PIPER—FLORA OF THE STATE OF WASHINGTON. 23 
of most of their moisture, the result being that the region to the east-. 
ward is not only much drier, even to semidesert conditions in limited 
areas, but also much warmer in summer. The plants and animals 
adapted to such conditions are necessarily very different, as a rule, 
from those that thrive in the moist region to the westward. 
It is very clear that the Cascades serve as a barrier, primarily 
because of the difference in humidity eastward and westward which 
they occasion, and not because of their altitude. Many of the passes 
over these mountains are but a little over 1,000 meters in altitude, not 
sufficiently high even in this period to prevent many plants from 
migrating through, especially in the wake of forest fires. As a matter 
of fact the rainfall influences the vegetation for a considerable dis- 
tance down the eastern slopes of the mountains, the flora of the main 
range down to about 1,000 meters altitude being largely composed of 
species of the coast region. 
In Washington proper no stream traverses the Cascade Mountains, 
but on the southern border is the great gap through which the 
Columbia River flows. This enormous gorge gives rise to peculiar 
local conditions, which find marked expression in the flora. Through 
this gap, too, the coastal flora, aided by the prevailing upstream 
humid winds, penetrates farther eastward than usual. Still, at the 
village of White Salmon there is an unmistakable dividing line 
between the humid and the semiarid floras. 
Nearly all of the many streams that arise in the Cascades flow 
through deep gorges, once occupied by glaciers. Indeed, many of the 
streams, especially those heading about the higher peaks, still find 
their birth in glaciers. 
The Cascade range north of the forty-seventh parallel is composed 
largely of granite and other metamorphic rocks. Mount Rainier and 
the entire range southward to its extremity in northern California is 
on the contrary almost entirely made up of volcanic rocks. Recent 
investigations in the geology of these mountains disclose in part a 
very complex history, but indicate that the principal uplift took place 
either in late Pliocene or early Pleistocene time, and subsequent to the 
great outpourings of lava that make up most of the region between 
the Cascades and the Rockies. 
From a biological point of view the changes brought about by the 
Cascade uplift were profound. Undoubtedly it transformed the 
climate of the region to the eastward from one relatively moist to one 
distinctly arid, and at the same time increased greatly the humidity 
of the region to the westward. This climatic change, particularly in 
the interior, must have been accompanied by a correspondingly great 
change in the flora. The peculiar make-up of the Columbia Basin 
flora of the present time indicates with more or less clearness some of 
the resultant effects of the Cascade uplift. 
