PORPHYRY is an igneous rock identified by its texture rather than its
mineral content, which is variable. Distinct crystals (phenocrysts) of
minerals are embedded in a matrix of fine-grained rock. The phenocrysts
formed before the main mass of the rock hardened.
Any igneous rock may have a porphyritic variety, such as granite
porphyry and rhyolite porphyry, although porphyries are most likely to
form in association with fine-grained igneous rocks.
Porphyry is found in Illinois only in glacial drift.