CLASSIFICATION AND RELATIeL.... -. 305 
mixture) is here proposed for the designation of all mixed formations, 
whether they arise from succession or from juxtaposition. Thus, the Ment- 
zelia-Elymus-mictium is the transition between the Mentzelia~-Pseudocymop- 
lerus-chahicium and the Elymus-Muhlenbergia-chalicium. Similarly, the 
Populus-Picea-mictium and the Pinus-Pseudotsuga-mictium are transition 
stages in the development of the Picea-hyliiun. On the other hand, the And- 
ropogon-Bulbihs-mictium 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. 82. A mixed formation of aspens and spruces (Populus-Picea- 
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. Bouteloua oligostachya, and And- 
ropogon scoparius extend irom 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 
