GLOSSARY OF GEOLOGIC TERMS. 
Alluvial fan. The outspread sloping deposit of alee. gravel, and sand left by 
a stream where it passes from a gorge out upon a plai 
Andesite. A lava of widespread occurrence, DN of darks ned color and inter- 
mediate in chemical composition between’ rhyolite and basa 
Anticline. Arch of bedded or layered rock suggestive in 3 of an overturned 
canoe. (See fig. 20, p. 102.) (See also Dome and Syncline. 
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 is 
almost impossible, hence the name. (See Pls. VI-IX, pp. 62-63. 
asalt. 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 
Breccia (pronounced bretch’a). A mass of naturally cemented angular rock frag- 
talline 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 crystalline es Its principal con- 
stituent minerals are feldspar, hese and usually olivine. Olivine is easily 
changed by weathering, and in ognizable. Augite 
is a mineral containing iron wend ia sehen and is similar % 5 nib ieasdi: 
Dike. A mass of igneous rock that has solidified in a wide fissure or crack in the 
earth’s crust. (See fig. 15, p. 95.) 
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. 2, p. 17. 
Dissected. Cut by erosion into hills and valleys. Applicable especially to plains 
or peneplains in process of erosion after an uplift. 
Dome. As applied to rock layers or beds, a short anticline, suggestive of an in- 
verted basin 
The rock fragments—soil, gravel, and silt—carried by a glacier. Drift i 
cludes the unassorted material known as till and deposits made by streams flow- 
ing from a glacier. oe a : s 
. The Wear Way f materials abu 
- tion idl Fistiniing wate, waves, moving ice, or winds, which use , rock pe 
grains as tools or abrasives. Erosion is aided by weathering. (See west 
