MINING. 
127 
modes have undoubtedly been operative under different circum¬ 
stances. 
But, again, there are other kinds of veins—those which have 
no communication above or below with a fluid or liquid 
matter. They are fissures which begin, and which end 
in the rock, and yet these fissures are filled. In furnishing 
an explanation of cases of this kind, we must recognize the 
existence of the same forces as those which I have alluded to. 
The fissures are produced by tension in a cooling or drying 
mass, and when they terminate in the rock the fissures are 
absolute vacuities—each fissure is a vacuum. The filling of 
such fissures is effected in a mode similar to that already inti¬ 
mated—by fluids charged with lime, or any matters contained in 
the rock which are soluble under the circumstances; for towards 
the fissure soluble matter will tend, and crystallization will 
take place, and the fissure will ultimately be filled with it. It 
may be inquired, how it happens that veins possess such a unifor¬ 
mity of width. It may be answered satisfactorily in this way: a 
given rock, mica slate or gneiss, possesses a great degree of uni¬ 
formity in texture, and hence the different parts of the mass 
expand or contract alike by equal increments of heat. A 
fissure may proceed from above downwards, as the outer sur¬ 
face will cool faster than the inner; but its subsequent exten¬ 
sion through this uniform mass of matter will form a fissure of 
nearly the same width throughout, though it may be success¬ 
ively formed. We may justly suppose, however, that in case 
a fissure proceeds from the outer surface inwards, the resistance of 
tension will be less in the interior and lower parts of the rock; 
hence a fissure may rend the lower stratum, as it were prema¬ 
turely to a great depth, the tenacity being proportioned to the 
state of consolidation. 
We can with difficulty resist the conclusion, that as fissures 
are formed by the cooling of the surface, the width of a fissure 
must necessarily be wider at the surface than at considerable 
depths in the interior. The true mode of representing fissures 
or veins in diagrams should be in accordance with this view. 
