Geology Reference
In-Depth Information
grains rather than around them
when it is struck. Pure quartzite
is white; however, iron and other
impurities commonly impart a
red or other color to it. Quartzite
is commonly used as foundation
material for road and railway
beds.
The name
greenstone
is ap-
plied to any compact, dark-green,
altered, mafic igneous rock that
formed under low- to high-
grade metamorphic conditions.
The green color results from the
presence of chlorite, epidote, and
hornblende.
Hornfels
is a common, fine-
grained, nonfoliated metamor-
phic rock resulting from contact
metamorphism consisting of
various equidimensional min-
eral grains. The composition of
hornfels depends directly on the
composition of the original rock,
◗
Figure 7.14
Migmatites A migmatite boulder in the Rocky Mountain National Park, near Estes
Park, Colorado. Migmatities consist of high-grade metamorphic rock intermixed with streaks or
lenses of granite.
and many compositional varieties are known. The majority
of hornfels, however, are apparently derived from contact
metamorphism of clay-rich sedimentary rocks or impure
dolostones.
Anthracite
is a black, lustrous, hard coal that contains
a high percentage of fixed carbon and a low percentage
of volatile matter. It is highly valued by people who burn
coal for heating and power. Anthracite usually forms from
the metamorphism of lower-grade coals by heat and pres-
sure, and many geologists consider it to be a metamorphic
rock.
While mapping the 440- to 400-million-year-old Dalra-
dian schists of Scotland in the late 1800s, George Barrow
and other British geologists made the fi rst systematic study
of metamorphic zones. Here, clay-rich sedimentary rocks
have been subjected to regional metamorphism, and the
resulting metamorphic rocks can be divided into different
◗
Figure 7.15
Nonfoliated Texture Nonfoliated textures are
characterized by a mosaic of roughly equidimensional minerals, as
in this photomicrograph of marble.
Metamorphism
Limestone
Marble
◗
Figure 7.16
Marble Results from the Metamorphism of the Sedimentary Rock Limestone or Dolostone
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