Metamorphic rock—rock formed from [igneous] or [sedimentary rocks] that are altered by heat, pressure, and [fluids] below the earth’s surface.
Miocene—the fourth [epoch] of the [Tertiary] [Period] (see [p. 3]).
Mississippian—the fifth [period] of the [Paleozoic] [Era] (see [p. 3]).
Nodule—a small, rounded mass or lump.
Octahedron—a solid that has eight triangular faces.
Opaque—no light can pass through.
Ordovician—the second [period] of the [Paleozoic] [Era] (see [p. 3]).
Paleozoic—an [era], one of the great divisions of geologic time (see [p. 3]). This era began at the end of [Precambrian] time and lasted until about 230 million years ago.
Parting—occurs when a mineral breaks along a flat surface that is not a true [cleavage] plane.
Pebble—a rock or mineral fragment that has a diameter between 4 and 64 millimeters (about ¹⁵/₁₀₀ and 2½ inches).