56 J. D. Whitney on changes in Mineral Veins. 
Beneath the black ore is the undecomposed portion of the vein, — 
showing, in two or three points, where I was able to see it at the — 
time of my visit (1853), a hard quartzose gangue with particles of 
copper pyrites scattered through it, and associated with a consid- 
erably larger quantity of iron pyrites. here seems no reason to 
suppose that the ore which originally existed in the upper part of | 
the vein, from whose decomposition the black ore was derived, was 
any different in nature from that found below, although there may 
_ have been bunches of it considerably richer in copper. The de- 
posit of black ore is insignificant in dimensions, compared with 
the mass of gossan which overlies it, and when we consider that 
a large portion of the copper which was once disseminated 
through perhaps a hundred feet of. overlying vein-stone is now © 
concentrated into the thickness of perhaps two or three feet, on 
an average, it will bé seen that it is not necessary to suppose that 
the Neti > portion of the vein which is above the bed of 
black ore, ‘‘ doubtless once consisted of yellow sulphuret of cop- 
r,” as Mr. Tuomey supposes to have been the case. Certainly 
hardly doubted. The concentration of the black ore in one stra+ 
tum seems to have been due to the percolation of the surface wa- 
ter which was constantly carrying it downwards to the point 
where it was stopped by the solid portion of the vein. 
That the subject of the decomposition of veins is one which 
is thoroughly understood should by no means be inferred from the 
preceding remarks: there is, on the contrary, much in these phe- 
nomena which has not, as yet, been satisfactorily explained. We 
know, indeed, that the changes of the sulphurets with oxydised 
combinations do occur, for we see them taking place under our 
own eyes, through the joint action of air and water holding car- 
bonic acid in solution; but why in some mining districts the met- 
alliferous veins should be thus effected, while in others no change 
whatever has occurred, is less easily understood. Burat has 
ealled attention to this circumstance, and cited some instances in 
which the sulphurets remain entirely unoxydised up to the very 
surface. Thus the cupriferous veins of Mouzaia in Algiers pro- 
ject out from the surface like walls, being more permanent than 
the adjacent rock and the first blow of the hammer reveals the 
pyritiferous ore in its natural state. The same thing may be ob- 
served in this country. Throughout the Northern states the py- 
ritiferous lodes remain apparently in their unaltered condition ; 
