Geologists classify rocks into these groups: igneous, sedimentary or
metamorphic. Representatives of all three have been described in the
preceding pages.
[Illustration: Arrowheads made from white, gray, pink, and black
chert. (Courtesy of Mr. A. A. Jeffrey, Columbia, Mo.)]
Sedimentary rocks are those whose particles settled down through the air
or water to form rocks in layers or beds; hence layered, bedded, or
so-called stratified rocks are sedimentary rocks. For instance, the sand
and mud settling out of the Gulf of Mexico (or ocean) after being
brought in by the Mississippi River is on its way toward becoming
sandstone and shale. Limestone is forming off the coast of Florida now.
All of these rocks are accumulating in layers. Where one sees
regularly-layered or stratified rocks in streams, road cuts, quarries,
bluffs or hillsides, he expects them to be sedimentary rocks. The
sedimentary rocks that have been described herein include:
Limestone
Dolomite
Chert
Shale
Fire clay
Flint fire clay
Diaspore and Burley clay
Sandstone
Quartzite
Travertine
Coal
Igneous rocks are those which solidified from a hot liquid which was
either forced into older surrounding rocks (intrusive) or discharged on
the earth’s surface as a lava flow or products from a volcano
(extrusive).
The examples given below illustrate the two types. Everyone knows about
the extrusive forms from accounts of present-day volcanoes and
occasional lava flows, like those of Vesuvius, Paricutin, and Mauna Loa.
An intrusion was injected beneath the Yellowstone Park area years ago,
and its heat, with steam and gases, is contributing to the unusual
natural features which are found in the park and which make it famous.
Igneous rocks in Missouri are:
Granite
Porphyry, Rhyolite, Rhyolite porphyry
Gabbro and Diabase
Basalt
Metamorphic rocks are rocks which have been changed through the effects
of tremendous pressure (enough to raise mountains) and high temperature
while in the solid state. In most cases a banded rock results. The
metamorphic rocks mentioned in this booklet are:
Gneiss
Marble
No doubt it has become apparent to the reader that rocks ordinarily
occur in great quantities, that they are composed of multitudes of
grains (mineral grains), and that their properties and compositions vary
with the different minerals which are present in the grains of the rock.
A _rock_ can, therefore, be different from a _mineral_. In fact, a rock
may be defined as “an aggregate of mineral particles,” or more broadly
“a typical part of the earth.” To focus closer attention on minerals we
may discuss them for their own sake below.