Page 34 - History of Rockingham Co
P. 34
ROCKINGHAM COUNTY
thrown out on the surface by the extrusive action of ancient
volcanoes. The material in its original form was dark or al-
most black in color, but subsequent alterations and metamor-
phic changes have usually converted it into rock varying in
color from a dark bluish-green to a light green, dependent on
the secondary mineralogical constituents. Where these min-
erals are secondary amphibole and chlorite the rock is bluish
green in color; and where epidote is the dominant altera-
tion product the color is light green. There are all grada-
tions between these different colors depending on the propor-
tions of these minerals. Originally the essential minerals
which made up the basalt was plagioclase feldspar, magne-
tite, and pyroxene, but subsequent alterations have almost en-
tirely broken down the original mineral components giving
rise chiefly to the three above mentioned, so that at
present the rock may properly be termed an epidosite where
the dominant mineral is epidote, an amphibolite where the
dominant mineral is amphibole, and a chlorite-schist where the
dominant mineral is chlorite. For all practical purposes the
tv/o latter types may rightly be thrown together and termed
amphibolite-chlorite schists. The general name for this ba-
saltic formation wherever it occurs in the Blue Ridge and the
Piedmont plateau is Catoctin Schist, so called because it
usually presents a slaty or schistose structure, induced by the
folding of the earth's crust and the development of the sec-
ondary minerals contingent upon such conditions. The rock
frequently shows material picked up by the liquid lava streams
as they flowed over the ancient land surface covered with the
material of older flows; and gas bubbles, or amugdules, are
common in the upper surfaces. These cavities have since
been filled by percolating waters carrying mineral matter in
solution, the resulting minerals being usually epidote, calcite,
and quartz. It is within the Catoctin Schist that the copper
deposits of the Blue Ridge occur. The rock has been as-
signed to the Algonkian Period and is therefore one of the
oldest formations occurring in the state, and certainly the
oldest in the county.
—25-
thrown out on the surface by the extrusive action of ancient
volcanoes. The material in its original form was dark or al-
most black in color, but subsequent alterations and metamor-
phic changes have usually converted it into rock varying in
color from a dark bluish-green to a light green, dependent on
the secondary mineralogical constituents. Where these min-
erals are secondary amphibole and chlorite the rock is bluish
green in color; and where epidote is the dominant altera-
tion product the color is light green. There are all grada-
tions between these different colors depending on the propor-
tions of these minerals. Originally the essential minerals
which made up the basalt was plagioclase feldspar, magne-
tite, and pyroxene, but subsequent alterations have almost en-
tirely broken down the original mineral components giving
rise chiefly to the three above mentioned, so that at
present the rock may properly be termed an epidosite where
the dominant mineral is epidote, an amphibolite where the
dominant mineral is amphibole, and a chlorite-schist where the
dominant mineral is chlorite. For all practical purposes the
tv/o latter types may rightly be thrown together and termed
amphibolite-chlorite schists. The general name for this ba-
saltic formation wherever it occurs in the Blue Ridge and the
Piedmont plateau is Catoctin Schist, so called because it
usually presents a slaty or schistose structure, induced by the
folding of the earth's crust and the development of the sec-
ondary minerals contingent upon such conditions. The rock
frequently shows material picked up by the liquid lava streams
as they flowed over the ancient land surface covered with the
material of older flows; and gas bubbles, or amugdules, are
common in the upper surfaces. These cavities have since
been filled by percolating waters carrying mineral matter in
solution, the resulting minerals being usually epidote, calcite,
and quartz. It is within the Catoctin Schist that the copper
deposits of the Blue Ridge occur. The rock has been as-
signed to the Algonkian Period and is therefore one of the
oldest formations occurring in the state, and certainly the
oldest in the county.
—25-
