41 
.. BARRIERS, 
It should be borne in mind “hät only the Wasatch,. Uintas and 
Rocky Mountains and southern Arizona floor were above the Ocean ai 
be north in pre-Tertiary times. Then the whole Plateau was ele 
vated: The great Plains emerged. The Navajo. Basin was drained. 
The lofty. plateau of the Great Basin dropped many thousands of feet 
to its present elevation.and all its valleys, drained to the Colorado. 
The ercsion of the Colorado tilted up the southern end of the Great 
Basin till the Colorado drainage ceased and the Basin filled up 
. with two vast lakes, Lahontan on the west and Bonneville on the 
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 west as far south as central Utah along the Uim 
tas, through lakes Bonneville and Lahontan to the Sierras, and 
from the Yellowstone region to the edge of the Columbia lake along 
the present Snake river valley which was an arm of that lake.a$ 
that time, and from there to the Cascades by water transportation 
The Rocky Mountains also at the south swung round by continuous 
flora from New Mexico by the Mogollons to the Kaibab and north 
ward along the Wasatch Plateau to the Bonneville region. It was 
only at the close of the great lakes period that the element of 
barriers became at all effective in stoppage of plant migration. 
LOCAL FLORAS. 
Local flores, a comprehensive name for plant formations, often 
containing many minor plant formations, are caused by barriers 
which may be temperature, humidity, alkalinity, acidity, light or 
soil, or mountain chains, wind movements and the like. 
In 1895 I published in my plant 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 four 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 westward and 
.gouthward to the Ocean and the Mogollons; the Pacific 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 sodded 
area, and extends to the Atlantic. The second is the region of win- 
- ter rains and 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. 
Fach one of these grand divisions is readily separable into 
"subdivisions according to conditions. 
I will take up a few of the more important here. 
The Navajo Basin is the most unique of all local floras. It was 
‘the floor of a very salt inland sea in the Jurassic which became less 
‘salt in the Cretaceous period. Upon it vast sandstones were laid 
down, and upon these vast clay beds. This Basin is formed by the 
watershed of the upper Colorado from the Grand Canon north. It 
really includes the Green River Basin of Wyoming which is cut 
eff from it by the Uinta Mountains, but which I keep distinct for 
