MARYLAND GEOLOGICAL SURVEY 147 



Igneous. — Formed from a molten state. This does not imply fire but in- 

 tense heat. 



Inlier. — A former outlier or uneroded portion of an older rock which 

 having- formed an island or an elevation owing' to some later deposit has 

 thus become embedded in a younger rock. E. g., Grays Hill near Elkton. 



Intrusive. — An igneous rock which has been injected into cavities in pre- 

 existent rocks and there cooled. 



Limonite. — A brown hydroxide of iron. Iron rust. A brown hematite. 



Magnesite. A white, often chalky mineral occurring in veins or coat- 

 ings on serpentine; often adheres to the tong'ue when moistened. A car- 

 bonate of magnesia. 



Magnetite. — The magnetic oxfde of iron, usually in minute black dia- 

 monds. 



Meta. — A prefix used to indicate that the rock has been formed by a 

 metamorphism of the rock to which the prefix is attached. 



M&tamorpliism. — Changes that go on in rocks due to re-crystallization 

 with or without alteration in the chemical composition of the mass. 

 These changes are due to modifications of the rock's environment which 

 render the original minerals unstable. 



Norite. — A dark-colored granular igneous rock composed essentially of 

 a basic lime-soda feldspar and an enstatite or a hypersthene. 



Olivine. — Pale yellowish-green vitreous silicate of iron and magnesium 

 formed in igneous rocks of low silica content. 



Opliitic. — The texture of rocks produced by the intergrowth of lath-shaped 

 feldspars and augite. 



Orogenic. — Due to the forces which have formed or are forming moun- 

 tains. 



Orthoclase. — A variety of feldspar, abundant in granites. 



Outliers. — A portion of rock-mass that remains in position while the 

 originally contiguous portions have been removed by erosion. For ex- 

 ample, the gravel cappings near Pleasant Hill. 



Pegmatite. — A very coarse-grained granitic rock composed of orthoclase, 

 quartz and mica, occurring as veins or dikes in other rocks. 



Peridotite. — A granular, igneous rock composed of olivine and other 

 iron and magnesian minerals. Usually changed to serpentine. 



Picrolite. — A columnar or fibrous variety of serpentine. 



Plagioclase. — The soda-lime and lime-soda feldspars including albite, oligo- 

 clase, labradorite, bytownite and anorthite. 



Porpliyritic. — A texture in which larger crystals are imbedded in a back- 

 ground of finer grain or glass. 



Pyroxenite. — A granular, igneous rock, composed essentially of iron and 

 magnesian minerals and differing from peridotites by a lack of olivine, 

 and from gabbro, by a lack of feldspar. 



Quartzite. — A very compact, granular quartz-rock, formed by the harden- 

 ing of a sandstone through the secondary deposition of silicious cement. 



