246 On Mineral Veins. 
from each other by long intervals of barren rock, and in 
Derbyshire by thick beds of basalt. 
Many very productive veins are met with which only 
penetrate a short distance from the surface of the earth. 
Flat veins are formed between the layers of limestone beds, 
and are frequently very rich. The galena deposit of Wis- 
consin, in the United States, is of this character ; and in 
many places is so regular as to allow of its being worked in — 
the same manner as a coal seam. 
The flat veins vf quartz in the diorite dykes do not pene- 
trate to any depth, and are really detached strings of quartz 
having no connection with each other. 
Detached masses of metallic ores are found in the cavities 
formed in hard crystalline rocks, when these cavities are not 
connected with any joint. 
Bands of granite are found impregnated with metallic 
ores, and these ores are also found disseminated in sedi- 
mentary strata. 
Recent deposits of ore have been formed in old workings, 
where they could only have been derived from the rocks 
bounding the vein. 
Tt will be seen how impossible it is in some of these cases 
—how improbable it is in others—for the minerals to have 
been derived from some source deep in the earth, and to — 
have been brought to the surface and deposited in open fis- 
sures, elther by” currents of water or by sublimation. 4 
On the other hand, how readily all the observed facts may 
be accounted for by adopting the theory which derives the 
mineral contents of the veins from the rocks bounding them, 
and assumes these contents to have been deposited on the 
joints or fractures of the rock, which have been enlarged by 
the aggregation of the minerals in them, operating by the — 
law of replacement whose action is so marked, and perhaps 
in some instances assisted by the magnetic tension. Ifa — 
steel plate can be removed atom by atom, and each atom be 
replaced by a corresponding atom of silver—a fact estab- 
lished by direct experiment—it will be readily seen thata 
mineral vein may be formed in the same way. 
Further evidence in support of this theory will be dis- 
covered in the fact that productive veins are rarely found in 
hard crystalline rocks, unless a portion of these rocks has 
undergone decomposition. Bischof has noted this, and — 
points out that “the removal of the constituents of a rock — 
is always preceded by its decomposition, and is facilitated by 
