Glossary 42 5 



Neve The mass of granular snow forming the upper part of a glacier. 



Obsidian Dense volcanic glass, having a conchoidal fracture. 



Oxidize To unite with oxygen. Many minerals and most metals oxidize 

 when exposed to air or water. 



Peneplain A land surface of slight relief and gentle slopes, worn down by 

 erosion almost to base-level. 



Period A unit of geologic time, division of an era. Example: Silurian 

 period, Devonian period, are units of the Palaeozoic era. 



Petrified wood Formed by replacement of wood by silica. Wood, shells, 

 bones, etc., embedded in sediments become converted into stone by the 

 gradual replacement of their tissues with infiltrated mineral material. 



Placer A mass of gravel, sand, or other loose material, resulting from the 

 crumbling and erosion of solid rocks, and containing particles or nug- 

 gets of gold, platinum, tin, or other valuable minerals, that have been 

 derived from rocks or veins. 



Plateau An upland, table-land, or elevated plain having a fairly smooth sur- 

 face and bounded on at least one side by an escarpment separating it 

 from lower country. 



Pleistocene The earlier of two epochs of the Quaternary period, also called 

 the glacial epoch. Also applied to series of sediments deposited during 

 that epoch. 



Plutonic Of igneous origin. Applied to those rocks that have crystallized 

 in the depths of the earth and have therefore assumed, as a rule, the 

 texture of granite. Formed under the influence of high heat and 

 pressure. 



Porphyritic A term applied to those rocks which have larger crystals 

 (phenocrysts), as feldspar, quartz, or augite, set in a finer ground-mass. 

 The latter may be crystalline or glassy or both. 



Pre-Cambrian All that part of geologic time represented by rocks older than 

 Cambrian; thought to be 3 times as long as all time since the earliest 

 Cambrian rocks were deposited. Also applied to all rocks earlier than 

 Cambrian. 



Quartz Crystallized oxide of silicon, SiO 2 . (See Silica.) 



Quartzite Metamorphosed quartz sandstone, formed by deposition of sec- 

 ondary silica between the grains. Called also granular quartz. 



Quartz-monzonite An igneous rock of granular texture containing quartz 

 with feldspars (orthoclase and plagioclase in about equal proportions), 

 and with these may be mica and horneblende. Often classed as granite. 



Radiolarian ooze Formed by microscopic animals known as Radiolaria, 

 which fashion their beautifully ornate shells from silica. 



