468 GEOLOGY, 



Arenaceous rocks- -either those which are mainly sand or those in which sand 

 is a notable accessory. 



Argillite — a clayey rock; usually applied to hard varieties only. 



Arkose — a sand or sandstone formed of disaggregated granite or similar rock 

 in which a notable part of the grains are feldspar or other silicate; sand when 

 undefined, is understood to be quartzose. 



Augitite — a rock mainly made up of augite. 



Basalt — a dark, compact basic igneous rock consisting of a mass of minute 

 crystals sometimes with more or less glassy base, often containing also visible 

 crystals; composed of plagioclase and pyroxene, with olivine, magnetite, or 

 titaniferous iron as common accessories; a basic lava in which the crystallization 

 has taken place rapidly; usually rich in crystallites or microlites; graduates 

 into dolerite and basic andesite. 



Bituminous coal — common soft coal, intermediate between lignite and anthra- 

 cite; contains much bituminous matter, i.e., hydrocarbons. 



Bowlders — rounded masses of rock, particularly those that have been shaped 

 by glaciers. 



Breccia — a rock composed of angular fragments, contrasted with pudding- 

 stone or conglomerate, in which the fragments are rounded. 



Buhrstone — a compact, flint-like silicious rock full of small cavities, so named 

 from use as millstones. 



Calc-sinter (calcareous tufa) — a loose cellular deposit of calcium carbonate 

 made by springs ; travertine is the better term, as tufa should be left for volcanic 

 elastics. 



Cannel coal — a very fine-grained homogeneous bituminous coal, giving off 

 much gas and burning with a candle-like flame. 



Chalk — a fine-grained soft rock composed essentially of calcium carbonate 

 derived from minute marine organisms. 



Chlorite schist — a schistose rock in which chlorite is a predominant mineral; 

 usually greenish, whence the name. 



Clastic rock — formed from the debris of broken-down rocks; the same as 

 fragmental or detrital rock. 



Clay — a term commonly applied to any soft, unctuous, adhesive deposit, but 

 in strict use confined to material composed of aluminum silicate; many so-called 

 clays are chiefly silicious silts or loams. 



Clay ironstone — a clayey rock heavily charged with iron oxide, usually limonite; 

 commonly in concretionary form. 



Clinkstone — a name applied to phonolite because of its metallic clinking 

 sound when struck; composed of orthoclase, with nephelite and one or more of 

 the ferromagnesian minerals as accessories. 



Chert— an impure flint, usually of light color, occurring abundantly in con- 

 cretionary form as nodules in certain limestones. 



Coal — a carbonaceous deposit formed from the remains of plants by partial 

 decomposition. 



Concretions — aggregates of rounded outlines formed about a nucleus; the 

 material is various: clay, iron ore, calcite, silica, etc. 



Conglomerate (pudding-stone) — a rock formed from rounded pebbles, con- 

 solidated gravel. 



