36 PHYSICAL

 

GEOGRAPHY.

 

from bituminous mainly in the greater metamorphism to
which it has been subjected; it contains a greater propor-
tion of carbon and less hydrogen and oxygen.

76. The Age of Reptiles.—In this age the ani-
mals and plants begin to resemble existing species.
The age is characterized mainly by the prepon-
derance of reptiles, many of which were very
large, as, for example, the plesiosaurus, an animal
with a long, snake-like neck and a huge body, or
the ichthgosaurus, with a head like a crocodile and
short neck and large body. Both of these ani-
mals were furnished with fin-like paddles, and
lived in the water. Huge pterodactyls, or bat-
like saurians, flew in the air or paddled in the
water. Mammals and birds also occur.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  
  

Fig, 28, ‘The Age of Reptiles. (A restoration.)

77. The Age of Mammals, or the Tertiary Age.
—Mammals, or animals that suckle their young,
occurred in great numbers, and, being the highest

 

 

 

 

 

 

 

 

 

 

 

== cae

Fig, 29. Mastodon giganteus, An Animal of the Mammalian Age

type of life, gave the name to the age. The ani-
mals and plants of the Mammalian Age closely
resembled existing species, though most of them
were much larger; as, for example, the dinothe-

rium, a huge animal, with a trunk like an ele-
phant, but with downward-turned tusks; the
paleotherium, and many others.

78. The Era of Man, or the Quaternary Age,
witnessed the introduction of the present animals
and plants and the creation of man.

79. Changes Now Occurring in the Earth's
Crust.— Geological time was characterized by ex-
tensive changes, both in the hind and luauriance
of life, and in the nature of its distribution.

The earth is still undergoing extensive changes,
which are caused by the following agencies:

(1.) By the Winds, which often carry sand
from a desert and distribute it over fertile plains:
in this manner the narrow tract of fertile land on
the borders of the Nile, in Egypt, receives much
sand from the Sahara. The winds are also piling
up huge mounds of sand along the sea-coasts,
forming what are called dunes, or sandhills,

(2.) By the Moisture of the Atmosphere, soak-
ing into porous rocks or running into the crevices
between solid ones. This water in freezing ex-
pands with force sufficient to rend the rock into
fragments, which are carried away by the rivers
or, when sufficiently small, by the winds. 2

(8.) By the Action of Running Water.— Rivers
wash away portions of their banks or cut their

 

i VAT
ENE START

Fig. 30, Curious Effect of Erosion,

their channels. This action is
It occurs even in the hardest

way throug
called erosgon.