[Illustration: Close view of a granite hand specimen. Feldspar
predominates. Quartz appears dark in the photograph, but shows
glistening edges and points. From Graniteville.]
Granite is a granular (coarse-grained) rock which has a glassy luster
and is too hard to be scratched appreciably by steel. It may be white to
gray, tan, brown, or pink to red in color, but pinkish to red granite
predominates in Missouri. Some black stone, referred to locally as
“black granite,” is usually a variety of gabbro. Most Missouri granite
is coarse-grained, so that the constituent mineral grains—quartz,
feldspar, and (less frequently) mica—can be readily recognized by anyone
familiar with those minerals. It makes up many of the mountains and
hills in Iron, Madison, and St. Francois counties and adjacent regions.
North of the Missouri River, or where the glacial deposits remain,
granite boulders may occur in the sandy and clayey glacial drift.
The mineral quartz is recognized in granite by its glistening, oily
luster, really more brilliant than the luster of glass, and by its
curved to irregular broken surface. Furthermore, the brilliant luster of
quartz is not dulled by exposure to weather.
The mineral feldspar, in granite, has a glassy luster on the tiny flat
cleavage faces where the individual grains are broken. Where weathered,
feldspar becomes dulled, and chalky to dusty or clayey. Fresh feldspar
may be glassy, white, buff, pink, red, in intermediate shades in color.
With the mica, it imparts most of the color to granite.
Mica is recognized by its softness and its ability to be split very
easily into tiny flakes. Other minerals may be found in granite under
the microscope, but they have little importance or significance.
[Illustration: Granite at the “Elephant Rocks,” Graniteville. The
large boulders now rounded by weathering are remnants of a higher
part of the large granite body which underlies this region.
(Photograph courtesy of Mr. Noel Hubbard).]
Granite is an intrusive igneous rock; that is, it solidified from a hot
liquid state (like lava) in a large body, beneath, or surrounded by
pre-existing rocks. Because of slow solidification a coarse-grained
texture was developed. In southeastern Missouri where granite is now
exposed at the surface (for example, the Elephant Rocks State Park at
Graniteville), that granite was covered originally by hundreds of feet
of rock at the time it solidified from a liquid. During the millions of
years which have elapsed since the granite solidified, its cover and the
upper part of the granite have been eroded away by streams and rain
after weathering to soil material. In fact, the ocean has covered the
area several times during its long geological history.
Missouri has a fine quality of granite in large quantity in southeastern
Missouri. Granite is used for building, structural and monument purposes
(see discussion under marble), for rubble stone, rip-rap, ballast,
gravel, paving blocks, crushed chicken gravel, and for other specialized
uses where favorably located. Chemical analyses of granite and porphry,
taken from Missouri Geological Survey Report, Volume VIII, 1895, follow.
Porphyry 6 miles east Granite 6 miles east
of Ironton of Ironton
SiO₂ 71.88 72.35
Al₂O₃ 12.88 13.78
Fe₂O₃ 3.05 1.87
FeO 1.05 0.36
CaO 1.13 0.87
MgO 0.33 0.42
K₂O 4.46 4.49
Na₂O 4.21 4.14
P₂O₅ 0.15 0.13
TiO₃ 0.22 0.44
Ignition loss 0.26 0.54
The glacial granite boulders found in central to northern Missouri also
solidified as intrusive rock in the northern United States or in Canada.
After being exposed at the surface they were picked up and carried down
by the geologically recent, continental ice sheet (glacier) that moved
down from Canada to across the northern half of Missouri. Scratches and
grooves may have been cut in some of these boulders, or flat faces
scoured and planed off as they were scraped against other hard rocks.
Quartz gravel is usually present, often in abundance, in glacial
deposits. Small specimens of native metallic copper, which come from
near Lake Superior, have been found in Missouri glacial deposits. Even
diamonds from an unknown source in the north were carried by the ice
down into the United States. The history of the glaciation is a
spectacular account of changes which our continent has undergone in the
geological past.
[Illustration: Glacial scratches on boulder carried by the large
glacier in northern Missouri long ago. Boulder from near Columbia.]