Vesicular basalt (scoria). Basalt is an extrusive (volcanic) igneous rock composed of feldspar, pyroxine, and occasionally olivine. Igneous rocks are formed by magma (molten rock) being cooled and becoming solid. The holes in this specimen are known as vesicles. They result from the expansion of trapped gas bubbles during cooling and solidification. Vesicles often contain deposits of other minerals such as calcite or zeolites.