GLOSSARY OF GEOLOGIC TERMS. 



Alluvial fan. The outspread sloping deposit of bowlders, gravel, and sand left by a 

 stream where it passes from a gorge out upon a plain. 



Andesite, A lava of widespread occurrence, usually of dark-gray color and inter- 

 mediate in chemical composition between rhyolitc and basalt. 



Anticline. An arch of bedded or layered rock suggestive in form of an overturned 

 canoe. (See fig. 3, p. 18.) (See also Dome and Syncline.) 



Badlands. A region nearly devoid of vegetation where erosion, instead of carving 

 hills and valleys of the familiar type, has cut the land into an intricate maze of 

 narrow ravines and sharp crests and pinnacles. Travel across such a region ia 



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almost impossible; hence the name. 

 Basalt. A common lava of dark color and of great fluidity when molten. Basalt ia 



less siliceous than granite and rhyolite, and contains much more iron, calcium, 



and magnesium. 

 Bolson (pronounced bowl-sowi/). A flat-floored desert valley that drains to a central 



evaporation pan or playa. 



Bomb. See Volcanic bomb. 



Breccia (pronounced bretch^a). A mass of naturally cemented angular rock frag- 

 ments. 



Crystalline rock, A rock composed of closely fitting mineral crystals that have 



formed in the rock substance as contrasted with one made up of cemented grains 

 of sand or other material or with a volcanic glass. 



Diabase. A heavy, dark intrusive rock having the same composition as basalt, but, 

 on account of its slower cooling, a more crystalUne texture. Its principal con- 

 stituent minerals are feldspar, augite, and usually oUvine. OUvine is easily 

 changed by weathering, and in many diabases is no longer recognizable. Augite 

 is a mineral containing iron and magnesium and is similar to hornblende. 



Dike. A mass of igneous rock that has solidified in a wide fissure or crack in the 



earth's crust. 

 Diorite. An even -grained intrusive igneous rock consisting chiefly of the minerals 



feldspar, hornblende, and very commonly black mica. If the rock contains 



much quartz, it is called quartz diorite. Quartz diorite resembles granite and is 



connected with that rock by many intermediate varieties, including monzonite. 



The feldspar in diorite differs from that in granite in containing calcium and . 



sodium instead of potassium. Hornblende is a green or black mineral containing 



iron, magnesium, calcium, and other constituents. 

 Dip. The slope of a rock layer expressed by the angle which the top or bottom of 



the layer makes with a horizontal plane. (See fig. 9, p. 51.) (See also Strike.) 

 Dissected, Cut by erosion into hills and valleys. Applicable especially to plains 



or peneplains in process of erosion after an uplift. 

 Dome. Ab applied to rock layers or beds, a short anticline, suggestive of an inverted 



basin. 



Drift. The rock fragments— soil, gravel, and eilt— carried by a glacier. Drift in- 

 cludes the unassorted material known as till and deposits made by streams flow- 

 ing from a glacier. 



Erosion. The wearing away of materials at the earth's surface by the mechanical 

 action of running water, waves, moving ice, or winds, which use rock fragments 

 and grains as their tools or abra^ves. Erosion ia aided by weathering. (See 

 Weathering.) ^^^ 



