of Mineral Species. 



97 



ryte. In the six-sided prisms of the same kind of formation 

 met with at Wheal Hope in Cornwall, generally a film of lead- 

 glance is also observed near the surface; but the crystals of 

 the suphuret in their interior are often much more curiously ar- 

 ranged. Partly they are simply composed of a mass of very com- 

 pact galena, partly also they present, when broken, the appear- 

 ance of being cleavable with great facility perpendicular to their 

 axis, and at the same time also parallel to the sides of the six- 

 sided prisms, and parallel also to the planes replacing their edges. 

 The smooth planes obtained in this manner, are actually the 

 faces of cleavage of the hexahedron peculiar to lead-glance. The 

 individuals of the sulphuret namely, gradually formed in the 

 crystal of the phosphate, assume such positions, that two of their 

 faces are parallel to the sides, and two to the terminations of 

 the six-sided prism ; the two remaining ones will be perpen- 

 dicular to the lateral and the terminal faces. The direc- 

 tion of them appears distinctly in the annexed sketch of the 

 transverse section of a crystal, as indicated by the lines parallel 

 and perpendicular to the sides of the hexagon. 

 On breaking the prisms, we obtain fractures 

 situated like the line abed, which I have 

 sometimes observed, giving a clear demon- 

 stration of the actual composition of the 

 crystal in the manner described. Generally 

 the portion adjoining the centre, as it were 

 the axis of the prism, consists of perfectly 

 compact lead-glance, provided the original species has entirely dis- 

 appeared ; then comes a more or less considerable stratum of the 

 cleavable mass, which, however, is often wanting ; and then a coat- 

 ing of a coarser texture. From the mere arrangement of the par- 

 ticles, it is placed beyond a doubt, that the crystals of the sulphuret 

 have not been formed in moulds from the phosphate. They are 

 probably the product of the gradual decomposition of the latter 



VOL. XI. PART I. 



