depending on their Internal Changes. 285 



plete account of the changes by which the oxidized species 

 are formed from the sulphuret. — Mem. Wern. Soc. iv. p. 508. 



Miners pretty generally have an opinion, that the contents 

 of metallic veins are not always the same, and that they are 

 often working such as are not yet ripe, or would have been 

 more productive, if attacked at a later period. This opinion 

 is founded chiefly on a belief, that blende is changed into 

 lead-glance. We are not entitled by observation to admit of 

 such a change ; and though in this manner it does not appear 

 that we can come too soon with our mining operations, we see 

 plainly that at least, as at Leadhills, we may come too late ; 

 for that vein which now contains the carbonates, and sul- 

 phates, and phosphates, must have been once replete with the 

 much more valuable sulphuret of lead. Evidently, also, those 

 among the Freiberg veins have been opened too late, which 

 now are found to contain the large six-sided prisms of iron- 

 pyrites, produced by the decomposition of that valuable ore, 

 the brittle silver, or prismatic melane-glance of Mohs ; this, 

 at least, is the only species to which we can attribute the shape 

 of those prisms, although they themselves remain in some 

 measure problematical. 



The changes are not at an end, even with the com pletede- 

 struction of the sulphuret. I must, in particular, mention 

 three cases, all of them in specimens from Leadhills, in the 

 cabinet of Mr Allan, in support of this observation. One of 

 them has distinctly the form of large, perfectly recognizable 

 crystals, with a rough surface, however, of the prismatic lead- 

 baryte. The whole of the substance of the crystals is a gra- 

 nular tissue of minute crystals of the di-prismatic lead-baryte. 

 The sulphate, Pb S 2 , containing 73.56 per cent, oxide of lead, 

 has been here converted into carbonate, Pb C 2 , which contains 

 83.52 per cent, of the same ingredient. The form in the se- 

 cond case is that of the low six-sided prisms of the axotomous 

 lead-baryte, with pretty smooth surfaces. Its substance is an 

 aggregated mass of crystals, likewise of the di-prismatic lead- 

 baryte, but presenting in their distribution much resemblance 

 to the mode in which the individuals of malachite are arrang- 

 ed, which replace the crystals of the blue copper. The sul- 

 phato-tri-carbonate has here given way to the carbonate of 



VOL. IX. NO. II. OCTOBER 1828. ' T 



