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 

 FufRciently 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 \Vhite 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. 



