The Afforestation of the desert Areas. 137 
everywhere. While a vast territory on the Atlantic side and a 
narrower belt on the Pacific toast, connected by a broad belt 
through the northern latitudes, was almost entirely under its 
undisputed sway, and while the back-bone of the continent, the 
crest and slopes of the Rocky mountains, was more or less in 
its possession, there still remained a vast empire in the interior 
unconquered. 
Of parts of this territory we feel reasonably certain from strong 
evidences that the forest once occupied them, but has been driven 
off by aboriginal man, the firebrand taking sides with the grasses 
and the buffalo probably being a potent element in preventing 
reestablishment. In other parts it is questionable whether the 
lines along the river courses, the straggling trees on the plateaus 
and slopes, are remnants of a vanquished army or outposts of an 
advancing one. Insome parts, like the dry mesas, plateaus and 
arroyos of the interior basin, and the desert-like valleys toward 
the southern frontiers, it may reasonably be doubted whether 
arborescent flora has more than begun its slow advance from the 
outskirts of the established territory. 
Certain it is that climatic conditions in these forestless regions 
are most unfavorable to tree-growth, and it may well be ques- 
tioned whether in some parts the odds are not entirely against 
the progress of the forest. 
Temperature and moisture conditions of air and soil, determine 
ultimately the character of vegetation, and these are dependent 
not only on latitude, but largely on configuration of the land, 
and especially on the direction of moisture-bearing winds with 
reference to the trend of mountains. 
The winds from the Pacific ocean striking against the Coast 
range are forced by the compression and subsequent cooling to 
give up much of their moistuge on the windward side; a second 
impact and further condensation of the moisture takes place 
on the Cascade range and Sierra Nevada. On descending, with 
consequent expansion, the wind becomes warmer and drier, so 
that the interior basin, without additional sources of moisture 
and no additional cause for condensation, is left without much 
rainfall and with a very low relative humidity, namely, below 50 
per cent. “The Rocky mountains finally squeeze out whatever 
moisture remains in the air currents, which arrive proportionally 
drier on the eastern slope. This dry condition extends over the 
plains until the moist currents from the gulf of Mexico modify 
