PERSISTENCE OF VEINS IN DEPTH. 
133 
Fig. 25. 
for about Torty feet, it may have an average width of eighteen 
inches, it bulges out and contracts at many points upon the line 
of exposure, and is four feet thick at one of the places of dip. 
It will be observed, also, that the vein is not perpendicular, the 
rent is not even or vertical, but one which is inclined; or in other 
words, it has a dip slightly variable at different points, but which, 
when followed down for one 
hundred or two hundred feet, 
or more, is found to be con¬ 
stant, so as to conform to 
those of the district. The 
direction of dip may be de¬ 
pended upon, and shafts are 
often sunk two or three or 
even four hundred feet, with 
the expectation of cutting it 
at one of those depths. Some¬ 
times a vein which dips east¬ 
ward at the surface, is found 
to dip westward at 80 or 100 
feet. The indication that 
such a change may be expect¬ 
ed consists in the character 
of the disturbances at the 
surface. The dip at the sur¬ 
face in those cases is entirely 
at variance with the common 
dips of the veins of the dis¬ 
trict, and the existence of 
dykes and faults, show that 
something unusual has taken 
place. The veins of a dis¬ 
trict usually dip in the same 
direction; and when a vein 
dips in a contrary one, under 
the circumstances I have stat¬ 
ed, there is ground for expecting that at a certain depth this vein 
will change its direction. 
