Fig. 518.
Fig. 519.
Let a b, [fig. 517.], be a line of fracture traversing a rock, and let a b, [fig. 518.], represent the same line. Now, if we cut a piece of paper representing this line, and then move the lower portion of this cut paper sideways from a to a', taking care that the two pieces of paper still touch each other at the points 1, 2, 3, 4, 5, we obtain an irregular aperture at c, and isolated cavities at d d d, and when we compare such figures with nature we find that, with certain modifications, they represent the interior of faults and mineral veins. If, instead of sliding the cut paper to the right hand, we move the lower part towards the left, about the same distance that it was previously slid to the right, we obtain considerable variation in the cavities so produced, two long irregular open spaces, f f, [fig. 519.], being then formed. This will serve to show to what slight circumstances considerable variations in the character of the openings between unevenly fractured surfaces may be due, such surfaces being moved upon each other, so as to have numerous points of contact.
Fig. 520.