MARYLAND GEOLOGICAL SURVEY 145 



Allanitc. — A pitch-brown silicate of the rare earths, occasionally found 

 in granite and other rocks, usually in minute specks. 



Allotriomorphic. — Applied to the constituents of igneous rocks whose forms 

 are determined by the earlier formed constituents. Antinque of idiomor- 

 phic. 



Amphibolite. — A grayish-green, fine-grained, metamorphic rock, com- 

 posed essentially of hornblende and plagioclase feldspar, sometimes called 

 hornblende-schist. It includes certain of the meta-gabbros. 



Amygdaloidal Structure. — A porous structure of certain igneous rocks in 

 which the elliptical holes formed by the escaping gas have been filled 

 with secondary minerals. 



AnthopliylHte. — A fibrous green silicate of magnesium and iron very 

 similar to actinolite. 



Apatite. — A vitreous greenish or colorless phosphate of lime. In Mary- 

 land rocks it is usually only in microscopic grains. 



Aphanitic. — A texture of igneous rocks in which the individual grains 

 are visible only with the microscope. 



Apophysis. — One of the lateral projections of certain intrusive igneous 

 rocks; often equivalent to dike. 



Basicity. — The relative amount of bases such as lime or iron, in igneous 

 rocks. The opposite of acidity. 



Biotite. — A brown, green or black platy silicate of iron and magnesium. 

 A member of the mica family and a iDrominent constituent in granites, 

 gneisses and other micaceous rocks. Sometimes called black " isinglass." 



Brucite. — A pearly white to grayish-green hydrate of magnesium, fre- 

 quently found in serpentine and marbles. 



Calcite. — Lime carbonate, usually colorless or white. It occurs in crys- 

 tals or as limestone, marble and stalactites. 



Glialcedony. — A waxy, transparent, close-grained quartz, often lining' or 

 filling cavities in serpentine. Sometimes found looking like hard glue. 



Chlorite. — A green tabular or scaly hydro-silicate of magnesium, alu- 

 minum, and iron. Due to changes since the formation of the original rock. 



Clinochlore. — A variety of chlorite. 



Conglomerate. — A sedimentary rock composed of rounded and water-worn 

 pebbles or fragments of pre-existing rock. (Puddingstone.) 



Creep. — Slow down-hill movement of surface-rock due to gravity. 



Dexoeylite. — An amorphous resinous white hydrous-magnesium silicate, 

 related to serpentine. 



Diabase. — A compact heavy igneous rock, dark gray, dark green, or 

 black, composed of plagioclase feldspar, pyroxene, iron oxide, and some- 

 times olivine. Frequently called trap. 



Diastrophic. — Eelating to the movement of the earth's crust, producing 

 continents, mountains, folds and faults. 



Dike. — A mass of igneous rock filling a fissure in other rocks into which 

 it has been intruded while in a liquid condition. 

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