416 Washington — Copper Crystals in Aventurine Glass. 



fifth of 360°. Similar forms have been described bj v. 

 Lasaulx* and W. G. Brown. f 



It has already been said that the tabular phenocrysts occur 

 in a vitreous groundmass which is almost absolutely free from 

 smaller crystals and microlites. The same is true in a certain 

 way of these smaller phenocrysts which have been just 

 described. They occur generally in parts of the glass that are 

 free from the tabular crystals but very thickly strewn with the 

 minute microlites which form the third group, being imme- 

 diately surrounded, however, by a more or less broad circular 

 zone (Hof) of perfectly clear glass basis, exactly as is seen to 

 be the case with magnetite and other minerals in volcanic 

 rocks. These microlites which are extremely small, running 

 from 0*005-0 -0005 mm or even less, are seen under high powers 

 to be sharp and perfectly symmetrical octahedra. Forms other 

 than octahedra were not seen even with the use of the highest 

 powers, and the angles are only rarely replaced by cubic planes. 



The imperfect glass shows under the microscope the same 

 colorless glass basis as the perfect, but is almost absolutely free 

 from the tabular crystals, only one or two being seen in the 

 slide, and these very small. The copper crystals are in an 

 overwhelming majority the microlites of the third group, 

 which are exactly like those just described. Crystals of the 

 second group are rare and these mostly of the simple octa- 

 hedral type, with a few cubo-octahedra and scarcely any twins. 



The thin greenish blue top layer already spoken of, which 

 is sharply distinct from the brown glass beneath, but which 

 throws projections into the latter, offers some special peculiari- 

 ties. The surface is, as has been said, covered with very 

 minute pits and small indented stellate forms with generally 

 six rays, then crossing in most cases at about 60°, but in others 

 at irregular angles, and some stars having fewer than six rays. 

 I had a slide from this prepared by Messrs. Yoigt and Hoch- 

 gesang, this smooth original surface being used as one face, 

 while the other was ground down parallel to it. 



With low powers under the microscope the centers of these 

 stars are seen to be occupied by holes, and the rays are seen to 

 be less regular than they appear to the naked eye or with the 

 lens. They are seen to be composed of long groups and 

 clusters of black opaque trichites which are either straight or 

 curved. With these are sometimes associated small irregular 

 specks of a pale brown non-pleochroic mineral, which shows 

 marked double refraction. Its precise nature could not be 

 determined, but it seems to be augite. In the pale blue ground- 



* Lasaulx, Ber. nied Ges., xxxix, 1882, 95. 



f W. G. Brown, this Journal, xxxii, 1886, p. 377. 



