POPULAR GEOLOGY

The black rock, “Cold Spring Black” (trade name), was quarried near Alma in Quebec, Canada; the dark gray diorite, “Academy Black,” was quarried northeast of Fresno, California in the Sierra Nevada foothills; and the light gray granite, “Rockville White,” was quarried near the small town of Rockville, Minnesota.

Dark gray and black granite-like rocks called “black granite” in the building stone industry are usually diorite or gabbro to geologists. Diorite is dark gray to blackish-gray igneous rock mainly composed of sodium-rich plagioclase feldspar, hornblende, pyroxene, and a small amount of quartz. Gabbro is dark-colored, coarse-grained igneous rock characteristically composed of calcium-rich plagioclase feldspar, pyroxene, and possibly olivine.

“Cold Spring Black” is a mafic (contains iron and magnesium) metamorphic rock that most likely formed during the early Precambrian over 2.5 billion years ago. Originally, it was probably a gabbro that was later subjected to low-temperature, low-pressure metamorphism as it was uplifted.

“Academy Black” contains crystals of hornblende and pyroxene interlocking with smaller crystals of plagioclase feldspar. It is part of the Knowles Granodiorite which formed during the early Cretaceous Period around 110 million years ago.

“Rockville White” comes from the approximately 1.8 billion-year-old Rockville Granite. The Rockville Granite is one of several granite bodies located in east-central Minnesota. This granite has been quarried since the early 1900s and varies in color from a dominantly pink to a dominantly white stone. It is coarse grained and consists of larger crystals of pale pink to white feldspar with smaller crystals of quartz and black mica.

Cross 300 South and Main Street and continue south down the west side of Main Street until you reach the Frank E. Moss United States Courthouse.