Glossary 423 



Fault A break in the rock body with movement on one side or the other, 



so that parts of one continuous stratum are now separated. 

 Feldspar A general name for a group of rock-forming minerals of wide 



extent, silicates of the metals calcium, aluminum, sodium, potassium, 



as orthoclase, plagioclase, etc. Feldspar is found in practically all igne- 

 ous rocks. It is quite commonly flesh-colored, but varies in color. 

 Felsitic Almost or wholly crystalline, but made up of crystals too small to 



be readily distinguished by the unaided eye; said of the texture of some 



igneous rocks. 

 Ferromagnesian Containing iron and magnesium. Applied to dark silicate 



minerals, as amphibole, pyroxene, biotite (mica) and olivine, and to 



igneous rocks containing them. 



Fissure An extensive crack, break, or fracture in the rocks. 

 Floodplain The flat ground along a stream, covered by water at the flood 



stage. The plain is generally made up of muds, sands, and gravel. 

 Foraminifera Minute one-celled animals that secrete a calcareous (lime) 



shell. In myriad numbers on shallower ocean bottoms these microscopic 



shells form globigerina ooze. 

 Formation Any assemblage of rocks which have some character in common. 



Sometimes applied to the groups of related strata that were formed in a 



geologic period. 

 Fossil Remains or impression of a plant or animal of past geologic ages 



preserved in the rocks. 

 Gabbro A finely to coarsely crystalline igneous rock, composed mainly of 



lime-soda feldspar (labradorite or anorthite), pyroxene and frequently 



olivine. 

 Geomorphogeny That part of geomorphology which treats of the origin 



and development of the earth's surface features. 

 Granite A granular igneous rock composed essentially of quartz, feldspar, 



and mica, often with horneblende. 



Granite family The group of crystalline, homogeneous or non-foliated igne- 

 ous rocks resembling granite, as syenite, quartz-monzonite, granodiorite, 



diorite, monzonite, and all varieties of granite itself. 

 Granitic Characteristic of, composed of, or resembling granite. 

 Granodiorite A term employed for the intermediate rocks between granites 



and quartz-diorites. The term is a contraction of granite-diorite. 



Called also granite. 

 Hardhead A large, smooth, hard, rounded stone, generally a glacial boulder, 



occurring in coarse gravels and morainal deposits. 

 Horneblende An important rock-forming mineral, a silicate of calcium and 



