146 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



Diorite. — A granular igneous rock of grayish-white or green color com- 

 posed essentially of plagioclase feldspar, and hornblende or mica. It 

 differs from monzonite in its lack of orthoclase feldspar and from gabbro 

 in the acidity of its feldspar. 



Dip. — The angle of inclination of strata to the horizon, measured at the 

 steepest point. 



Epidote. — A yellowish-green granular or fibrous aluminum-silicate of 

 lime. The epidote group includes zoisite, epidote, piedmontite, and allan- 

 ite. 



Extrusive. — An igneous rock which has cooled at the surface from a 

 molten state; a lava. 



Feldspars. — A group of light-colored silicates of aluminum and lime, 

 soda, or potash. The species vary with the composition of the mass in 

 igneous rocks and form the basis for a classification of these rocks. The 

 species are: orthoclase microcline and sanidine, the potash feldspars; albite, 

 the soda feldspar; anortliite, a lime feldspar; oligoclase, labradorite and 

 bytownite intermediates between albite and anorthite, i. e., soda-lime and 

 lime-soda feldspars. The feldspars of the albite-anorthite series are col- 

 lectively called plagioclase. 



Gabbro. — A granular, igneous rock of dark color, composed essentially 

 of lime-soda feldspar and a pyroxene. Sometimes sold as black granite — 

 popularly called " nigger head." 



Garnet. — A transparent, generally reddish-brown silicate of iron, lime 

 and alumina frequently found in small crystals. 



Gneiss. — A metamorphic rock, composed of feldspar, quartz, and mica, 

 arranged in banded layers. When appended to the name of an igneous 

 rock it implies that the latter has been metamorphosed with the rear- 

 rangements of the constituents into parallel lines. E. g., granite-gneiss. 



Granite. — A granular aggregate of feldspar and quartz with accessory 

 mica or hornblende which has resulted from the crystallization of a 

 molten mass under conditions of high temperature and high pressure. 



Granular. — Composed of irregularly-shaped grains of approximately equal 

 size. 



Greenstone. — A common term applied loosely to various kinds of green- 

 ish rocks generally of ig'neous origin. 



Hematite. — Red oxide of iron. 



Hornblende. — A greenish-black, often fibrous, iron magnesium silicate 

 formed by crystallization from a molten state, as in the hornblende- gran- 

 ites; or by crystallization from pyroxene through metamorphism, as in 

 meta-gabbro. 



Hydro-magnesite. — A basic magnesium carbonate occurring as a tufted 

 or chalky crust on serpentines. 



Hypersthene. — A pearly, dark-colored, brittle iron magnesium pyroxene, 

 frequently found in gabbro and allied rocks. 



Idiomorphic. — Applied to constituents of igneous rocks which have formed 

 under conditions permitting the development of their characteristic crystal 

 forms. 



