158 
AMERICAN GEOLOGY. 
OF THE CHANGES WHICH TAKE PLACE IN THE AURIFEROUS 
VEINS OF SULPHURETS OF COPPER AND IRON IN DEPTH. 
§ 94. The condition of the materials at and near the surface 
differs materially from that below. Those sulphurets which are 
gold-bearing seem to be more subject to decomposition than 
those which are not. At a variable depth, then, the matrix of 
gold consists of a porous quartz, colored brown upon the surface 
by a superficial deposit of oxide of iron. This partially fills 
the cavities. It is a friable dry powder, or it may have become 
consolidated, in which case it resembles the oxide of iron with 
a resinous luster, or the common brown hydrous oxide of iron, 
or brown hematite. This condition of the iron is due to the 
decomposition of the mixed sulphurets of iron and copper. The 
sulphur is discharged, and usually disappears entirely, and 
leaves the rock; but not always, for in a few instances it is 
retained in the cavities, and has crystallized in octahedrons 
with rhombic bases. The decomposition proceeds from the 
outside to the inside, as the inside of a mass of oxide often con¬ 
tains an undecomposed one of sulphuret of iron or copper. 
Where the decomposition is complete, the gold is attached to 
the quartz in irregular plates, or is also retained in the midst 
of the oxide in grains or thin scales. But it is a still more 
interesting fact, that the gold occasionally crystallizes, and 
appears under the form of octahedrons and dodecahedrons, or in 
skeleton crystals, the general form of which is developed, but 
the faces are deeply striated. Beautiful productions of this 
kind were quite common for a time in the Ward mine in David¬ 
son county, North Carolina. The figure of one of these striated 
or skeleton crystals appears on the title page of this work. 
Regular octahedron! have been obtained in the mines of Burke 
county in the same state. But crystals of gold are extremely 
rare in North Carolina. 
