BASALT is the most widely distributed volcanic rock. Pyroxene, feldspar,
magnetite (an iron ore), and in some instances olivine, biotite, and
hornblende, compose this rock. The dark green, gray, or black color is
due to the dark-colored minerals that make up much of the rock. The
minerals in basalt are fine grained and are packed closely together.
Phenocrysts of olivine, pyroxene, and hornblende may be present. Basalt
is easily identified by its color and fine-grained texture.
The glaciers brought basalt into Illinois along with other igneous
rocks.