Chapter 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)