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



of simple cubes we may have either one of the commoner tetra- 

 hexahedrons or a combination of one or more of them with 

 each other or with the cube, octahedron and dodecahedron. 

 Moreover, we have sometimes an open series of cubic twins, 

 and from these we pass to more compact forms and finally to 

 specimens which are simple plates with broad octahedral sur- 

 face showing the characteristic spear-shaped twins only occa- 

 sional]}^ at the edges and with the hexagonal lines of growth 

 only slightly accentuated. In the latter cases the octahedral 

 surface is commonly covered over, often very thickly, with 

 hexagonal elevations formed by one of the common tetrahexa- 

 hedrons as already described. Still further, a number of these 

 branching dendritic growths may be grouped in slightly diverg- 

 ing position about a common center, thus producing arbores- 

 cent crystallizations of great beauty. 



The twinning is also often somewhat complex ; for it may be 

 not simply a case of a single twin growth with the upper sur- 

 face in twinning position with respect to the lower, but on the 

 same side any one of the tectonic axes may be taken by a 

 series of cubic twins in reversed position to those adjoining, or 

 a single crystal in twinning position may appear in the midst 

 of the others. 



Fig. 51 represents (fths natural size) one of the finest of these 

 remarkable specimens. It illustrates admirably the method of 

 growth and the remarkable complexity of the resulting forms. 

 The spear-head forms at the extremities are here well seen. It 

 is to be noted also, in illustration of a point just made, that for 

 the lower part of the specimen the crystals are in twinning 

 position with reference to the majority of the others, and this 

 is true also of isolated cubes at various other points as at a, a. 

 Fig. 52 shows one of the tabular forms in which the octahedral 

 plane predominates, and it is only on the central and branching 

 ribs that the other — tetrahexahedral — forms are distinct. The 

 surface is thickly covered with hexagonal plates and low 

 pyramids, which the drawing only in part represents. 



In fig. 53 a partiallv satisfactory representation is given of a 

 very delicate moss-like dendritic crystallization which is 

 especially interesting because careful examination proves that 

 the tectonic axes here are situated diagonally with reference to 

 the common method, in other words it is like the forms figured 

 by Kose. The specimens are nearly an inch in length and 

 very thin and fragile. The octahedral surface is prominent, 

 though cubes in thin plates, or in normal forms, project from 

 it sometimes in one position, sometimes reversed — over one 

 surface there are a number of tetrahexahedrons with rhombo- 

 hedral (scalenohedral) development implanted on the octahe- 

 dral surface; these are also in both positions. 



