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. (See PI. XLII, p. 188.) 



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

 mediate in chemical composition between rhyolite and basalt. 



Anticline. An arch of bedded or layered rock suggestiA'e in form of an overturned 

 canoe. (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 ra\dnes and sharp crests and pinnacles. Travel across such a region is 

 almost impossible, hence the name. 



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

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

 and magnesium. 



Bolson (pronounced bowl-sown^. A flat-floored desert valley that drains to a central 

 evaporation pan or playa. 



Bomb. See Volcanic bomb. 



Breccia (pronounced bretcVa). A mass of naturally cemented angular rock frag- 

 ments. 



Ci-ystalline 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 haA^ing the same composition as basalt, but, 

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

 stituent minerals are feldspar, augite, and usually olivine. Olivine is easily 

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



x= ^ miu«rai comammg iron and magnesium and ia 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 

 felds^par, hornblende, and very commoidy 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 m diorite differs from that in granite in containing calcium and 

 BQdium 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 also Strike ) 



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

 or peneplains m process of erosion after an uplift 



^""iZm^ ^^^^'""^ ^'^ '"^'^ ^^^^"^ ""' ^''^'' * '^°'* anticline, suggestive of an inverted 



^"^^ ^^V"^^ fnigments-soil. gravel, and silt^arried by a glacier. Drift in- 

 cludes the unassorted material known as tiU and deposit, made by streams flow- 



ing from the glacier. ^ . -^ 



^TJ^r* ^f^' ''''"°° '"^^^ ""^ °^^''^^^ "* '^"^ ^^''^'^ ^^"rface by the mechanical 



and'linr'rT ""'T' '^'''''' r '"^^ '''' '' ^^"*^^' ^^-^"^h ^^^ -oek fragments 

 and grains a. tools or abrasives. Erosion is aided by weathering. See Weathering. 



