﻿E. S. Dana — Crystallization of Native Copper. 425 



velopment. The front edge is sometimes sharp and jagged in 

 the way described, by the repetition of the lower angle of the 

 trigonal twin. Figure 34 shows this point well ; it is drawn 

 (like the simple figure 40) with the twinning-plane parallel to 

 the plane of the paper instead of normal to it, as in all the fig- 

 ures previously spoken of. These simpler specimens often 

 show traces of the complex growth along lines at angles of 60°, 

 and thus pass into such forms as that represented in fig. 51. 

 Other specimens show other forms of the same type, but differ- 

 ing most widely according to the planes present and according 

 to their relative development. In almost all of these the ten- 

 dency to develop in orthorhombic symmetry is strongly marked. 

 The tetrahexahedron e is an interesting case, since it gives 



a prismatic form, 230 0--J-), terminated above and below by 

 a brachy-pyramid 133 (1-3). This form, which is not an un- 

 common one, and appears as a spear-head crystal flattened 

 parallel to the twinning-plane, is especially noteworthy because 

 it is in this position a normal orthorhombic form, not hemi- 

 morphic like the others. The same form may come without 

 twinning from the shortening of the tetrahexahedron in the 

 direction of an octahedral axis and a simultaneous elongation 

 parallel to the middle pair of planes (compare fig. 21 and p. 

 421). 



Another type of twin crystal, somewhat related to those just 

 described, but of very different aspect, is shown in fig. 44. The 

 first crystal studied proved to be a problem of some difficulty, 

 especially as it was very small and only a few planes gave dis- 

 tinct reflections. At first sight it appeared to be a square 

 prism terminated somewhat acutely by an octagonal pyramid 

 and with several small modifying planes. The measurement 

 of a few angles served to unravel the form. The pyramidal 

 angles of two corresponding pairs of planes, measured with fair 

 precision, were found to be 



38° 55' and 39° V- 



the angles of the other approximately measured gave 



30° to 31°. 



The first angles given correspond to the angle between two 

 adjacent planes of a twinned octahedron : 



o a = 38° 56'. 



The other pyramidal angle corresponds to that (30° 27') between 

 two planes of the tetrahexahedron k (520, i-g). Other angles, 

 measured with more or less accuracy, served to show that the 

 planes present included the cube (a), octahedron (o), dodecahe- 

 dron (d) and tetrahexahedron k (520, %-§). Figure 44 shows it 

 in ordinary projection, and in fig. 43 a basal projection is 



