The copper era., 1907-10-24
Formation of IeneOus Rock.

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basalt and lava are typical igneous
rocks, while sand and sandstones,
clays and states are examples of
sedimentary rocks.
As the name indicates, the sedi-
mentary rocks consist of sediments
laid down by water. The sediment is
composed usually of fragments of
other rocks, although, as in the case
of limestone, large rock masses are
sometimes wholly built of organic
remains.
The term igneous, derived from a
Latin word meaning fire, is applied
tn rocks in whose formation heat has
been a chef factor. Any rock when
raised to a temperature sufficiently
high, becomes molten or liquid. The
presence of water lowers the point
at which liquefaction takes place.
Press usually increases the amount
of heat necessary to produce lique-
faction, or, in other words, raises the
is called rock magma. Chemically, a
rock magma is solution, each of the
substances of which it is composed
being dissolved to some extent in
each of the others. According to one
theory the entire mass of the earth
was at one time molten, but the idea
that the interior is molten at the
present time has been abandoned.
The interior is known to be as rigid
as steel.
Igneous rocks are produced by the
cooling and qualification of rock
magmas. If the cooling is rapid a
rock of uniform composition, texture
and color, resembling glass or smell
ter Stag is formed. Such rocks are
known as volcanic glasses. If the
cooling is sufficiently slow, the sub
stances forming the magma separate
and combine in Crystalline form as
different minerals. In granite, for
example, the capitalization is COM
plete. There are also rocks, includ-
ing some of the porDhyries, repre-
senting all stages of partial crys-
tallizat ton.
Sedimentary rocks, when buried to
sufficient depth, may become crys-
talline under the action of heat and
pressure. Indeed, the change may
continue until the rock produced
differs in no respect from typical
igneous rocks. Usually, however,
traces of qualification and the origi-
nal structure of these altered sedi-
mentaries remain, and they are
known as metamorphic rocks. The
term metamorphic is also applied to
altered igneous rocks, especially to
those that assume the appearance OF
qualification.
Everywhere underlying the sedi-
mentary rocks, and exposed without
a sedimentary covering over great
areas, especially in mountainous re-
gions is the great mass of crystalline
igneous rocks extending to unex-
plored depths and forming a large
part of the known crust of the earth
This class of igneous rocks is called
fundamental or massive. Some of
them are very likely altered sedi-
mentary rocks.
Driving their heat from crystal
movements or more deeply seated
changes; forced up through assures,
chimneys or other openings; filling
their avenues of escape, overlooking
the surface or crowding between lay
ers of sedimentary rocks, and then
cooling at different rates according
to conditions, are the various kinds
of eruptive igneous rocks. At times,
when the excess of stress in the
earth crust obliges it to break,
molten rock comes up by the new
openings. The liquid mass of rock is
known under the name of eruptive
before it can reach the surface and
the magma solidfes slowly in crys-
talline form. At other times the
molten rock finds d assure which
brings it to the surface of the earth,
and it may be elected with great
violence from the chimney of d vol
cano.
Igncous rocks can be studied from
two main points of view, widely diff
terent from one another. One is
their mode of formation, how they
have solicited in large masses, or in
long and narrow bikes, or how they
have been poured over the country
as flows of lava. The other side of
the study of igneous rocks deals with
their chemical composition. 1t con-
siders their intimate structure and
their constituent minerals.
When the stress in the earth be-
comes too great, it obliges it to fold
1t. elf into a mountain range, or to
move large blocks, perhaps of several
thousand square miles, perhaps
thousands of feet up or down. The
heat produced by such movements, or
heat from other sources, may be suf-
ficient to fuse portions of deep seat
ed rocks, and the molten magma
takes advantage of every opening to
come up towards the surface of the