Rhyolite

Rhyolite is an extrusive igneous rock …igneous meaning from fire

and extrusive meaning the rock was volcanically expelled as lava

or magma. Rhyolite is very closely related to granite. The

difference is that Rhyolite has much finer crystals, having cooled

much more rapidly than granite. These crystals are so small that

they cannot be seen by the naked eye. The minerals that make up

Rhyolite are quartz, feldspar, mica, and hornblende. There are

primarily three colors of Rhyolite in Douglas County: pink, gray

and buff (tan).

The Rhyolite in Douglas County came from volcanic activity far to

the south of Castle Rock. Two main Rhyolite quarries existed in

Castle Rock, the Madge Quarry started by Silas Madge in 1872

South of present day Castle Rock and the O’Brien Quarry

Southeast of Castle Rock.

Examples of Rhyolite buildings: Castle Rock First National Bank

(300 Wilcox, Street-side facades), Castle Hotel, the original parts

of the Court House, Cantril School, Castle Rock Depot (Now the

museum). The Molly Brown House in Denver was built with Castle

Rock gray Rhyolite.