Porphyry and granite are the two most abundant igneous rocks in
southeastern Missouri (Iron, Madison, and St. Francois counties, and
adjacent country). The porphyry there is a compact, very fine-grained,
almost glassy, hard, brittle rock that varies in color from light gray
through pink and red to dark purplish red and almost black. It always
breaks with a horny, flinty fracture. Small mineral crystals of
glistening quartz and usually reddish feldspar are generally scattered
throughout the dense background (groundmass). The crystals are commonly
about one-sixteenth of an inch in cross section and ordinarily
constitute from about ten to twenty per cent of the rock. Other names,
somewhat more specific than simple porphyry, which are applied
technically to certain phases of the rock are rhyolite, and rhyolite
porphyry.
[Illustration: Rhyolite porphyry showing phenocrysts (light
“freckles”) of quartz and feldspar. From near Ironton.]
The porphyry of southeastern Missouri is igneous rock which in the main
poured out as lava flows, millions of years ago. Volcanic dust or “ash”
was erupted during the same period, and layers of it, now strongly
cemented, are found in association with the flow rock.
The Missouri rhyolite porphyry has about the same chemical composition
(see page 40) as Missouri granite, but whereas granite is
coarse-grained, the porphyry has an extremely fine-grained to almost
glassy ground-mass. This difference in texture (grain size) is due to
the difference in rate of solidification. The porphyry lava flows
chilled and solidified very rapidly, thereby freezing the liquid to
glassy and extremely fine-grained rock, except for the scattered larger
crystals (phenocrysts) which had developed prior to eruption. Granite,
on the other hand, solidified very slowly under a thick cover of rock
which acted as a heat insulator, and during the long time of
solidification large or coarse grains of minerals could grow and develop
by crystallization so that a coarse-textured rock (granite) was formed.
The relative ages of the Missouri igneous rocks are of interest to
geologists and to most persons who recognize the different types within
a small area. It has been found that the prophyry was invaded by the
granitic liquid, that both the porphyry and granite were cracked after
solidification, and that liquid basalt rose and filled the cracks. Hence
the porphyry is the oldest, the granite next in age, and the basalt is
youngest. In fact, it may be mentioned in passing that some basalt and
allied dikes have been found cutting through the sedimentary sandstone,
shale, and limestone which overlie the igneous granite and porphyry and
are much younger.
Missouri porphyry has little use or value other than of bulk or crushed
stone.