LAKES. 69

 

- and by the Elbe and the Weser into the North
Sea. It is drained toward the south and east by
the Ural and the Volga into the inland basin of
the Caspian; and by the Don, the Dnieper, and
the .Dniester into the Sea of Azov and the Black
Sea.

All the peninsulas have streams traversing them. The
Seine, the Loire, and the Garonne from France, and the
Douro, the Tagus, and the Gaudiana from Spain and Por-
tugal, empty into the Atlantic. The Ebro from Spain,
and the Po from Italy, empty into the Mediterranean.

185. Asia possesses the most extensive inland
drainage of all the-continents. The plateaus are
surrounded by lofty mountains containing but
comparatively few breaks, and their waters, there-
fore, can find no passage to the sea. The outer
slopes, however, are-drained by some of the
largest rivers in the world.

The Great Northern Plain drains into the
Arctic, mainly through the Lena, the Yeniset,
and the Obe.

~The Eastern Slopes drain into the Pacific
through the Amoor, the Hoang-Ho, the Yang-tse-
Kiang, and the Cambodia.

The Southern Slopes drain into the Indian
Ocean through the Irrawaddy, the Brahmapootra,
the Ganges, the Indus, the Tigris, and the Eu-
phrates.

The principal drainage-centre in Asia is the Plateau of
Thibet, from which descend the Hoang-Ho, the Yang-tse-
Kiang, the Cambodia, the Irrawaddy, the Ganges, the
Brahmapootra, and the. Indus.

186. Africa, being low in the interior, with
high mountain-walls on her borders, is charac-
terized, like the Americas, by the union of her
smaller river-systems into a few large streams,
which drain nearly the entire continent.. These
embrace the Nile, emptying into the Mediterra-
nean; the Zambezi, into the Indian Ocean; and
the Orange, the Congo, the Niger, and the Senegal,
into the Atlantic.

187. Australia —The Murray, which drains the
south-eastern part of the continent into the Indian
Ocean, is the only considerable stream.

188. Principal Oceanic Systems—A careful
study of the river-basins of the different oceans
discloses the following fact: v

The Atlantic and Arctic. Oceans receive the
waters of nearly all the large river-systems of the
world.

The cause of this is as follows: The predomi-
nant systems being situated nearest the deepest
ocean, the long, gentle slopes descend foward the

9 |

smaller, shallower oceans (the Atlantic and the -
Arctic), which thus receive the greatest drainage.

For details of the various river-systems—such as the
length, area of basin, etc.—see Table, page 174.

—026400—_

CHAPTER VI.
Lakes.

189. Lakes are bodies of water accumulated in
depressions of the surface of the land.

They are connected either with the systems of
oceanic or of inland drainage. The waters of
lakes draining into the ocean are fresh; those
having no connection with the ocean are salt.

Depth—From their mode of formation lakes
which occur in mountainous districts are, as a
class, deeper than those found on the great low
plains, since the former occupy the basins of nar-
row but deep valleys, and the latter the depres-
sions of the gentle undulations of the plain.

In mountainous districts the depths of the depressions
are sometimes so great that the bottom of the lake is con-

siderably below the sea-level. Lake Maggiore in the Swiss
Alps extends about 2000 feet below the level of the sea.

Lake Superior. ‘
Lake Huron.

    
 

600 feet.
500 feet.
400 feet.
300 feet.
200 feet.

 

100 feet.

 

Sea Level. 4

WN 7b
100 feet.
200 feet.
300 feet.
400 feet.

 

Fig, 67.- Elevations and Depressions of Lakes,

One of the most remarkable series of depressions in the
general land-surface of the world is that occupied by the
waters of Lakes Superior, Michigan, Huron, Erie, and On-
tario. Superior and Huron, though some 600 feet above
the level of the ocean, reach, in their greatest depths, far
below its surface ; the former being 270 feet, and the latter
about 400 feet, below the general level of the Atlantic.

When a lake is connected with a river-system,
the place where the principal stream enters is
called the head of the lake; the place where it
empties is called the foot of the lake.

190. Geographical Distribution. — The large