_Chart _Rock_ _Remarks_
No._
1. GLASSY appearance (rock is dark, smooth, and shiny)
Obsidian Entire rock is glassy
Vitrophyre Crystalline grains are scattered
through the dark glassy mass
2. COMPACT, DULL, OR STONY appearance (individual grains too small to be
recognized)
A. Can be scratched by a fingernail
Chalk Dilute hydrochloric acid fizzes on it
Clay Earthy odor when breathed on
Diatomite Crumbly
Rock gypsum Made up of the mineral gypsum
Soapstone Soapy or greasy feel
B. Cannot be scratched by a fingernail but can be scratched by
ordinary pocket knife
Dolomite Dilute hydrochloric acid may fizz
slightly on it; will fizz if rock is
powdered
Limestone Dilute hydrochloric acid fizzes on it
Serpentine rock Commonly some shade of green
Shale Breaks in flat, thin flakes; earthy odor
C. Cannot be scratched by an ordinary pocket knife
Basalt Dark colored and heavy
Chert Hard, smooth, and porcelain-like
Rhyolite Light to dark colored; may show streaks
or flow structure
3. GRANULAR appearance (at least some of the individual grains are large
enough to be seen without a magnifying glass)
A. Can be scratched by an ordinary pocket knife
Limestone Dilute hydrochloric acid will fizz on it
Marble Dilute hydrochloric acid will fizz on
calcite marble, and it may fizz
slightly on dolomite marble
Rock gypsum Made up of the mineral gypsum
Rock salt Has a salty taste; made up of the
mineral _halite_
B. Generally cannot be scratched by an ordinary pocket knife (some
schist is softer)
Grains are of about equal size (equigranular)
1.
Granite Quartz and feldspar grains interlock
Pegmatite Large interlocking grains; commonly
feldspar, quartz, mica
Quartzite Rock breaks across the quartz grains
Sandstone Rock breaks through the cement but
around the sand grains
Easily seen grains are scattered through a mass of finer grains
2.
Basalt Dark colored, heavy rock
Llanite Rusty pink feldspar and blue quartz
grains embedded in a brownish rock mass
Rhyolite porphyry Light to dark colored rock; may show
streaks or flow structure
Grains are arranged in layers
3.
Gneiss Interlocking grains are in straight or
wavy bands
Schist Splits in thin layers; some schist is
soft enough to be scratched by a knife
4. FRAGMENTAL appearance (rocks are made up of fragments that are either
loose or cemented together)
Breccia Angular, gravel-size fragments that are
cemented together
Conglomerate Rounded, gravel-size fragments that are
cemented together
Coquina Shells and shell fragments that are
cemented together
Gravel Loose fragments
Pulverulent Loose, powdery fragments; dilute
limestone hydrochloric acid fizzes on them
Sand Loose fragments no larger than a pinhead
Sandstone Sand fragments that are cemented
together
Volcanic ash Loose, fine, gritty particles
[Illustration: Physiographic outline map of Texas.]