84 GEOLOGY 



feldspar labradorite (or anorthite), and diallage (or a related ferromagnesian 

 mineral), with magnetite or titaniferous iron as a common accessory. 



Gangue. A term applied to the crystalline material in which ores are 

 imbedded. 



Ganister. Essentially a quartz silt, pulverized quartz, or silicious fire clay 

 suitable for lining iron furnaces. 



Garnetite. A rock composed largely of garnets. 



Geyserite. The silicious sinter deposited about hot springs. 



Gneiss. A foliated granite, consisting normally of quartz, feldspar, and 

 mica; the feldspar typically orthoclase. 



Granite. A granular crystalline aggregate of quartz, feldspar, and mica; 

 the feldspar typically orthoclase; popularly and properly used for any dis- 

 tinctly granular crystalline rock. 



Granulite. A fine-grained granite with little or no mica. 



Graphic granite. (See pegmatite.) 



Greensand. A sand or sandstone containing a notable percentage of 

 grains of glauconite. 



Greenstone. A comprehensive term used to designate igneous and 

 metamorphic crystalline rocks of greenish hue and of intricate and often 

 minute crystallization; they are mostly dolerites, diabases, and diorites; a 

 convenient term for field use where the constituents cannot be determined, 

 and for general use when the variety is unimportant. 



Greywacke. A sand rock in which the grains are largely basic silicates 

 instead of quartz. 



Hornblendite. A rock essentially composed of hornblende. 



Hornstone. A very compact, silicious rock of horn-like texture, allied 

 to flint; term also applied to flinty forms of felsite. 



Hydraulic limestone. (See waterlime.) 



Hypogene rocks. Those formed deep within the earth under the influence 

 of heat and pressure. 



Ironstone. A rock composed largely of iron, usually applied to clayey 

 rocks having a large iron content. 



Infusorial earth (tripolite). An earthy or silt deposit consisting chiefly 

 of the silicious secretions of diatoms. 



Itacolumite. A flexible sandstone whose pliability is due to an open 

 arrangement of sand grains which are held together by scales of mica. 



Jasper. A reddish variety of chalcedonic quartz. 



Lapilli. Small fragments of lava ejected from volcanoes; volcanic cinders. 



Lava. A molten rock, especially applied to flows upon the surface, 

 whether from vents or from fissures; also applied to the solidified product. 



Lignite, (brown coal). A soft, brown, impure coal. 



Limestone. A rock composed primarily of calcium carbonate, though 

 magnesium sometimes replaces a part of the calcium. (See dolomite and 

 marble.) 



