THE ORIGIN AND DESCENT OF ROCKS. 469 



Coquina — a rock formed almost wholly of small and broken shells; especially- 

 applied to a shell limestone of Florida. 



Dacite (quart z-andesite) — an andesite (q.v.) with quartz. 



Diabase — a dolerite (q.v.) which has undergone alteration; consists essentially 

 of plagioclase feldspar and augite, with magnetite or titaniferous iron as a common 

 accessory; one of the greenstones. 



Diatom ooze — a soft silicious deposit found on the bottom of the deep sea, 

 made largely or partly of the shells of diatoms ; similar deposits are formed from 

 the shells of radiolaria. 



Diorite — an igneous rock usually of dark - greenish color, consisting of 

 plagioclase feldspar and hornblende; often speckled from the commingling of 

 light feldspar and dark hornblende. 



Dolerite — a fine-grained igneous rock composed of plagioclase feldspar (labra- 

 dorite or anorthite) and augite (or related ferromagnesian mineral, as enstatite, 

 olivine, or biotite), with magnetic or titaniferous iron as common accessories; 

 crystals usually of medium size, assuming the ophitic structure; embraces many 

 of the greenstones; graduates into basalt on the one hand and gabbro on the 

 other. 



Dolomite — a magnesian limestone. 



Drift — in common American usage, a mixture of clay, sand, gravel, and bowl- 

 ders formed by glacial agencies. 



Eolian rocks — deposits formed by wind, embracing esiDecially dunes and 

 one variety of loess. 



Felsite (felstone) — a light-colored aphanitic rock composed of feldspar often 

 with quartz, in which the crystallization is very imperfect or obscure, gi\'ing a 

 close-gi'ained texture with conchoidal fracture and flinty aspect; certain varieties 

 are called petrosilex and halleflinta. 



Flint — a compact dark chalcedonic or lithoid form of quartz. 



Freestone — a sandstone of uniform grain without special tendency to split 

 in any direction. 



Fulgurites — glassy tubes, produced through fusion by lightning in penetrating 

 sand, earth, or rock. 



Gabbro (euphotide) — a crystalline rock composed of the plagioclase 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. 



Gannister — essentially a quartz silt or pulverized quartz used for lining iron 

 furnaces. 



Garnetite — a rock composed largely of garnets. 



Geest — residual earth or clay left by the decomposition of rocks, especially 

 limestones. 



Geyserite — the silicious sinter deposited about hot springs. 



Globulites — minute spherical bodies embraced in volcanic glass. 



Gneiss — a foliated granite, consisting typically of quartz, feldspar, and mica; 

 the feldspar typically orthoclase. 



Granite — a granular crystalline aggregate of quartz, feldspar, and mica; the 



