Washington — Copper Crystals in Aventurine Glass. 413 



melting pot shows a smooth uppermost layer, about 02 mm 

 thick, of a clear dark greenish blue color, the surface of this 

 being covered with minute rounded pits and indented six- 

 rayed stars. 



Microscopically the perfect glass may be first described. 

 This shows under the microscope a porphyritic structure, the 

 groundmass being composed of a perfectly clear and colorless 

 glass basis, which in the parts where the phenocrysts are best 

 developed is quite clear and free from microlites, in other 

 places and in streaks being " dusty " through the presence of 

 large numbers of the minute octahedral microlites to be 

 described presently. 



The crystallized portion of the mass consists entirely of 

 copper and may be divided sharply into three distinct groups, 

 large phenocrysts, small phenocrysts, and microlites, which 

 differ greatly from each other both in the size and the habit of 

 the individuals. 



All of the large phenocrysts, which range in diameter from 

 - 05-0'12 mm , are tabular and extremely thin, the thickness 

 scarcely exceeding O002 mm and often less than half this, being 

 perfectly opaque notwithstanding their excessive tenuity. 

 Most of them are hexagonal in outline, the hexagons being of 

 almost ideal symmetry, and equilateral triangles, which occa- 

 sionally show truncated angles, also occur. They show a 

 rather yellowish copper color and in reflected light a brilliant 

 metallic luster. The edges are perfectly sharp and straight, 

 except those of the plates lying obliquely in the section and 

 which are due to the making of the slide, which are rough 

 and somewhat granular or ragged. Seen " end on " the edges 

 show, when thick enough, a face not quite normal to the plane 

 surface of the crystals, though they are too thin to make 

 measurements at all exact. 



The faces of these hexagons and triangles are in general 

 perfectly smooth and plane, but a number show a peculiar 

 appearance which is represented in figs. 1-5. In such cases 

 we see that the central part of the face is depressed forming a 

 very shallow hexagonal or triangular pit, symmetrical with 

 the crystal outline, and surrounded by a salient edge about 

 0"004 mm broad, and with sloping and rounded sides. The 

 angles of these shallow depressions are generally rounded, so 

 much so in some cases that they assume an almost circular 

 form (fig. 2). It may be remarked that such shallow pitted 

 surfaces with salient edges are not rare in native copper crys- 

 tals and quite common in gold. 



This peculiar form, which seems to be due to a skeleton 

 growth, is almost always accompanied by projections of greater 

 or less length at the outer angles of the tabular crystals, as 



