422 Adventures in Scenery 



Boulder A fragment of rock, usually large and rounded in shape, generally 

 but not always brought from a distance by natural means. 



Breccia A rock composed of angular fragments. Distinguished from con- 

 glomerate as not water-worn. 



Canyon A valley, usually precipitous; a gorge. 



Chert A compact, siliceous rock formed of chalcedonic or opaline silica, 

 one or both, and of organic or precipitated origin. Flint is a variety 

 of chert. 



Cirque A steep-walled, amphitheatre-like recess in a mountain side, the 

 starting-place of a glacier. 



Clay Principally decomposed feldspar, aluminum silicate. The most abun- 

 dant of the materials derived from the decomposition of igneous rocks. 

 Clay and silt differ in fineness of particles largely, those of clay being 

 microscopically small. Many so-called clays are chiefly siliceous silt. 



Conglomerate An aggregate of rounded and water-worn pebbles and boul- 

 ders cemented together into a coherent rock. 



Consequent (river) A river having a course determined by the form and 

 slope of the surface of the land. 



Coulee A deep gulch or water channel, usually dry. 



Dacite An igneous rock (generally volcanic) containing essential (plagio- 

 clase) feldspar and quartz, with or without horneblende and biotite; 

 quartz-andesite. 



Delta An alluvial deposit at the mouth of a river. 



Diabase A basic igneous rock usually occurring in dikes or intrusive sheets, 

 composed essentially of (plagioclase) feldspar and augite, with small 

 amounts of other minerals. 



Diatom A minute plant (of microscopic size) which is provided with a 

 siliceous envelope. Diatomaceous earth may be composed of nearly pure 

 silica from the frustules of the microscopic plants called diatoms. 



Dike A thin body of igneous rock forced in molten condition into a fissure 

 in older rocks, and there solidified (cooled). May vary in thickness 

 from an inch or two to 300 feet. 



Diorite An intimate mixture of crystals of horneblende and (plagioclase) 

 feldspar, with biotite (mica) or augite. A granitic rock. If consider- 

 able quartz is present it is called quartz-diorite. 



Drift Earth materials, as boulders, gravel, sand and clay, transported by a 

 glacier and deposited from the melting ice. 



Epoch Division of a period. The time during which a formation or group 

 of strata was deposited. 



Era The largest division of geologic time. The geologic eras are: Archaean 

 or Proterozoic, Palaeozoic, Mesozoic, and Caenozoic. 



