STRUCTURE OF MINERAL VEINS, ETC. 
135 
of this change is always regarded as a favorable indi¬ 
cation, and an encouragement at least to pursue it to a 
greater depth. A vein is often laminated. The metal and 
rock being arranged in parallel stripes, it possesses the parallel 
structure of gneiss. This structure is regarded as one of the 
most favorable changes,—one which will ultimately lead to the 
formation of a productive vein. The lamination of a vein is 
unlike the lamination of gneiss and mica slate; it is always verti¬ 
cal. The structure is sometimes developed so prominently that a 
question arises respecting the periods of its formation. The indi¬ 
cations suggest the probability that the vein was filled by succes¬ 
sive openings. At each opening it received an accession of new 
matter from beneath. I have already stated that the first 
fissuration may not have communicated with the metalliferous 
matter, and hence is merely a drainage fissure, a receptacle of 
stony matter. For example, we often find that an auriferous 
vein is composed of two parts; one of copper, which lies next to 
the hanging wall, and which is most stony, and a lower mass, 
which lies against the foot wall, and which is metalliferous. 
In an example of this kind the fissure may have been widened 
by the same force which produced it in the first instance. It is 
upon the foot wall that we look for the greatest quantity of 
metal. But the metal may pass from the foot to near the hang¬ 
ing wall; and it is frequently central, as in the ftossie lead 
mine. From the foregoing remarks it will not escape the 
notice of the reader, that a vein consists mostly of stony matter 
or gangue, the metal being distributed through it in elongated 
branches, which run together; and there may be two or more 
such confluent ranges of metal arranged in parallel stripes, and 
which are separated from each other by an excess of stony 
matter. 
The foregoing statements may be taken as a general descrip¬ 
tion of the structure of all productive veins, without regard to 
the kind of mineral which forms the gangue. 
