Geological Survey Bulletin 1309 The Geologic Story of Isle Royale National Park

An attempt has been made to minimize the use of technical terms in this volume, but some jargon is inevitable in any discussion of technical matters. Most of the strictly geologic terms that are apt to be stumbling blocks are defined where they first appear in the text; for ease of reference, some of them have been summarized in this short glossary. Geologic time terms have not been included. (See the section on "Time as a Geologic Concept.")

Alluvial fan. A sloping fan-shaped mass of loose rock material deposited by a stream where it emerges from an upland into a broad valley or plain.

Alluvium. A general term for clay, silt, sand, and gravel deposited by running water, such as a stream.

Amygdule. A gas cavity or vesicle in volcanic rock that has been filled with secondary minerals such as calcite or quartz.

Basalt. The most common type of volcanic rock, generally fine grained, dark, heavy, and with a silica (SiO₂) content of about 50 percent or less.

Basement. The assemblage of metamorphic and igneous rocks underlying the stratified rocks in any particular region.

Breccia. A rock consisting of consolidated rock fragments like a conglomerate, except that most of the fragments are angular rather than rounded.

Chlorastrolite. A variety of the mineral pumpellyite, a complex hydrous calcium silicate. (See section on "Chlorastrolite—Michigan's State Gem.")

Chlorite. A green complex silicate mineral, similar in structure to the micas and usually of secondary origin.

Cleavage. The tendency of a mineral to break along definite planes, producing smooth surfaces.

Columnar jointing. Parallel prismatic columns, polygonal in cross section, formed by contraction during cooling in some lava flows.

Conglomerate. A rock, the consolidated equivalent of gravel.

Drumlin. A streamlined hill of glacial till elongate in the direction of flow of the glacier that formed it. The crag-and-rail variety is streamed out behind a bedrock knob.

Erratic (glacial). A rock fragment, usually of a large size, that has been transported from a distant source, especially by the action of glacial ice.

Fault. A break in rock strata along which displacement has occurred.

Felsite. A fine-grained light-colored volcanic rock, generally reddish. (See section on "Volcanic Rocks" for fuller description.)

Flood basalt (plateau basalt). A basaltic lava flow or successive lava flows that spread over very large areas from fissure eruptions.

Formation. In geology, an assemblage of rocks that have some character in common that distinguishes them from adjacent rocks.

Greenstone. (1) Informal name for the mineral chlorastrolite (pumpellyite); (2) a volcanic rock with a greenish hue, such as makes up Greenstone Ridge.

Igneous rock. A rock formed by solidification of hot molten material, either at depth in the earth's crust (plutonic) or erupted at the earth's surface (volcanic).

Keweenawan Supergroup. A sequence of geologic formations of which the Portage Lake Volcanics and the Copper Harbor Conglomerate are a part; Keweenawan is also used in formally to denote that part of Precambrian time during which the rock sequence was deposited.

Metamorphic rock. Rock changed materially in composition or appearance by heat, pressure, or infiltrations at depth in the earth's crust.

Moraine. An accumulation of glacial till with an initial topographic expression of its own, usually a ridge, unrelated to the surface upon which it lies. Several varieties have been recognized; for example, a recessional moraine is one formed at the margin of a receding ice sheet during a pause in its recession.

Ophite. A volcanic rock with a mottled texture, often exhibiting a knobby surface when weathered. (See section on "Volcanic Rocks" for fuller description.)

Pegmatite. A volcanic rock in which elongate plagioclase laths produce a matted appearance. (See section on "Volcanic Rocks" for fuller description.)

Playa. A level or nearly level area that occupies the lowest part of a completely closed basin and that is covered with water at irregular intervals, forming a temporary lake.

Porphyrite. A volcanic rock with a texture produced by well-defined plagioclase crystals scattered through a finer grained matrix. (See section on "Volcanic Rocks" for fuller description.)

Prehnite. A calcium aluminum silicate mineral. (See section on Prehnite—The Little Pink Pebbles.")

Pyroclastic rock. Rock formed by consolidation of ash or other fragmental material explosively ejected from a volcano.

Rhyolite. A light-colored commonly reddish volcanic rock with a high silica (SiO₂) content of 70 percent or more.

Sedimentary rock. Rock formed by consolidation of sediment deposited at the surface of the earth, chiefly in, or through the action of, water.

Silica (SiO₂). Occurs as the natural mineral quartz, including various fine-grained varieties such as chert, chalcedony, agate, and others. It also occurs in most rock-forming minerals (silicates) and is a major factor in the chemical classification of volcanic rocks.

Stratigraphy. The arrangement of rock strata, especially as to geographic position and chronologic order of sequence.

Syncline. In simple form, a concave-upward, or troughlike, fold in stratified rock.

Talus. An accumulation of coarse angular rock fragments derived from and resting at the base of a cliff or very steep slope.

Till. Glacially transported material deposited directly by ice, without transportation or sorting by water.

Trap. A fine-grained dark-colored volcanic rock. (See section on "Volcanic Rocks" for fuller description.)

Tuff. Rock formed from consolidation of volcanic ash.

Vesicle. Small cavity formed by the expansion of bubbles of gas during solidification of a volcanic rock.