92 
MINERAL VEINS. 
been introduced, as in the ‘‘ box-crystals ” from the Virtuous Lady 
Mine. Many again have been produced by the gradual substitution, 
particle by particle, of one mineral for another, and a large group is 
tracealde to the chemical action, which taking place under the admis¬ 
sion of air and water, is naturally met with near the surface of the 
ground or in the shallower workings of the mines. This action it is 
which in many districts causes all the lead ore, found superficially, to 
.be coated with or actually converted into cerussite (carbonate), anglesite 
(sulphate), or pyromorphite (phosphate); which has changed iron pyrites 
into brown oxide, and so decomposed the sulphuretted ores of copper 
that sometimes no copper mineral remains, whilst in others, oxides, 
arseniates, and carbonates bear testimony to the character of the 
metamorphosis. 
It is in this latter group that we have to range the gossan,” that 
ferruginous mass which constitutes the upper part of most of the lodes, 
and from which the miner has often to judge, without seeing any 
actual ore, of the promise offered to his enterprize in depth. Under 
the name of the eiserne Hut (iron hat) it has been for centuries well 
known to the German miners ; and on the continent as in Cornwall, 
the peculiar tints of brown and red, and the cellular character and 
lightness of the gossan are carefully noted and compared, and many a 
previously observed specimen has to contribute its results before an 
opinion can be formed on the value of a particular variety as indicative 
of a special metallic ore. The old miners’ saying, 
tf)ut nie ciu ©ang so gut 
Sbaun cv tvdgt uic^t eiu’ etgemen 
has been translated into the language of the bal,” where grass signifies 
the surface of the ground, 
A lode is never like to be fat. 
If he carry at grass no iron hat. 
A number of examples of gossan have been selected for the museum 
from particular mines, in Vv^hich the workings having been pushed 
down through a few fathoms of this material have opened into enor¬ 
mous stores of wealth. 
Lastly, a special phase of these chemical changes remains to be 
illustrated, viz., that in which the decomposition of the original ore is 
followed by the precipitation of a pure metal. Thus, native silver and 
copper are frequently found associated with gossan, and here and there 
an observant e}^ will detect the same kind of process obviously going 
on in connexion with some of the operations of the miner, as the iron 
ladder stave of a mine in St. Agnes entirely encrusted with metallic 
copper ; portions of the pumps of a Cornish mine lined with a rich 
spongy deposit of the same metal ; and a very curious conglomerate 
from the rubbish heaps of Wheal Leisure, in Perran, where copper has 
been thrown down amid a series of changes to which the original ores 
have been exposed. The metallic state, with copper especially, cannot 
be considered a condition of rest, it must be the commencement of a 
fresh series of alterations ; and the brief series of specimens thus 
exhibited will satisfy the attentive inquirer, that the constant flux and 
