84 PHYSICAL GEOGRAPHY.

 

 
      
  

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Fig, 25. Stratified Rock,

In Fig. 25 the different layers or strata are shown by
the shadings. Stratified rocks are the most common form
of rocks found near the earth’s surface.

Stratified rocks are largely composed of fragments of
older rocks; for this reason they are sometimes called
fragmental rocks.

(2.) Unstratified Rocks, or those destitute of
any arrangement in layers. They are of two kinds:

(1.) Igneous, or those which were never stratified.

(2.) Metamorphic, or those which were once
stratified, but have lost their stratification by
the action of heat.

Unstratified rocks are sometimes called crystad-
line rocks, because they consist of crystalline
particles.

68. Fossils are the remains of animals or plants
which have been buried in the earth by natural
causes. . Generally, the soft parts of the organism
have disappeared, leaving only the harder parts.
Sometimes the soft parts have been gradually re-
moved, and replaced by mineral matter, generally
lime or silica; thus producing what are called
 petrifactions. At times the mere impression of
the animal or plant is all that remains to tell
of its former existence.

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Fig, 26, Fossil Encrinite,

When the remains of an animal or plant are exposed to
the air or buried in dry earth, they generally decompose
and pass off almost entirely as gases; but when buried
under water or in damp earth, their preservation is more
probable. Therefore, the species most likely to become
fossilized are those living in water or marshes, or in the
‘neighborhood of water or marshes.

69. According to the Presence or Absence of
Fossil Remains, rocks may be divided into two
classes :

 

(1.) Fossiliferous Rocks, or those which con-
tain fossils. They are stratified and are of
aqueous origin. Metamorphic rocks, in very
rare instances, are found to contain fragments
of fossils.

(2.) Non-fossiliferous Rocks, or those destitute
of fossils. They include all igneous rocks and
most of those that are metamorphic.

70. Paleeontology is the science which treats of fossils. .

Paleontology enables us to ascertain the earth’s condi-
tion in pre-historic times, since by a careful examination
of the fossils found in any rocks we discover what animals
and plants lived on the earth while such rocks were being
deposited. The earth’s strata thus become the pages of a
huge book; and the fossils found in them, the writings
concerning the old life of the world. By their careful
study geologists have been enabled to find out much of
the earth’s past history.

71, Division of Geological Time.—A compari-
son of the various species of fossils found in the
earth’s crust discloses the following facts:

(1.) The fossils found in the lowest rocks bear
but. a slight resemblance to the animals and
plants now living on the earth.

(2.) The fossils found in the intermediate strata
bear a resemblance to existing species, though
this resemblance is not so strongly marked as in
the upper strata.

(3.) ‘The fossils found in the upper strata bear
a decided resemblance to existing species.

It is on such a basis that the immense extent
of geological time is divided into the following
shorter periods or times:

(1.) Archean Time, or the time which wit-
nessed the dawn of life. This time included an
extremely long era, during most of which the con-
ditions of temperature were such that no life could
possibly have existed. Toward its close, however,
the simplest forms of life were created.

The lower Archean rocks resulted from the
original cooling of the molten earth, and cover
its entire surface, including the floor of the ocean.
On these rest less ancient Archean rocks, formed
as sedimentary deposits of the older rocks.

The rocks of the Archean Time in North America in-
clude the Laurentian, the lowest, hamed from the river
St. Lawrence, near which they occur, and the Huronian,
named from their occurrence near Lake Huron.

(2.) Paleozoic Time, or ancient life, included
the time during which the animals and plants
bore but little resemblance to those now living.

(3.) Mesozoic Time, or middle life, included
the time during which the animals and plants
began to resemble those now living.

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