414 Washington — Cojppev Crystals in Aventurine Glass. 



shown in the figures, the projection being always in the direc- 

 tion of the bisectrix of the angle. These projections are in 

 general rounded and show no crystal faces, but in a few cases 

 (fig. 4) they are seen to be minute octahedra, an apex being 

 outward and two of the dodecahedral interaxes being parallel 

 to the plane of the tabular crystal. Some of these small octa- 

 hedra show replacements of the solid angles by cubic planes, 

 and they all gradually run into the salient edges of the main 

 crystal. 



Pigs. 1-12. — Copper Crystals ia Aventurine Glass, x &^. 



In addition to these more regular forms some skeleton crys- 

 tals are to be seen (figs. 5, 6, 7) which are made up of narrow 

 ridges and show that the pit-like depressions are due to incom- 

 plete growth, and not to corrosion. A curious and quite 

 unique form is shown in fig. 8. This is a " Siamese twin " of 

 the hexagonal tables joined together by a narrow ridge. 



The tendency of copper to forms flattened in the direction 

 of a normal to an octahedral plane is well known and has been 

 clearly brought ont by Prof. E. S. Dana, in an article on the 

 copper of Lake Superior.* These flattened forms are shown 

 by him to be generally twins with the twinning plane an octa- 

 hedral face. Though owing to the small size and especially 

 the extreme thinness of the tabular crystals in the aventurine 

 glass no direct evidence of twinning, such as reentrant angles 

 on the edges, is to be seen, yet from analogy with the copper 

 crystals described by Dana and others it is probable that they 

 are also twins. Vogtf expresses the opinion that the plates 

 are simple distorted crystals, but gives no reasons for this view. 



*Dana, On the Crystallization of Native Copper. This Journal, xxxii, 1886, 

 p. 413. 



•j- Op. cit., p. 238, note 2. 



