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ROCK CLASSIFICATION: Igneous Rocks are formed from a molten state and are either igneous or plutonic. Igneous rocks are formed from extrusive lava which spilled out onto the surface or from intrusive lavas, also know as magmas, which have solidified quickly. Extrusive igneous rocks include obsidian, felsite, and basalt. Intrusive igneous rocks include porphyry, and diabase. These are fine-grained mixes of unrecognizable minerals that show flow lines. Pegmatites are composed of large crystals of quartz, feldspar, and mica, formed in the late stages of cooling in granitic magmas. Pegmatites in Maine often contain such minerals as beryl, tourmaline, lepidolite, apatite, quartz, and garnet.

Plutonic Rocks have cooled and solidified at some depth and much more slowly. Plutonic rocks include granite, diorite, gabbro, and peridotite or pyroxenite. These are course-grained and composed of common primary mineral such as quartz, feldspar, mica, and dark mineral.

Sedimentary Rocks form when wind, water and ice break down rock formations, carry them off, and deposit them in layers where they pile up thicker and thicker, gradually forming rigidly cemented masses. Feldspars change to clay, metals dissolve in water. Sedimentary rocks include arkose, conglomerate, breccia, sandstone, shale, limestone, dolomite, salt and coal. These are low temperature minerals banded, stratified, and often fossiliferous.

Metamorphic Rocks are formed by the pressure and heat that accompany deep burial of sedimentary rocks creating an environment like the original one deep under the crust or in volcanoes. Moisture, oxygen and carbon dioxide are forced out and the sediments making up the shale, sandstone, or limestone begin to change. For example, the clay of shales will revert back to the original feldspar and mica, become metamorphosed or changed. Metamorphic Rocks include slate, phyllite, schist, gneiss, quartzite, marble and Hornfels. These are high temperature minerals banded, stratified and concentrated in single mineral formations.

—SUSAN VARNEY




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