of Mineral Species. 85 



other. These prisms are sometimes more than an inch in 

 diameter, but are usually smaller. The copper-glance, which 

 originally occupied the regularly limited space, has been suc- 

 ceeded by variegated copper. The arrangement of the por- 

 tions of both species in successive coats, shews that the decom- 

 position has proceeded from the surface. 



On breaking some of the six-sided prisms here alluded to, I 

 found a stratum of copper-pyrites, of its usual bright yellow co- 

 lour, contiguous to their surface, while the rest consisted of va- 

 riegated copper. The original form had here still been preserv- 

 ed ; but a new change in the chemical constitution had con- 

 verted the variegated copper into copper-pyrites. The peculiar 

 twin-crystals, discernible in groups of six-sided plates, crossing 

 each other at nearly right angles, and the distinct form of the 

 six-sided plates themselves, leave no doubt that two of Mr AL- 

 LAN'S specimens, consisting entirely of copper-pyrites, owe their 

 origin to the decomposition of copper-glance. One of them is 

 covered with a black pulverulent oxide ; but the surface of the 

 other is perfectly bright, and of a fine brass-yellow colour. It 

 presents to the observer the deceitful and puzzling appearance 

 of copper-pyrites crystallized in nearly regular six-sided plates. 

 No cleavage can be traced ; but this being not easily obtained 

 in any of the species, it cannot form, in the present instance, a 

 sufficient distinctive character between the simple and com- 

 pound minerals. 



The variegated copper itself occurs in distinct crystals, mostly 

 small, which are hexahedrons. Some larger ones, but with curved 

 and irregularly formed faces, occur in regular compositions, si- 

 milar to those of fluor, twins being produced by two individuals, 

 which may be supposed in transverse position to each other, in re- 

 ference to one of the rhombohedral axes of the hexahedron. Each 

 of these groupes contains in its interior a six-sided prism, whose 

 smooth surfaces may be relieved from the surrounding homoge- 



