THE ORIGIN OF MOUNTAINS. 67 
less upwards or downwards (Fig. io), it is termed a 
fault. 
Faults may be small, and the difference of height 
between the two sides only a few inches. On the 
other hand, some are immense. In the case of one 
great fault described by Ramsay, the difference is no 
less than 29,000 feet, and yet so complete has been 
the denudation that the surface shows no evidence 
of it, and one may stand with a foot on each side, 
unconscious of the fact that the stratum under the 
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