I COARSE-GRAINED ROCKS
A. Rock consists of interlocking grains or crystals, easily seen; too hard to scratch with a knife 1. Crystals aligned in one direction a) Crystals in parallel bands with layers of quartz and feldspar separated by mica and other minerals Gneiss ([6]) b) Crystals in thin parallel bands; tends to split into thin sheets parallel to banding; some varieties may be scratched with a knife Schist ([6]) 2. Crystals not aligned in any particular direction a) Light gray, pink, red, or tan with only a few dark minerals; feldspar and quartz principal minerals Granite ([1]) b) Dark to medium gray; composed of feldspar and dark minerals with little quartz Gabbro ([2]) c) Dark green to black; essentially dark minerals, may have some feldspar; quartz generally lacking Peridotite ([5]) d) Light color; similar to granite in texture but lacks quartz; composed of feldspar and some dark minerals Syenite e) Large, easily seen crystals set in a fine- to extremely fine-grained background; any color Porphyry ([3]) f) Essentially quartz; grains may be identifiable; specimens break through rather than around grains Quartzite ([9]) B. Rock composed of individual rock particles or fragments, non-interlocking crystals, cemented or not cemented together; may or may not be scratched with a knife 1. Particles or fragments not uniform in size; a mixture of pebbles, sand, and smaller materials a) Solid rock consisting of particles or fragments generally rounded and cemented together Conglomerate ([7]) b) Solid rock consisting of particles or fragments, generally angular and cemented together Breccia c) Fragments ranging in size from clay to large boulders; may be compacted, but not cemented; much clay generally present; may effervesce Glacial till d) Loose particles of many sizes, not cemented together; some particles may effervesce Gravel 2. Rock particles or fragments, about the size of grains of sugar (2 to .05 mm) a) Loose particles consisting largely of quartz Sand b) Solid rock consisting largely of quartz; can be separated easily into individual particles; granular; breaks around rather than through grains Sandstone ([8])