GLOSSARY 



Acidic A descriptive term applied to those igneous rocks that contain more 

 than 65% of silica. 



Alkali The opposite of acid; forming salts with acids. Alkalies have the 

 property of corroding organic substances. The term is applied to the 

 hydroxides of potassium, sodium, and ammonium. Vegetable matter 

 corroded leaves the carbon, hence the term "black alkali." 



Alluvial fan The outspread sloping deposit of boulders, gravel, and sand left 

 by a stream where it spreads from a gorge upon a plain or open valley 

 bottom. 



Andesite A volcanic rock of porphyritic or felsitic texture, whose crystal- 

 lized minerals are (plagioclase) feldspar and one or more of biotite 

 (mica), horneblende, or augite. 



Antecedent (river) A river that holds its early course in spite of crustal 

 movement of uplift across its course. 



Anticline A fold or arch of rock strata, dipping in opposite directions from 

 the axis. 



Arkose Material derived from the disintegration of granite. A sandstone 

 rich in feldspar fragments, as distinguished from the more common 

 richly quartzose varieties. 



Basalt The term is used to include all dark, basic, volcanic rocks. When 

 poured forth upon the surface it spreads in thin sheets. In cooling basalt 

 tends to take on a columnar structure. The leading minerals are 

 (plagioclase) feldspar and horneblende (pyroxene). Basalt is classed as 

 basic (low in silica). 



Base-level The level below which a land surface cannot be reduced by 

 running water. 



Basic A descriptive term for those igneous rocks that are comparatively low 

 (less than 50%) m silica. 



Batholith Huge irregular masses of plutonic rocks that have been forced 

 up from the interior of the earth in molten condition, and have cooled 

 and crystallized under a depth of overlying rocks, and have only been 

 exposed by erosion. 



Beach The wave-washed shore of a sea or lake. 



Bed-rock Called also basement complex. The series of slates, schists, and 

 associated igneous rocks, including the auriferous slate series, comprising 

 all the sedimentary formations from pre-Cambrian through Palaeozoic 

 and Jura-Trias. 



