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THE CRUST OF THE EARTH. 35

 

 

(4.) Cenozoic Time, or recent life, included the
time during which the animals and plants bore
decided resemblance to those now living.

These times are divided into ages.

Archean Time includes—

(1.) The Azoic Age;

(2.) The Eozoic Age.

Paleozoic Time, or, as it is sometimes called,
the Primary, includes—

(1.) The Age of Invertebrates, or the Silurian ;

(2.) The Age of Fishes, or the Devonian;

(8.) The Age of Coal-plants, or the Carbon-
iferous.

Mesozoic Time, or, as it is sometimes called,
the Secondary, includes the Age of Reptiles.

Cenozoic Time includes—

(1.) The Tertiary, or the Age of Mammals;

(2.) The Quaternary, or the Age of Man.

Where no disturbing causes existed, and the
land remained under the seas, the rocks deposited
during these periods were thrown down in regu-
lar strata, one over the other. The Archean
were the lowest; above them were the Paleozoic,
then the Mesozoic, and finally those of the Ceno-
zoic. Generally, however, frequent dislocations
of the strata have disturbed the regular order
of arrangement.

72. The Azoic Age included all the time from
the first formation of the crust to the appearance
of animal and vegetable life.

The Eozoic Age is that which witnessed the
dawn of life. The sedimentary rocks of this age
are so highly metamorphosed that nearly all traces
of life have been obliterated. Among plants, the
marine alge, or sea-weeds, and among animals,
the lowest forms of the protozoa, were probably
the chief species.

73. The Age of Invertebrates, or the Silurian,
is sometimes called the Age of Mollusks. Among
plants, algw, or sea-weeds, are found; among ani-
mals, protozoa, radiates, articulates, and mollusks,
but no vertebrates. Hence the name, Age of In-
vertebrates. Mollusks were especially numerous.

The name Silurian is derived from the ancient Silures,
a tribe formerly inhabiting those parts of England and
Wales where the rocks abound.

74, The Age of Fishes, or the Devonian.—
During this age all the sub-kingdoms of animals
are found, but the vertebrates first appear, being
represented by fishes, and from this fact the name
has been given to the age. Land-plants are also
found. Immense beds of limestone and red sand-
stone were deposited.

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The name Devonian is derived from the district of Dev-
onshire, England, where the rocks abound.

75. The Age of Coal-Plants, or the Carbonif-
erous.—The continents during this age consisted
mainly of large, flat, marshy areas, covered with
luxuriant vegetation, subject, at long intervals, to
extensive inundations. The decaying vegetation,
decomposing under water, retained most of its
solid constituent, carbon, and formed beds of coal.
All the sub-kingdoms of animals were represented
and reptiles also existed. The comparatively few

-land-plants of the preceding age now increased

and formed a dense vegetation.

To favor such a luxuriant vegetation the air
must have been warm and moist. Since all the
coal then deposited previously existed in the air
as carbonic acid, the Carboniferous Age was nec-
essarily characterized by a purification of the
atmosphere.

   

Fig, 27, Carboniferous Landscape, (A restoration.)

Formation of Coal.—In every 100 parts of dry vege-
table matter there are about 49 parts of carbon, 6 of hydro-
gen, and 45 of oxygen. The carbon is a solid; the hydro-
gen and oxygen are gases. Itis from the carbon that coal
is mainly formed. When the decomposition of the vege-
table matter takes place in air, the carbon passes off with
the hydrogen and oxygen as various gaseous compounds;
but when covered by water, most of the carbon is retained,
together with part of the oxygen andhydrogen. Although
every year our forests drop tons of leaves, no coal results,
the deposit of one year being almost entirely removed
before that of the next occurs. F

It has been computed that it would require a depth of
eight feet of compact vegetable matter to form one foot.of
bituminous coal, and twelve feet of vegetable matter to
form one foot of anthracite coal. Anthracite coal differs