In the rock identification charts on pages 40-41, the Texas rocks
described in this book are arranged in four major groups according to
their texture.
1. _Glassy_ (the rocks are smooth, dark, and shiny)
2. _Compact, dull, or stony_ (the rocks are smooth and dull, but the
individual grains are too small to be recognized)
3. _Granular_ (at least some of the individual grains of the rocks are
large enough to be seen without a magnifying glass)
4. _Fragmental_ (the rocks are made up of fragments that are either
loose or cemented together)
Consult Rock Chart 1, if the rock is glassy; Chart 2, if it is compact,
dull, or stony; Chart 3, if it is granular; and Chart 4, if it is
fragmental.
Two of the rock charts are subdivided. In Rock Chart 2, the compact,
dull, or stony rocks are arranged according to hardness as follows:
A. Rocks that can be scratched by a fingernail
B. Rocks that cannot be scratched by a fingernail but can be scratched
by an ordinary pocket knife
C. Rocks that cannot be scratched by an ordinary pocket knife
In Rock Chart 3, the granular rocks also are arranged according to
_hardness_ into:
A. Rocks that can be scratched by an ordinary pocket knife
B. Rocks that cannot be scratched by an ordinary pocket knife
These harder rocks are subdivided into three groups:
1. Those that have grains of about equal size
2. Those with large easily seen grains that are scattered through a
mass of finer grains
3. Those rocks whose grains are arranged in layers
In the “remarks” column of the rock identification charts are included
further tests that will aid you in identifying the rock.
For a more complete rock determination chart, you can consult a
textbook, such as _Rocks and Rock Minerals_, by L. V. Pirsson and A.
Knopf.