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Hexagonal The name given to one of the six systems of crystals. It is 

 characterized by seven planes of symmetry. The forms of the hexagonal 

 system are referred to four axes, three of which are equal and similar, and 

 cross one another at an angle of 60 ; while the fourth (the chief axis) 

 is unequal and dissimilar, and stands at right angles to the plane of the 

 other three. 



Syn. Ehombohedral, Monotrimetric, Drei-und-einaxiges. 



HexakiSOCtahedron (Ge>\ Acht-und-vierzigflachner) The most general of the 

 forms belonging to the regular system. It is contained by 48 scalene 

 triangles ; and each face cuts one axis at unit distance, one at a distance 

 measured by a rational quantity m, and the third at a distance measured 

 by a rational quantity n. By the variation of the values for m and ?i, 

 the hexakisoctahedron may be made to approximate successively to all 

 the simpler forms of the regular system (octahedron, cube, rhombic, 

 dodecahedron. &c.). 



Hollow SphePulites (Ger. gekammerte Spheruliten) A generic name for such 

 spheroidal bodies, occurring in rocks, as have a cavernous interior. 

 Syns. Chambered spherulites, Lithophysse (q.v.). 



HolOCPystalline (Ger. vollkrystallinisch) Composed wholly of crystalline 

 material ; having no interstitial, glassy, or microfelsitic matter. 



HolohedPal The term applied, in contradistinction to hemihedral, to crystals 

 which present the full number of faces required by the symmetry of their 

 system. 



HomOBOCrystalline A term applied by some authors to a granitic structure 

 when the minerals are developed in equal proportions. 



Homomikt (Ger.} See Monogenous. 



HOUP glass Structure (Ger. Sanduhrglassstructur) A remarkable case of zonal 

 growth presented by the augites (less often by the hornblendes) of some 

 basic rocks. Viewed under crossed nicols, a section of the crystal, cut 

 parallel to the clinopinacoid, appears divided into four parts, of which 

 those lying opposite to one another show similar optic orientation, thus 

 producing a striking resemblance to the shape of an hourglass. A section 

 parallel to the orthopinacoid behaves in a like manner; while sections 

 perpendicular to the vertical axis show only the usual zonal growth. 

 This structure appears to be produced by the formation of a skeleton 

 crystal shaped like an hour-glass, the central depression of which is after- 

 wards filled in by augitic material of a slightly different chemical com- 

 position, and extinguishing therefore at a different angle. 



Hyaline Of the nature of glass. 

 Hyalopilitic See Pilotaxitic. 

 HydatOgeniC See Aqueous. 



Hydrothermal A term applied to phenomena which are caused by the com- 

 bined action of heat and water (steam). 



Hypidiomorphic (Rosenbusch, Die massigeu Gesteine, p. 11) Composed in 

 part, only, of idiomorphic crystals. See Idiomorphic. 



Hypocrystalline Composed partly of crystalline, partly of amorphous, matter 

 (glassy or micro-felsitic base). 



Hypometamorphic A term used by Callaway for the transitional forms 

 between the slates and shales on the one hand, and the schists on the 

 other. 



