674 REPORT OF NATIONAL MUSEUM, 1897. 



take place about a certain axis wliicli will bring tlieni into a reversed 

 or twinning position. 



Twin crystals are classed as contact twins and penetration twins. 



Parallel growths are those in which the molecular arrangement is 

 completely parallel in the component individuals; that is, the corre- 

 sponding planes are i^arallel throughout all the individuals, and they 

 are only separated from each other by their external planes. 



The following are illustrations of the methods of twinning and of 

 parallel growths: 



renetration twin : Fluorite, Cnuiberlaiid, Knglaiid. (Cat. No. 49946, U.S.N.M.) 

 Contact ttoin: Cak-itc, Woodcock luiiie, Grauby, Missouri. (Cat. No. 81254, 

 U.S.N.M.) 



Carlsbad twin : Orthoclase, near Leadville, Colorado. (Cat. No. 45179, U.S.N.M.) 

 Baveuo twin: Orthoclase, Baveno, Lomliardy, Italy. (Cat. No. 82410, U.S.N.M.) 

 Mannehacher twin : Orthoclase, Tanagama-Yama, .lapan. (Cat. No. 51147,11. S.N.M.) 

 Metagenic twin : Rutile, Magnet Cove, Arkansas. (Cat. No. 51723, U.S.N.M.) 

 Varanevic twin: Fluorite, Cumberland, England. (Cat. No. 49946, U.S.N.M.) 

 (lenictilafed twin: Rutile, Christiana, Lancaster County, Pennsylvania. (Cat. 

 No. 8190.5, U.S.N.M.) 



TriUinff: Staurolite, Morgantou, Fannin County, Georgia (Cat. 48029, U.S.N.M.). 



Fourling : Staurolite, Morgantou, Fannin County,Geovgia (Cat. No.48030,U.S.N.M.) 



Contact twi7i8: Cassiterite, Schlagenwald, Bohemia (Cat. No. 82083, U.S.N.M.); 



Calcite, Guanajuato, Mexico (Cat. No. 50077, U.S.N.M.); Aragonite, Herreugrund, 



Hungary (Cat. No. 82241, U.S.N.M.); Gypsum, Racamulto, Sicily (Cat. No. 51743, 



U.S.N.M.). 



Penetration twins: Pyrite, Tavistock, Devonshire, England (Cat. No. 82054, 

 U.S.N.M.) ; Quartz, Warstein, Germany (Cat. No. 82044, U.S.N.M.) ; Staurolite, Fannin 

 County, Georgia (Cat. No. 82573, U.S.N.M.); Harmotome, Stroutian, Argylshire, 

 Scotland (Cat. No. 48730, U.S.N.M.) ; Microcline, Pikes Peak, Colorado (Cat. No. 

 48564, U.S.N.M.). 



Parallel (jrowths: Quartz, Rauris, Salsburg, Austria (Cat. No. 51831, U.S.N.M.); 

 Hematite, Elba (Cat. No. 3315, U.S.N.M.); Adularia coated with chlorite, Viesch, 

 Switzerland (Cat. No. 51295, U.S.N.M.); Albite, Topsham, Maine (Cat, No. 83488, 

 U.S.N.M.); Witherite, Cumberland, England (Cat. No. 84116, U.S.N.M.); Calcite, 

 Guanajuato, Mexico (Cat. No. 50087, U.S.N.M.); Pyrite, French Creek, Chester 

 County, Pennsylvania (Cat. No. 83608, U.S.N.M.). 



IMPEUFECTIONS OK CRYSTALS. 



A crystal is theoretically a symmetrical polyhedron having its 

 homologous faces, angles, and dimensions in the direction of like axes 

 equal. This ideal condition is rarely attained by the crystal; the form 

 may be distorted, the planes may be irregular, or the crystal may 

 contain internal impurities. 



The crystal, then, may fall short of perfection by distortion of form, 

 imperfection of crystal planes, internal impurities. 



Examples : 



Distortion of form, due to a curvature of dynamic origin: Galena, .Jopl in, Missouri 

 (Cat. No. 81766, U.S.N.M.); Gypsum, Friedrichrode, Thuringia (Cat. No. 82413, 

 U.S,N.M.); Quartz, Crystal Mountain, Hot Springs, Arkansas (Cat. No. 45208, 

 U.S.N.M.); Zircon in WoUastouite, Natural Bridge, Lewis County, New York (Cat. 



J 



