On M imeral Veins. 239 
and divisional planes, and not unlikely that the magnetic 
tension may have at least assisted in the opening of vein- 
fissures in the harder rocks. 
I have met with one instance where the rock on the north 
side of the vein presented a singular appearance, which 
might arise from this cause. At the Beldi hill mine—an 
east and west vein in the mountain limestone formation— 
a drive was taken into the north wall in the twelve fathom 
lime ; on the wall of the vein the rock was hard and diffi- 
cult to work, with a peculiar knotted appearance as if it had 
been compressed by some enormous force, and the drive was 
carried several fathoms before the limestone resumed its 
usual character. A lower drive was carried in the same 
direction in the plate or shale beds underneath the lime- 
stone. T’hese beds are nearly horizontal, and the sedimentary 
planes correspond ; but on the north wall of the vein there 
was a vertical cleavage which had nearly obliterated the 
sedimentary planes, and as in the limestone drive above, 
the bed only attained its ordinary character at some distance 
from the vein. The compression of the sedimentary beds in 
the schistose rocks when these beds are at right angles to 
the cleavage may possibly arise from the same cause. In 
numerous cases, however, the conditions point more to the 
gradual replacement of the rock by the constituents of the 
vein, and the force of tension could only have acted as an 
auxiliary. For instance, a mineral like quartz distributed 
through a mass of alumina, as it is found in the schistose 
rocks, has a tendency to segregate itself from the clay and 
accumulate on certain lines or points, where it will replace 
the previously existing rock atom by atom by an action 
similar to that by means of which pseudomorphic crystals 
are formed. An interesting experiment bearing on this 
point is described by Becquerel :—A plate of steel was en- 
closed in a case communicating through a fissure at one end 
with a weak solution of nitrate of silver—-kept at the same 
level by occasional additions to the solution—and left in this 
state for eight years. At the end of that time it was found 
that one half of the steel plate was changed into very pure 
silver, the volume of silver being the same as that of the 
steel removed. 
After a careful analysis of the observed facts I have come 
to the conclusion that mineral veins have not been open 
fissures caused by faults in the sense usually understood, 
but that they have formed gradually on the existing joints, 
