41 

 BARRIERS. 



It should be borne in mind that only the "Wasatch^ TJintaa and 

 Kocky Mountains and southern Arizona floor were above the Ocean at 

 ■ 0!' north in i)re-Tertiary times. Then the whole Plateau was el» 

 vated. The great Plains emerged. The Navajo Basin was drained. 

 The lofty i iateau of the Great Basin dropped many thouixands of f'.'et 

 to its present elevation and all its valleys drained to the Colorado. 

 The ercsicn of the Colorado tilted up the southern end of the Great 

 Basin till the Colorado drainage ceased and the Basin filled up 

 with tvo vnst lakes, Lahontan on the west and Bonneville on th» 

 east. The Columbia region was filled by Lake Columbia. 



These facts had an important bearing on the later distribution 

 of the flora of the Great Plateau, from the Rockies to the Sierras. 



Toward the close of the Ice age there was easy access of plants 

 from the east and we:it as far south as central Utah along the Uin- 

 tas, through lakes Bonneville and Lahontan to the Sierras, and 

 from the Yellowstone region to the edge of the Columbia lake along 

 the present Snak'.^ river valley which was an arm of that lake at 

 that time, and from there to the Cascades by water trausportatioa. 

 The Rocky Mountains also at the south swung round by continuous 

 flora from New Mexico by the Mogollons to the Kaibab and nortti- 

 ward along the Wasatch Plateau to the Bonneville region. It. v/as 

 only at the close of (the great lakes perioid that the element of 

 barriers became at all effective in stoppage of plant migration. 



LOCAL FLORAS. 



Local floras, a comprehensive name for plant formations, often 

 containing many minor plant formations, are caused by barrien 

 which may be temperature, humi'lity, alkalinity, acidity, light or 

 soil, or mountain chains, wind mnvpments and the like. 



In 1895 I published in my pl-:nt schedules a complete list of 

 these local floral regions in all the life zones, of the Great Plateau. 



As I have stated humidity is the greatest factor in determining 

 plant formations and local floras. 



The Great Plateau is readily divisible into faur well marked 

 groups. The Rocky Mountain region which extends from Santa Fe 

 New Mexico along the Atlantic watershed to the far north; The Paci- 

 fic slope region extending from Central Montana westvv-ard and 

 southward to the Ocean and the Mogollons; the Pncific Coast region 

 from San Francisco southward; The Albuquerque and the Arizona-Mexi- 

 can plateau region from the lower Colorado to Texas and southward. 

 The first is a region of spring and early summer rains, the so*dded 

 area, and extends to the Atlantic. The second is the region of win- 

 ter rains fnd midsummer showers or none, the sodless region. 

 The fourth is the region of fall and late summer rains. The third 

 is the region cf winter rains. All these general floras are remarkably 

 distinct. 



Each on'^ nf these grand divisions Is readily separable into 

 subdivisions according to conditions. 



I will ^.nke up a few of the more important here. 



The NavTiio Basin is the most imique of rll loc.il floras. It wa» 

 the floor of n very snlt Inland sea in the Jurassic which became les-^ 

 salt in the Cretaceous period. Upon it vast sandstones were laid 

 down, and upon these vast clay beds. This Basin Is formed by th* 

 watershed of the upper Colorado from the Grand Canon north. It 

 really includes the Green River Basin of Wyoming which is cut 

 off from it by the Uinta Mountains, but which I keep distinct for 



