428 



Crystallite This term, first proposed by Sir James Hall, has been adopted 



by modern petrographers with slightly varying meanings. By some it 

 is used as a general designation for all the indefinitely crystalline or 

 incipient forms of individualization of minerals, and is thus made to 

 include such forms as microlites ; while by others it is restricted to such 

 products of incipient crystallization as are not sufficiently individualized 

 to exert any definite action on polarized light. It would thus embrace 

 the terms globulite, longulite, margarite, trichite, but not microlite, which 

 is applied to those forms which can be recognized by their optic behaviour 

 as belonging to a definite mineral species. Thus we speak of microlites of 

 felspar, augite, &c. Syn. Mikromorphit (Giimbel). 



CrystallOgenesiS The development or growth of crystals. 



Crystalloid A term applied by Vogelsang (Die Krystalliten, Bonn, 1875, 

 p. 43) to microscopic bodies which are intermediate in development between 

 crystallites and microlites ; i.e., they are bodies which, without possessing 

 crystallographic contours, react on polarized light. 



Cube (Ger. Wiirfel) One of the simplest forms belonging to the regular system 

 of crystals. It is contained by six equal squares, each of which cuts one 

 axis at right angles, and runs parallel to the other two, thus coinciding 

 with the three chief planes of symmetry of this system. 

 Syn. Hexahedron. 



Cubic See Regular system. 



Cuboidal (Ger. quaderformig) Applied to the rudely rectangular masses into 

 which certain igneous rocks, especially granite, weather. Their form is 

 determined by an intersecting system of horizontal and vertical joint- 

 planes. 



Cumulite A term applied to cumulous accumulations of globulites in vitreous 

 rocks. 



Cuneiform, or Cuneate Wedge-shaped ; as, for example, sphene. 



Cup-and-ball Structure A cross-jointing of columnar igneous rocks, accom- 

 panied by a depression on one side of the joint-plane and a corresponding 

 elevation on the other, the two parts fitting into one another. Basalt 

 furnishes a familiar example. 



Cuspidate Spear-shaped ; tapering abruptly. Twin- crystals of rutile often give 

 cuspidate sections. 



Damascened A term suggested by Eutley (Study of Books, 1879, p. 181) " to 

 describe the structure shown in some obsidians, in which streaks or 

 threads of glass are contorted in a confused manner, which somewhat 

 resembles the workings on Damascus sword-blades or the damascening on 

 gun-barrels." 



Dendritic A term applied to certain arborescent growths of mineral matter, 

 occurring often on the surfaces of joints and fissures in rocks, or pervading 

 certain limestones, as, for instance, the Gotham or landscape-marble. 



Derivative See Aqueous. 



Detrital A term applied to rock-material resulting from the disintegration of 

 pre-existing rocks. 



Deuterogenic See Aqueous. 



Deuteropyramid ; deuteroprism See Pyramid and Prism. 



