1901] VEGETATION OF WESTERN TEXAS i rg 
masses. These various phases in the process of wearing away 
the former plain and leveling up the denuded areas constitute 
the edaphic conditions which determine the general types of 
vegetation formation prevailing in the several provinces. These 
might serve almost equally well as designations of vegetation 
provinces. The provinces here referred to are the following: 
the Rio Grande plain; the Great plains region, embracing the 
Edwards plateau, the Grand prairie, the granite area, the Car- 
boniferous area, the Red beds prairies, the Staked plains, Toyah 
basin, and the Stockton plateau; the south plateau of the Rocky 
mountains, embracing isolated mountain masses and cafions, 
grass plains, and bolson deserts. 
PLANT FORMATIONS. 
The classification of plant formations emploved in the fol- 
lowing pages is based chiefly upon local conditions of soil, geo- 
logic structure, and physiographic features, that is the forma- 
tions are edaphic. For example, rock formations, forest forma- 
tions, and salt basin formations exist because of local soil 
structure or content. In the case of grass formations, climatic 
factors, especially moisture, play an important part, not only in 
determining the existence of a grass formation as opposed to a 
forest formation, but also in determining the special association 
of species in the different formations. For example, although 
the physical structure of the Staked plains is most favorable to 
forests, such formations are naturally excluded by scantiness of 
rainfall; and in the Rio Grande plain the pigmy forest of chap- 
arral succeeds the dense mesophytic forests of the Atlantic 
coast plain because the factor of moisture has suffered so great 
reduction. In every case the particular type of formation exist- 
ing upon a given local area depends upon the local condi- 
tions of physiography and geology. For example, of the forest 
formations the post oak type is always present upon sand and 
gravel beds. The streamway cafion, hill bluff, and escarpment 
forests are all products of the soil conditions prevailing where 
they occur; for, although their differences are due to differences 
