MATERIALS OF THE EARTH 83 



Chert. An impure flint, usually of light color, occurring abundantly in 

 concretionary 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 (puddingstone). A rock formed from rounded pebbles; 

 consolidated gravel. 



Coquina. A rock formed almost wholly of small and broken shells; 

 especially applied to a shell limestone of Florida. 



Dacite (quartz-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 secretions of diatoms. 



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 

 (labradorite 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 

 bowlders formed by glacial agencies. 



Eolian rocks. Deposits formed by wind, embracing especially dunes 

 and much of the loess. 



Felsite (felstone). A light-colored aphanitic rock composed of feldspar, 

 often with quartz, in which the crystallization is very imperfect or obscure, 

 giving a close-grained texture with conchoidal fracture and flinty aspect; 

 certain varieties are called petrosilex and halleflinta. 



Fire clay. A clay capable of standing a high degree of heat. The per- 

 centage of iron, calcium, magnesium, and the alkalies must be very low. 

 Often found in association with coal. 



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 pen- 

 etrating sand, earth, or rock. 



Gabbro (euphotide). A crystalline rock composed of the plagioclase 



