158 Distribution of the North American Flora. -[March, 
From the Missouri the ascent is very gradual to the elevated 
region of the Rocky mountains, which consist of a complicated 
series of rocky ridges rarely exceeding 14,000 feet elevation, 
occupying a belt 300 miles broad from east to west. These 
ridges inclose very large, well-watered, open grassy valleys, called 
Parks, the rivers from which usually discharge from the range 
through narrow gorges, called cañons. 
The parks and valleys to the east of the mountainous belt 
present the gray-green (grassy) vegetation of the prairie, those on _ 
the west, the hoary sage-bush (Artemisia) vegetation of the dry 
country to the westward; and these often intersect, so that a 
transverse ridge may separate a green and well-watered park from 
a hoary and dry one. 
The descent from the Rocky mountains on the west is on to a 
tract elevated upwards of 4000 feet above the sea, extending for 
400 miles to the foot of the Sierra Nevada. This tract is inter- 
sected by several short ranges 8000 feet high and upwards; its 
climate is dry, its soil saline, and many of its rivers lose them- 
selves in salt lakes and marshes, whence the local names of Great 
basin, and of the Sink, Salt-lake and Desert regions. The Sierra 
Nevada succeeds, rising steeply to an elevation of 12,000 and 
sometimes of 15,000 feet. Under various names it traverses 
America, with little interruption, from Alaska to Southern Cali- 
fornia, at a distance of one hundred to one hundred and fifty 
miles from the Pacific; but its breadth is nowhere so great as 
that of the Rocky mountains. The descent from it to the west- - 
ward is into the great valley of California, whose floor is raised 
but little above the sea-level, and between which and the Pacific 
are the low and narrow coast ranges, of which the southernmost. 
in Southern California unites with the Sierra Nevada. 
Turning now to the flora of North America north of the tropic, 
we find that the distribution of its plants is in remarkable con- 
formity with its geographical and climatal features, being in mer- 
idional belts from the Arctic ocean to the gulf of Mexico; the 
botanical components of these belts differing more and more in 
advancing south, till in the principal parallel that we have traced, 
the diversity between the eastern and western belts is greater 
than between any two similarly situated regions on the globe. 
Polar Area. —Commencing i in the Polar area, the Arctic Ameri- 
can flora, though on the wagle a uniform one, is dadota divini 
Rupee Pt ary S E E a T a TaD tear Nn Aer A AE ORE ES S A a Meany eee? AA EA a S EN ¥ 
