132 
AMERICAN GEOLOGY. 
tions, and hence be prolonged still farther. Vein fissures extend 
beneath the soil farther than the indications upon the surface 
warrant us to expect. In the direction of the strike they may 
plunge down beneath the surface on a gentle or steep slope. 
The outcrop is lost—the fissure does not extend to the surface— 
the lamina of rock are in contact—and yet the vein is prolong¬ 
ed, and may reappear in an outcrop, several miles from the points 
where it is known. The argentiferous galena vein of Davidson 
county was struck in an excavation six miles northeast of its 
principal workings, and yet the surface gave no signs . of its 
presence beneath. In instances of this kind, the vein plunges 
downward to an unknown depth, when it takes an upward 
movement and ascends to the surface. The fissure may be fill¬ 
ed, however, with veinstone only, and hence excites no atten¬ 
tion. The foregoing statement proves the existence of vein 
fissures which do not reach the surface. The same fact has been 
observed in dykes; they penetrate the inferior layers of a rock, 
but the fissure not extending to the surface, and the stony mat¬ 
ter having no power of itself to form a passage, stops at the 
upper limit of the rent. Branches of veins, too, are often cut 
in sinking shafts at many levels, which have no connection 
with the surface. These facts illustrate the direction and mode 
in which those vein fissures have been filled, and clearly point 
to their igneous origin. 
As the quantity of metal which a vein carries is variable at 
different points on its line of bearing, so it is also variable at 
different depths. It may be variable in consequence of the 
diminished amount in the vein, which at the bottom preserves 
its ordinary width, or it may be diminished by a contraction of 
the whole vein in width. It may be pinched out, and the vein¬ 
stone, together with the metal, 'becomes a mere trace or string, 
retaining its position between the walls. The variation of the 
latter kind is well illustrated by the terminal outcrop of the Rossie 
lead mine, which is exposed in an uplift of the mass of gneiss 
in which the vien is enclosed, as is shown in the annexed cut, 
Fig. 25. It will be observed that while the vein is exposed 
