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any of a group of dark gray to black intrusive igneous rocks that generally occur as dikes (tabular bodies inserted in fissures). Such rocks are characterized by a porphyritic texture in which large crystals (phenocrysts) of dark, iron-magnesium (mafic) minerals are enclosed in a fine-grained to dense matrix (groundmass). The abundance, large size, well-formed crystal outline, and brilliantly reflecting cleavage faces of the mafic phenocrysts give the rock a striking appearance. Mafic minerals, including biotite, hornblende, augite, or olivine, not only constitute virtually all of the phenocrysts but occur in the groundmass as well, together with much potash feldspar, plagioclase, or feldspathoid.