668 REPORT OF NATIONAL MUSEUM, 1897. 



those which contain similiir j^lanes only. Combinations are those in 

 which dissimilar planes occnr, and are made up of those simple forms 

 which would result from the extension of one set of similar planes till 

 tlie others disappear. In certain compounds only complete forms occur ; 

 in others crystal faces occur wliich correspond in position to jjlanes of 

 comjdete forms, but there has been a regular suppression of one-half to 

 three-(iuarters of the required number of planes. These conditions are 

 known respectively as holohodral, hemiliedral, and tetartohedral accord- 

 ing as there is the full, one-half, or one-quarter of the required number 

 of planes present. 



The number of possible combinations of crystal forms is large; but 

 they may all be grouped under six .systems. 



CRYSTAL SYSTEMS. 



''A system is the sum of all the possible crystal forms whose planes 

 can be referred to the same kinds of axes." The systems of crystalliza- 

 tion are: (1) Isometric; (2) tetragonal; (3) hexagonal; (4) ortho- 

 rhombic; (5) monocliuic; (6) triclinic. The first system includes 

 all forms referable to three axes of equal length which intersect at 

 angles of 90 degrees. The second and third systems include all forms 

 referable to one ])rincipal or vertical axis which is perpendicular to 

 and different in length from the lateral axes. The fourth, fifth, and 

 sixth include those forms having no principal axis. 



Isometric system. — Includes all forms referable to three axes of equal 

 length, and which intersect at angles of 9w degrees. 



Examples: 



The octahedron : Fluorite, Jefferson CoiTiity, New York (Cat. No. 49947, U.S.N. M.) ; 

 magnetite, Lake Itknl, Russia (Cat. No. 493(J6, U.S.N.M.). 



The cube: Pyrite, Fishkill, New York (Cat. No. 14136, U.S.N.M.) ; fluorite, Wear- 

 dale, England (Cat. No. 49597, U.S.N.M.) ;' galena, Mineral Point, Wisconsin (Cat. 

 No. 49110, U.S.N.M.). 



The dodecahedron : Garnet, Salida, Chaffee County, Colorado (Cat. No. 81216, 

 U.S.N.M.); almandite, Tyrol (Cat. No. 82462, U.S.N.M.) ; grossularite, Xaloostoc, 

 Morelos, Mexico (Cat. No. 50116, U.S.N.M.). 



The ikositetrahedron : Garnet, Burke County, North Carolina (Cat. No. 81402, U.S. 

 N.M.); Icucite, Vesuvius, Italy (Cat. No. 16692, U.S.N.M.) ; analcite. Pinnacle 

 Island, Minas Basin, Nova Scotia (Cat. No. 49457, U.S.N.M.). 



Combination of the octahedroi and cube: Pyrite, Burgess, Ontario, Canada (Cat. 

 No. 84098, U.S.N.M.); hauerite, Miineo, Catania, Sicily (Cat. No. 51128, U.S.N. M.j. 



('omhlnation of the cube and octahedron : Galena, Alston Moor, Cumberland, England 

 (Cat. No. 49592, U.S.N.M.). 



Combination of the dodecahedron and cube: Cuprite, Cbessy, France (.Cat. No. 

 47395, U.S.N.M.); magnetite, Mineville, Essex County, New York (Cat. No. 47830, 

 U.S.N.M.). 



Combination of the cube, pentarjonal dodecahedron, and octahedron : Pyrite, Texas, 

 Lancaster County, Pennsylvania. (Cat No. 81.580, U.S.N.M.) 



Combination of the dodecahedron and ikonitetrahedron : Garnet, Stickeen River, 

 Alaska. (Cat. No.21209, U.S.N.M.) 



