128 
AMERICAN GEOLOGY, 
The depth, however, to which mines penetrate the earth’s crust 
is exceedingly small compared with the earth’s semi-diameter, 
or even with that of fifty or one hundred miles. We have no 
occasion, therefore, to attempt to illustrate this principle by a 
different mode than the one which is usually employed. 
But another fact may require a word of explanation. A 
fissure or vein is shifted, or is jogged out of its line of bearing. 
W 7 e may suppose in such a case, that while cooling, the stratum 
is subjected to an unequal tension of its parts, or to a tension 
in two directions. Hence, we may infer that a shift in the posi¬ 
tion of a part on one side of the fissure may take place at the 
moment, the tenacity of the rock yields to this force in another 
direction. Or the shift may take place at a period long subse¬ 
quent to the first fissuration, by the tension in an opposite direction 
to the first; the shift taking place by an unequal support of the 
mass. I can not conceive that the force of the entrance of the 
matter of the vein, tends to the displacement of the stratum. 
Its entrance acts equally upon the sides of the fissures; and 
though it is evident that there is considerable friction upon 
the sides by the striation of the walls and the vein-stone, still, 
it may be due to the weight of the mass resting upon an unsta¬ 
ble foundation. 
If a fissure opens to the surface from a great depth, but does 
not extend to the molten mass beneath, it becomes a water 
course, a drainage fissure, upon the sides of which incrusting 
matter will be deposited. This is called veinstone, or the 
gangue, and with its metallic associates forms the vein. The 
upper part of a vein fills, or is filled, in part, by veinstone 
intermixed with metal in specks or small lumps, which are 
diffused through it very sparingly at or near the top, but with 
an increase of depth increases in quantity. 
In addition to the function of drainage, fissures may become 
galleries of sublimation, in which the sulphurets, chlorides, &c., 
will be deposited. Metallic zones and stripes of metal will tra¬ 
verse the gangues wherever they are penetrable by subterranean 
exhalation. The exhalations passing upwards through the crater 
