﻿E. 8. Dana — Crystallization of Native Copper. 429 



A novel and inteivsting method oi grouping, different from 

 those described, is represented by several fine specimens. The 

 crystal individuals taking part in it have already been partially 

 described (see figs. 43, 44, 45). The larger specimen, as shown 

 in the drawing, fig. 54, consists essentially of a long slightly 

 curved line, deeply grooved along the center, from which pro- 

 ject above and below, in two directions crossing at angles of 70° 

 32' and 109° 28', a series of small (2 mm ) partial crystals. This 

 line of elongation is one edge of the twinning octahedral plane. 

 The crystals above and below form the two parts of fig. 45. 

 Those below terminating in the cubic twinning-edge have a 

 rather constant habit as shown in fig. 46. The crystals above, 

 however, which terminate in an octahedral twinning-edge, vary 

 much, and have a marked tendency to develop in forms similar 

 to those below by elongation to one side in a direction normal 

 to the cubic edge. This is suggested by the forms in fig. 46 ; 

 but other crystals are very much more elongated and incline to 

 one side at a sharp angle. Others of these crystals are broad 

 plates showing but few planes except at the edges. 



The smaller specimens of this type show also more distinctly 

 a second similar line about the same axis and crossing the 

 others at the octahedral angle of 70° 32'. Fig. 55 gives a 

 cross-section in outline, and makes clear what the relations of 

 the two series are. The individuals I and II are in twinning 

 position with the octahedral edge above. The individuals III 

 and IV are also in twinning position with the adjacent octahe- 

 dral plane as twinning-plane, and in such a manner that the 

 first twinning-plane is parallel to the octahedral face of III 

 which forms the edge of III and IV. Another specimen 

 showed the same method of complex growth, though the com- 

 ponent crystals were simpler, some of them being simple spinal 

 twins like fig. 39. The lines in all these cases when naturally 

 terminated, taper off to a rather fine point. 



In concluding this paper, upon a subject which cannot by 

 any means be regarded as exhausted, the writer would ex- 

 press his thanks to Mr. Clarence S. Bement for his kindness 

 in allowing him the free use of his valuable collection for 

 several months, and also for the substantial support which 

 has made so liberal a degree of illustration possible. 



Am. Jour. Sci. — Third Series, Vol. XXXII, No. 192. — December, 1886. 

 27 



