26 BOTANY OF THE DEATH VALLEY EXPEDITION. 



To sum up, we find on the eastern side of the Sierra Nevada a desert 

 region whose lower altitudes lie in a zone called the Lower Sonoran, 

 marked below by the presence of Larrea tridentata, and above by a nar- 

 row treeless belt characterized by several shrubs and in many parts 

 by an abundant growth of Yucca arborescens; and next, a zone known 

 as the Upper. Sonoran, covered by a scant forest of Finns monophylla 

 and Juniperus californica utahensis. On the western slope of the. Sierra 

 Nevada there occurs first, at its base, a foothill belt characterized most 

 prominently by the presence of Qucrcus douglasii, and above this a 

 chaparral belt to be identified by a dense growth of many shrubs and 

 small trees, notably Quercus chrysolepis and Cercocarpus parvifolius. 

 These two belts are correlated respectively with the Lower and Upper 

 Sonoran zones of the desert. Above all these belts, and extending 

 from the western slope of the Sierra Nevada in California to the 

 Charleston Mountains of Nevada, but at an altitude so great as to cut 

 only a few of the higher mountains, lies the Transition zone, marked 

 in the Sierra Nevada by the presence of Finns ponderosa and P. Jeffrey i, 

 in the desert mountains by Finns ponderosa scopulorum or, in the ab- 

 sence of that tree, by Juniperus oceidentalis monospermy and Cercocar- 

 pus Icdifolius. Above the Transition comes the Boreal zone, represented 

 only in the highest of the desert peaks, and occupying all the space 

 between the Transition zone and tiniberdine. Its principal character- 

 istic trees in the Sierra Nevada are Finns monticola, F. baifouriana, 

 and F. murrayana, and in the desert mountains Pinus flexilis and Finns 

 aristata. 



GEOGRAPHIC RELATIONSHIP OP TnE FLORA OP THE HIGH SIERRA 

 NEVADA, CALIFORNIA. 



The two great mountain systems of the western United States are, 

 first the Kocky Mountains, and second the Sierra Nevada and Cascade 

 Mountains. The former stretches from New Mexico northward, through 

 Colorado, Wyoming, Idaho, and Montana, into British America; the 

 latter, from southern California, at about 35° N. lat., through that 

 State, Oregon, and Washington, likewise into British America. Be- 

 tween these two barriers lies the Great Basin, merging at the south 

 into the desert. At their southern ends the ranges are widely separated 

 and the broad desert intervenes ; but in British Columbia the two systems 

 draw closer together, the depressed plains between them become studded 

 with other mountains, and the cold of the northern latitude allows the 

 boreal flora of the mountain crests to descend more nearly to the base 

 level of the country. Under these conditions we should expect the 

 boreal flora of any portion of one system to be most nearly similar to 

 that of other portions of the same system ; and, indeed, that each system 

 would constitute a distinct line of southward boreal migration. To test 

 the validity of this surmise, a comparative examination of the flora of 



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