﻿418 E. S. Dana — Crystallization of Native Copper. 



that it was possible to obtain measurements accurate enough to 

 admit of their determination. The question as to whether they 

 were to be identified with already known forms had usually to 

 be left undecided. 



Irregularities of structure of the simple forms. — Native copper 

 is like gold in the frequency with which its crystals show hol- 

 low and cavernous forms and other related peculiarities of 

 structure. One specimen consists of a group of simple dodeca- 

 hedrons, the faces of which show deep irregular cavities. In 

 another the forms are hardly more than skeletons, the crystal, 

 although nearly perfect, being in fact a mere shell. In other 

 cases the edges of the crystal were salient and the faces deeply 

 depressed. One example is given in fig. 15 where the cubic 

 face is depressed, the sides of the depression being taken by the 

 dodecahedral planes. The faces of the cubic planes especially 

 are often thickly covered with quadrilateral pits formed by the 

 dodecahedral planes (fig. 15) or by those of one of the common 

 tetrahexahedrons. Striated faces and faces with wavy irregu- 

 lar surface are also frequently observed. Other irregularities, 

 sometimes of an accidental nature, might be mentioned. 



It is also common to observe cases of more or less distinct 

 and regular elevations, triangular or quadrilateral or hexagonal, 

 upon the larger faces. Sometimes these can be referred to 

 known forms, but in others they are too indistinct to be deter- 

 mined. A vicinal tetrahexahedron on the cubic faces is occa- 

 sionally noted. A prominent example of distinct and regular 

 elevations is given in fig. 13, where at the extremities of the 

 cubic axes there are the projecting pyramids formed by the 

 faces of the tetrahexahedron I (530, £-f-), a form which, as already 

 stated, bears ,< peculiar relation to h. 



Figure 13 also illustrates another point in the peculiarities of 

 crystalline structure. One angle of the cube is here cut off by 

 a broad octahedral face, at the center of which the planes of the 

 tetrahexahedron h appear forming a small six-sided pyramid. 

 Other crystals show the same thing in greater or less symmetry 

 of development, sometimes several of the octahedral angles 

 being treated in the same way. The octahedral face may be 

 made up of a series of plates with parallel edges, upon which a 

 number of minute pyramids are present, formed in the same way. 

 This peculiarity is interesting because it introduces us to a point 

 immediately to be considered, the tendency of the crystals to 

 develop with rhombohedral symmetry. Thus a number of 

 specimens consist essentially of flattened plates, corresponding 

 to an octahedral plane, whose surfaces are covered with six- 

 sided tabular and pyramidal elevations, the latter often very 

 small and thickly crowded together. These pyramids are 

 always formed by the six planes of a tetrahexahedron, which 



