16 LANDS OF THE ARID REGION OF THE UNITED STATES. 
tains by storms, are frequently turned from their courses and divided into 
many channels running near the surface. Thus a subterranean watering 
is effected favorable to the growth of trees, as their roots penetrate to suffi- 
cient depth. Usually this watering is too deep for agriculture, so that for- 
ests grow on lands that cannot be cultivated without irrigation. 
Fire is the immediate cause of the lack of timber on the prairies, the 
eastern portion of the Great Plains, and on some portions of the highlands 
of the Arid Region; but fires obtain their destructive force through climatic 
conditions, so that directly and remotely climate determines the growth of 
all forests. Within the region where prairies, groves, and forests appear, 
the local distribution of timber growth is chiefly dependent upon drainage 
and soil, a subject which needs not be here discussed. Only a small por- 
tion of the Rocky Mountain Region is protected by climatic conditions 
from the invasion of fires, and a sufficiency of forests for the country 
depends upon the control which can be: obtained over that destructive 
agent. <A glance at the map of Utah will exhibit the extent and distribu- 
tion of the forest region throughout that territory, and also show what 
portions of it are in fact occupied by standing timber. The area of stand- 
ing timber, as exhibited on the map, is but a part of the Timber Region as 
there shown, and includes all of the timber, whether dense or scattered. 
Necessarily the area of standing timber has been generalized. It was 
not found practicable to indicate the growth of timber in any refined way 
by grading it, and by rejecting from the general area the innumerable 
small open spaces. If the area of standing timber were considered by 
acres, and all acres not having timber valuable for milling purposes rejected, 
the extent would be reduced at least to one-fourth of that colored. Within 
the territory represented on the map the Timber Region has an extent of 
18,500 square miles; that is, 23 per cent. belongs to the Timber Region. 
The general area of standing timber is about 10,000 square miles, or 12.5 
per cent. of the entire area. The area of milling timber, determined in the 
more refined way indicated above, is about 2,500 square miles, or 34 per 
cent. of the area embraced on the map. In many portions of the Arid 
Region these percentages are much smaller. This is true of southern Cali- 
fornia, Nevada, southern Arizona, and Idaho. In other regions the percent- 
