The fine-grained equivalent of granite. Its broad, gentle slopes are usually formed by basaltic lava flows or rhyolite ash. This rock contains albite, alkali feldspar and an least one mafic mineral. This bulge is caused by pressure build-up between the cooler, slower moving crust of the flow and the hotter, more fluid lava beneath the surface. The minerals involved are commonly augite and olivine.
Vesicles are gas bubbles in magma or lava.
Bates, Robert L., and Julia A. Jackson. Glossary of Geology , third edition. American Geological Institute, 1987.