GRANITE is one of the most widespread intrusive (originating deep within the earth) igneous rocks. It consists chiefly of feldspar and quartz with small amounts of biotite, muscovite, or hornblende. Most granite is light colored, but it can be white, gray, yellow, pink, or deep red. The texture ranges from medium grained to coarse grained. Granite pebbles or boulders are the most common igneous rocks found in glacial deposits in Illinois. They are not native to the state but were brought here by the great ice sheets or glaciers that advanced from southern Canada to cover much of northern United States during Pleistocene time. Native granitic rock probably lies very deep beneath the entire state. It has been found in deep oil-test drillings along the western and northern margins of Illinois. GABBRO (2) GABBRO is another intrusive igneous rock, but it is heavier and darker than granite. It is composed mainly of feldspar and dark iron-bearing minerals that give the rock a dark color. It is coarse grained and contains little or no quartz. Mineral crystals of gabbro are especially tightly interlocked, making the rock very difficult to break. Weathered gabbro is a rusty color on the surface, because the iron in gabbro changes color just as a piece of metallic iron becomes coated with rust when left out of doors. Like other igneous rocks found near the surface in Illinois, gabbro was carried into Illinois by the glaciers and deposited as glacial debris. PORPHYRY (3) 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. BASALT (4) 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. PERIDOTITE (5)