The names chert and flint have in some regions been used for the same
hard, fine-grained rock found so abundantly in Missouri, but correct
usage employs chert for the white and gray varieties, and flint for the
black variety. Flint may be thought of as slightly impure chert, a chert
which is colored black by a small amount of pigment, usually fine
carbon, or perhaps iron sulphide, scattered through it like fine dust.
[Illustration: Chert, fossiliferous and slightly speckled. Note
typical sharp edges, smooth surfaces, and conchoidal fracture. From
near Columbia.]
Chert is characterized by being harder than glass, brittle, very
fine-grained, and by breaking with a smooth, rounded or hollowed clam
shell-like (conchoidal) fracture and sharp edges. It was used by Indians
to make arrow heads. It accumulates in abundance both in stream beds as
gravel which has been more or less rounded by wear, and on the hillsides
within the soil and sub-soil. Yellow and red iron oxides may stain and
penetrate weathered chert gravel so that it becomes reddish, rusty, tan,
yellow or brown.
Chert remains abundant because of its extreme resistance to weathering.
It is so hard that stream action wears it only very slowly. Its chemical
composition is silica, SiO₂, a substance which is but little affected
chemically by ground water. Where chert has contained fine grains of
calcite scattered through it, the calcite may be removed in solution,
leaving pores, and a zone of porous, light weight, tripolitic chert,
harsh to the feel and enveloping an unaltered interior (See WEATHERED
CHERT). Not uncommonly, fossil remains of calcite which were embedded in
chert have been dissolved, leaving their hollow impressions.
Chert in Missouri originally occurs chiefly in limestone formations,
where it is found as nodules, lenses, stringers, and irregular forms in
and between the limestone beds. Chert and flint may be deposited
directly from silica in solution, or they may replace (substitute for)
wood, fossils, or older rock where silica-bearing solutions contact and
react with the replaced substance. For example, petrified wood usually
is wood which has been replaced molecule by molecule with silica. This
statement applies equally to the brightly colored petrified wood in the
Petrified Forest in Arizona and to that with comparatively drab coloring
in Missouri. Many other siliceous fossils, notably animal remains, are
replacements of calcite (limestone) by silica.
In anticipation that the reader may have difficulty understanding how
silica may go into solution if chert (silica) is hardly attacked by the
weathering process, it should be explained that silica is freed in
solution predominately during the weathering of complex
silica-combinations, silicates, rather than from uncombined silica. For
instance, feldspar and pyroxene from granite or gabbro weather in ground
water to a soil-forming clay mineral and release some silica in solution
in the ground water. After this silica is redeposited in an uncombined
form, like chert, it becomes highly insoluble.
An observation in regard to flint is that the metallic “flints” which
are used to ignite gas burners or cigarette lighters are not black
chert, SiO₂. Instead, they are special alloys containing rather uncommon
elements which possess the useful characteristic of emitting a brilliant
hot spark when harshly scratched.
Chert is the chief source of natural gravel in Missouri because it
accumulates in stream beds and on hillsides on account of its resistance
to weathering. The piles of “chats” in the Joplin region, containing
thousands of tons of crushed chert, have been used in part in road
surfacing material.