MINING. 
129 
of a volcano, carry up with them ferruginous combinations, which 
condense on spicula of lava, or colder pieces of rock, which 
project into the passage. The sulphuret of lead melts in the 
furnace, and in cooling returns again to its original sulphureted 
state. Both lead and copper volatilize by furnace heat, and may 
be condensed again; so subterranean exhalation will condense 
upon the cold surfaces of fissures. These facts leave, however, 
other facts observed in mines unexplained. The Rossie mines 
furnish large, fine crystals of sulphurets as well as crystalline 
masses enclosed in crystals of calc spar; a fact which seems to 
sustain the view that the materials were mingled together in the 
great furnace beneath, and were ejected bodily into the vein. 
In all those cases where lime and the sulphurets are intermin¬ 
gled in this way, it is evident that their fusion took place 
under great pressure, otherwise they would both have been 
decomposed, and it is highly probable sulphates would have 
been formed; as it is, we have reason to conclude that the mine¬ 
rals fused without parting with their sulphur or carbonic acid. 
From the foregoing views, we are justified in the belief that 
vein fissures are not filled by one mode only, but that they may 
have been filled by two or more modes conjointly; the upper 
portion by endosmosis in part as a drainage fissure, and the 
lower by injection, or by pressure, or sublimation. The copper 
veins of Cornwall rarely contain copper at less than a hundred 
feet from the surface; yet there is a fissure with its veinstone. I 
have said nothing of the electro-magnetic force as an agent, 
for I conceive that the detection of this agent is not proof that it 
has been operative in the modes assigned to it. 
* I have probably presented the simplest view of the formation 
of metallic veins; and if no other agencies were operative than 
that of the cooling of the earth’s crust, the business of mining 
would be less complicated and more certain than it is. It is to 
be recollected, however, that consolidated rocks have been sub¬ 
jected to many disturbing agencies at different times, and it is 
well known that a mining district is always one in which those 
agents have been particularly active. An undisturbed district 
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