﻿K S. Dana — Crystallization of Native Copper. 419 



meet at the extremity of an octahedral axis. When the tetra- 

 hexahedron is the form A, the angles (supplement) of the sea- 

 lenohedral pyramid are 61° 56' and 19° 45', while the angles at 

 the base of the pyramid are 87° 55' and 152° 12' respectively. 

 This is shown in fig. 14. When, however, the elevations are 

 formed by planes of the tetrahexahedron e (210, i-2), the pyra- 

 mid is a regular hexagonal pyramid with a pyramidal angle of 

 36° 52', and each of the angles at the base is 120° ; the latter is 

 more common ; it is illustrated in fig. 52. 



A secondary growth over an original crystal is sometimes 

 observed. One remarkably symmetrical case of this has been 

 already spoken of as described by vom Eath. Usually the 

 result is to partially obliterate the original form. Thus in one 

 specimen the crystals show numerous tuberose sproutings, sug- 

 gestive of a fungus growth ; in another two large dodecahedral 

 crystals are partially enclosed, each by a rough mass of copper 

 showing only a trace of crystalline form. 



Distorted forms and those showing pseudo- symmetry. — Cases 

 showing irregular distortion are common, but among the simple 

 crystals hardly more so than is the case with many other crys- 

 tallized species. The distortion, on the contrary, usually 

 exhibits a certain degree of regularity, being in the direction of 

 one of the inter-axes of the crystal and thus giving rise to forms 

 with marked pseudo-symmetry. Elongation in the direction of 

 a cubic axis is frequently observed and where symmetrical, 

 produces pseudo-tetragonal forms as shown in fig. 16 ; compare 

 also figures 48 and 49 described later. Where the elongation 

 is in the direction of one extremity of a cubic axis the forms 

 are hemimorphic in type and more or less irregular. 



A symmetrical development of a crystal about an octahedral 

 axis gives rise to forms with rhombohedral pseudo-symmetry 

 and these cases are so frequent and interesting that they are 

 described at length below. Elongated wire- and band-like 

 forms of varied shape, often much curved and twisted, and 

 showing some crystalline markings on the surface, are common 

 but these forms are generally very indistinct, and are hardly 

 to be spoken of as crystals. 



Crystals with rhombohedral symmetry. — It is an interesting 

 fact in connection with the crystallization of the native metals, 

 gold, silver and copper, that they so often show a tendency to 

 develop with rhombohedral instead of isometric symmetry. 

 Many cases of this kind have been noted; one striking exam- 

 ple has been recently described by the writer,* in which crys- 

 tals of native gold had essentially the form of an acute rhom- 

 bohedron (4/?) ; these were arranged in parallel position so as to 

 form slender strings of rhombohedrons developed in the direc- 

 * This Journal, xxxii, 132, August, 1886. 



