1906] SHANTZ—VEGETATION OF THE MESA IgI 
been in use for a longer or shorter time they are generally aban- 
doned. The soil is hard, in fact has never been broken, but since 
there is no vegetation, an opportunity is afforded for the entrance of 
new plants. These trails have not been used in wet weather, and 
they are therefore never cut up and no loose soil is formed. The 
succession here is first ruderals like Boebera papposa, Amaranthus 
‘blitoides, or Verbena bracteosa. These seem to be most successful 
Fic. 12.—Trail invaded by Boebera papposa; Bouteloua formation at the sides. 
mvaders of such trails. The grasses of the formation come in slowly, 
Muhlenber gia gracillima generally in advance of, or with Schedon- 
nardus paniculatus, Sitanion elymoides, Athero pogon curtipendulus, 
and ultimately Bouteloua oligostachya, It is not an uncommon thing 
to find these old trails only distinguishable by the depression of 
Surlace and completely covered by the Muhlenbergia gracillima 
Consocies. Where the same trail leads through the purer growth of 
Boutelowa oligostachya, it is not so rapidly covered, and when it 
pésses through the Bouteloua hirsuta consocies it remains open for a 
Stl longer period, 
