46 PHYSICAL

 

GEOGRAPHY.

 

 

Fig, 42, On the Plains,

The Rocky Mountain System forms the great
watershed of the continent, the eastern slopes
draining mainly through the Mississippi into the
Atlantic, and the western slopes draining through
the Columbia and the Colorado into the Pacific.
It slopes gradually upward from the Arctic Ocean
toward the Mexican plateau, where it attains its
greatest elevation in the volcanic peak of Pepo-
catepetl, 17,720 feet above the sea.

The System of the Sierra Nevada and Cascade)

Mountains extends, in general, parallel to the
Rocky Mountain System. It takes the name of
Sierra Nevada in California and Nevada, and of
the Cascade Mountains in the remaining portions
of the continent. It reaches its greatest eleva-
tion in Mount St. Elias, in Alaska, 19,500 feet
above the sea. This is the culminating point of
the North American continent.

In the broadest part of the plateau of the Pacific system,
between the Wahsatch Mountains on the east, and the
Sierra Nevada and Cascade ranges on the west, lies the
plateau of the Great Basin. Its high mountain borders
rob the winds of their moisture, and the rainfall, except
on the mountain-slopes, is inconsiderable. The Great
Basin has a true inland drainage.

The heights of all mountains, except those much fre-
quented, must generally be regarded as but good approxi-
mations, since the methods employed for estimating heights
require great precautions to secure trustworthy results.
Even the culminating points of all the continents have
not, as yet, been accurately ascertained.

_ 112. The Secondary Mountain-Systems of North
America comprise the Appalachian system, the

 

Plateau of Labrador, the Height of Land, and
the Arctic Plateau. The last three have but an
inconsiderable elevation.

The Appalachian Mountain System consists of
a number of nearly parallel chains extending
from the St. Lawrence to Alabama and Georgia.
It is high at the northern and southern ends, and
slopes gradually toward the middle. The highest
peaks at either end have an elevation of about
6000 feet.

The Appalachian system is broken by two deep depres-
sions, traversed by the Hudson and Mohawk Rivers. Be-
tween the foot of the system and the ocean lies a low coast
plain, whose width varies from 50 to 250 miles.

118. The Great Low Plain of North America
lies between the Atlantic system on the east and
the Pacific system on the west. It stretches from
the Arctic Ocean to the Gulf of Mexico.

Near the middle of the plain the inconsider-
able elevation of the Height of Land divides it
into two gentle slopes, which descend toward the
Arctic Ocean and the Gulf of Mexico. <A gen-
tle swell extending from north-west to south-east
divides the northern portion of the plain into
two parts. The eastern and western basins, so
formed, are connected by a break in the water-
shed, through which the Nelson River empties
into Hudson Bay.

The southern part of the plain is traversed, in

its lowest parts, by the Mississippi River.

The tributaries of this river descend the long, gentle
slopes of the Atlantic and Pacific systems.

114. The Relief Forms of a Continent are best
understood by ideal sections, in which the base
line represents the sea-level, and the scale of
heights on the margin represents the elevation
of the various parts.

In all such sections the vertical dimensions of the land
are necessarily greatly exaggerated.

 

Fig. 48, Section of North America from East to West.
1, St. Elias; 2, Sierra Nevada; 3, Rocky Mountains; 4, Mississippi
Valley; 5, Appalachian System.

115. Approximate Dimensions of North America,
Area of continent, 8,400,000 square miles.

Greatest breadth from east to west, about 3100 miles.
Greatest length from north to south, about 4500 miles,
Coast line, 22,800 miles.

Culminating point, Mount St. Elias, 19,500 feet.