

MATERIALS OF THE EARTH 85 



Loess. A very fine porous silicious silt containing some calcareous 

 material which often collects in nodules (Loss Kindcheri) or in vertical tubules ; 

 characterized 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. 



Mantle rock. (See regolith.) 



Marble. Typically a granular crystalline limestone or dolomite pro- 

 duced 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 secretions of plants, 

 notably the stoneworts ; 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 metamorphic 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 

 in conjunction with water. 



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-sj^enite). 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. 



Nodules. Concretionary aggregations of rounded form. 



Novaculite (hornstone, oilstone). A very fine-grained, hard sandstone 

 or siltstone, 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, varie- 

 gated with delicate colors due to iron and manganese. 



Oolite. A limestone or dolomite composed of small concretions resem- 

 bling the roe of fish. 



Ooze. A soft deposit of the deep sea; characterized usually by a micro- 

 scopic shells from which it is mainly derived; as diatom ooze, globigerina 

 ooze, etc. 



