Basalt is a fine-grained, dark-gray, dark-green, or greenish-black rock
which is hard enough to be scratched with difficulty by steel. It
originated by the solidification of lava. Today, basalt rock is forming
where lava at the Hawaiian volcanoes solidifies.
The relatively small amount of basalt in southeastern Missouri
solidified mostly in cracks within other rocks through which it rose.
Those occurrences—that is, fillings in nearly vertical cracks—are called
dikes. The basalt dikes in southeastern Missouri have been exposed by
the weathering and erosion of rocks which previously covered them.
[Illustration: A dark dike of basalt in granite near Silver Mines.]
In northern Missouri, boulders of basalt may be found in deposits of
glacial clay, sand, and gravel (glacial drift), where they were left
after the melting of the great ice sheet which brought the basalt down
from ancient dikes and igneous bodies cropping out in the northern
United States and Canada. Many of the boulders have been rounded by
weathering, and their shape, together with their dark color, has
stimulated the local name “niggerhead” for them.
[Illustration: Basalt in hand specimen.]
Basalt is a strong, tough, well-knit rock that will withstand heavy
blows from a sledge hammer, which usually rebounds upon striking. Except
for use as rubble stones, basalt has no commercial value. It weathers
characteristically to a yellowish, brownish, or reddish surface coating
of iron oxide and clay.