44 OF METALLIC VEINS AND BEDS OF VALUABLE MINERALS. 
rock, containing a metallic ore thickly disseminated through it, 
is sometimes raised and worked under the name of ore. The 
gneiss and mica slate rocks, enveloping crystals of magnetic oxide 
of iron, of the western counties of North Carolina, are an example. 
We shall treat first, and at greatest length, of veins. 
It appears that the strata constituting the crust of the globe, 
have been fractured since their consolidation, the sides of the 
fissure separated from each other, and the vacant space filled 
with a foreign substance, sometimes enveloping a quantity of me- 
tallic ore, and in that case called a vein. The whole mass of the 
vein must have been either, poured into the fissure from above, 
forced into it from below, or transferred into the position in which 
it is found by agents and methods of which we have no accurate 
knowledge. Circumstances to be noticed hereafter, prove that in 
most cases at least, veins cannot be coeval with the rocks they 
traverse. When the substance occupying the fissure is stone or 
clay, it is called a dyke, of which the natural walls of Rowan are 
perhaps examples. Werner defined a vein to be <; the mineral 
contents of a vertical or inclined fissure ', nearly straight, and 
of indefinite length and depth." 
The thickness of veins varies from the fraction of an inch to 
many feet. It is not uniform throughout the whole extent of the 
same vein, but increases or diminishes, both laterally, and as the 
vein descends into the earth. The great vein of silver ore at 
Guanaxuato, in Mexico, which has yielded a larger amount of 
that metal than any other that has been explored by man, is twen- 
ty-two feet across at the surface, and much more at a considerable 
depth. There is no instance on record, where a vein has been 
wrought to the bottom and exhausted, though the quantity of ore 
has sometimes become so small as to cause it to be abandoned. 
It is probable that in many cases of this kind, the mine would 
become richer at a greater depth, as the productiveness of every 
vein is continually varying. Veins cut through the strata and 
descend into the earth, at an angle with the plane of the horizon, 
which is different in different mines, but always considerable. It 
is sometimes a right angle. If at any time they appear to occupy 
a bed between contiguous strata, they may commonly be traced to 
a vein nearly or quite perpendicular to those strata, with which 
they are connected, and of which they appear to be branches. 
Amongst the workmen, the perpendicular, or such as descend at a 
large angle are called rake veins, and the horizontal, or such as 
follow the direction of the strata, flat veins. In England the 
large and productive veins most commonly run east and west, 
and it has been thought that there is a tendency to the same direc- 
tion in other parts of the world. The principal vein of the most 
valuable of the North Carolina gold mines, (Capps'), does not vay 
much from the meridian, and descends into the earth at an angle 
of about seventy-five degrees. Such at least, were the indica- 
tions at the surface, and when it was first opened. 
