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



simpler of these forms, as true to nature as possible; figure 35 

 gives another quite small tapering crystal. In fig. 37 the 

 extremity of a more complex form is represented ; here there is 

 a cluster of cubes in parallel position on one side of the form. 

 Other crystals have similar clusters on both sides in their respec- 

 tive twinning positions; they thus serve to reveal the true 

 nature of the terminal pyramid which, taken by itself, might 

 be a serious puzzle, appearing as it does far off from normal 

 isometric forms. The front edge, as seen in fig. 37, is often 

 made up of a series of jagged points formed by a repetition of 

 the lower extremity of the trigonal twin. One of these crystals 

 is so complex a combination of jagged projections on the edges, 

 front and rear and on both lateral edges as to hardly admit of 

 adequate representation. 



Another specimen of scarcely less beauty than that just de- 

 scribed consists of a group of twin crystals of the same type, 

 but more complex and irregular. In them the common tetra- 

 hexaheclron h (410, iA) is prominent, four planes of this form, 

 those about the cubic face 100, namely 410 (A), 410 (h z ), 401 

 (A n ), 40 1 (^m), correspond respectively in the rhombic position 



to the pyramids 355 (1-|), 533 (f |), 436 (| -f) and 452 (|-|) J 

 compare figure 30. Each of these crystals, as is also true of 

 the smaller crystals on the specimen before mentioned, demands 

 a special study before its form can be understood, and this is 

 no easy matter, since exact measurements are out of the ques- 

 tion. An adequate illustration of the subject would require an 

 almost indefinite number of ideal as well as artistic figures, 

 since the variety of form is so great. 



In fig. 28 the trigonal cubic twin is shown in combination 

 with the octahedron, not an uncommon form, though the crys- 

 tals of this type are also usually more or less irregularly elon- 

 gated somewhat as in figures 33, 35 and 37. Figure 31 shows 

 a similar trigonal twin combined with the cube and part of the 

 planes of the tetrahexahedron h ; this in the orthorhombic 

 projection appears as in fig. 29. Fig. 30 is another more com- 

 plex trigonal twin similar to fig. 31, and fig. 32 exhibits the 

 same habit, but instead of the tetrahexahedron we have the 

 hexoctahedron x already described. 



Crystals of the type represented in figs. 29 and 30, when 

 developed after the orthorhombic type, form acute, flattened 

 spear-head crystals, as shown in the drawings (figures 36 and 38). 

 In these cases the crystals are flattened in a direction normal to 

 the twinning-plane, but in other cases the flattening is parallel 

 to it, as described in the next paragraph. Very frequently on 

 one or both sides some trace of the cube is shown by which the 

 form can be orientated, and not infrequently the tetrahexahe- 

 dral planes can be seen on the cubic faces in their normal de- 



