THE ORIGIN AND DESCENT OF ROCKS. 471 



terized by a peculiar competency to stand in vertical walls; held by some to be 

 eolian, by others to be fluvial or lacustrine, and by still others to be partly eolian 

 and partly aqueous. 



Marble — typically a granular crystalline limestone or dolomite produced by 

 metamorphic action; but the term is variously applied to calcareous and even 

 to other rocks that are colored ornamentally and susceptible of polish. 



Marl — an earth formed largely of calcium carbonate, usually derived from the 

 disintegration of shells; or the calcareous accretions of plants, notably the 

 stone worts; term also sometimes applied to glauconitic and other fertilizing 

 earths. 



Melaphyre — a term of varying usage; most commonly applied perhaps to 

 an altered basalt (q.v.), especially an olivine-bearing variety. 



Meta-diabase — a term sometimes used for a metamophic diabase; in like 

 manner meta is prefixed to dolerite, syenite, etc.; not in general use. 



Meta-igneous rock — a metamorphosed igneous rock. 



Metamorphic rock — a rock which has been altered, particularly one which has 

 been rendered crystalline, or recrystallized by heat and pressure. 



Meta-sedimentary rock — a metamorphosed sedimentary rock. 



Microgranite — a very fine-grained granite. 



Microlites — incipient crystals found in glassy lavas; usually needle-shaped, 

 or rod-like; occurring singly and in aggregates. 



Millstone — see buhrstone. 



Minette (mica-syenite) — a rock consisting essentially of orthoclase and mica, 

 or a syenite in which mica replaces hornblende or predominates over it. 



Monzonite — a granitic rock composed of orthoclase and plagioclase in nearly 

 equal proportions, with ferromagnesian minerals; a rock intermediate between 

 syenite and diorite. 



Mudstone — solidified mud or silt, shale. 



Nephelinite — a rock composed essentially of nepheline and augite, with magne- 

 tite and other accessories. 



Nevadite — a variety of rhyolite of granitoid aspect due to an abundance of 

 porphyritic crystals. 



Nodules — concretionary aggregations of rounded form. 



Norite — a fine-grained rock consisting of plagioclase and hypersthene. 



Novaculite (honestone, oilstone) — a very fine-grained, hard sandstone or silt- 

 stone, used for whetstones. 



Obsidian — a typical form of volcanic glass usually of the acidic class. 



Onyx — a variety of chalcedonic quartz having colored bands alternating 

 with white; the ''Mexican onyx" is a crystalline calcium carbonate, variegated 

 with delicate colors due to iron and manganese. 



Oolite — a limestone or dolomite composed of small concretions resembling the 

 roe of fish. 



Ooze — an exceedingly soft watery deposit of the deep sea; characterized 

 usually by microscopic shells from which it is mainly derived, as diatom ooze, 

 globigerina ooze, etc. 



Ortnophyre (orthoclase porphyry) — a rock consisting of crystals of orthoclase 

 in an aphanitic base. 



