OP METALIC VEINS. 45 
Veins are often separated from the rock they intersect, by a 
thin wall or lining of clay or some other mineral. Sometimes 
the ore extends in a compact mass from one side of the fissure to 
the other, but more commonly it is imbedded in a matrix, gangue, 
or vein-stone, or forms layers alternating with it, suggesting the idea 
of their having been deposited in succession upon the sides of the 
fissure, until the cavity was at length filled. The gangue is occa- 
sionally of the same nature with the lining of the vein just men- 
tioned ; it is always different from the rock it traverses. Quartz, 
carbonate, and fluate> of lime, and sulphate of baryta, are the most 
common substances. Two or more of these, or varieties of the 
same substance of different colour or texture, are found associated 
in the same vein, and applied in successive layers, some of which 
contain ore, whilst others are barren. In the North Carolina 
gold mines, the gangue is cellular quartz. Veins undergo many 
changes as they descend, and this relates not only to their pro- 
ductiveness, but sometimes also to the kind of ore they yield. 
There are veins in Cornwall, which yield tin at the surface and 
afterwards prove rich in copper. 
When two veins, or a vein and a dyke cross each other, as not 
unfrequently happens, the working of the vein brings to light the 
appearances attendant on such an intersection. On cutting through 
the dyke, the continuation of the vein is not found in the line of 
its former direction, but elevated considerably above or depressed 
below, or shifted to the right hand or the left, of its former course. 
These shiftings of the strata are called by the workmen faults, and 
are commonly regarded as proving that when the crust of the 
earth was ruptured, and the sides of the fracture separated, it was 
done with great violence so as to derange and displace the strata 
to a great extent. Sometimes, veins cross each other without 
any change in the direction or appearance of either, or of the 
rocky strata in which they are both included. 
27. Werner supposed that all veins and dykes were produced 
by the shrinking of the materials of which the mountains are 
composed, and the subsequent introduction of the substances con- 
stituting the vein, in a state of solution, from above. That fissures 
in the crust of the earth have sometimes been so filled, is probable 
from the fact that they contain substances that have apparently 
been washed in from the surface, such as rounded stones and un- 
decayed vegetable matter, but that the materials of metallic veins 
have not been thus poured in, in a state of solution, from above, 
is rendered certain by all we know respecting the properties of 
those substances, and their distribution through the crust of the 
earth. There is no form of matter known to us that would act as 
the common solvent of all the contents of many veins — of the 
quartz and the sulphurets of copper, lead, and zinc, for example, 
of the lead mines at Southampton, in Massachusetts, even though 
we suppose it capable of dissolving one of them. The greater 
part of the metallic ores occur onlyin veins, whereas, had they 
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