis of 



Fig. S2. A mixed formation of aspens and spruces (Populus-Picca- 

 mictium), preceding the final spruce forest of a burn succession. 



origin, but at present this is unnecessary. Both in classification and in de- 

 scription they should be considered between the formations which give rise 

 to them, and this will at once indicate their origin. 



Puzzling cases of mixture resulting from position occur toward the limits 

 of facies which occupy extensive areas. Bout dona oligostachya, and Aiid- 

 ropogon scoparius extend from the prairies through the sand-hills and plains, 

 and into the foot-hills of the Rocky mountains. Their abundance at once 

 raises a question as to the validity of the prairie, sand-hill, plain, and foot- 

 hill formations. If these two grasses were controlling, and equally charac- 

 teristic throughout, then the entire stretch would have to be regarded as a 



