120 



SUPPOSED FORMER INTENSITY 



[Ch. VH. 



That the more impressive effects of subterranean power 

 fl1, " tl,a "?heaval of monntain-chains, may have been due 



convulsions of moderate intensity rather than 

 to a few paroxysmal explosions, will appear the less impro- 

 bable when the gradual and intermittent development of 



multiplied 



4- 



times 



It 



now very generally conceded that these eruptions have their 

 source in the same causes as those which give rise to the 

 permanent elevation and sinking of land ; the admission, 

 therefore, that one of the two volcanic or subterranean pro- 

 cesses has gone on gradually draws with it the conclusion 

 that the effects of the other have been elaborated by succes- 



sive and gradual efforts. 



Faults. - 



t faults. 



or those striking instances of the upthrow or downthrow of 

 large masses of rock, which have been thought by some to 

 imply tremendous catastrophes wholly foreign to the ordinary 

 course of nature. Thus we have in England faults in which 

 the vertical displacement of the rocks amounts sometimes to 

 several hundred, and in other cases to 3,000 feet, while the 

 fissures extend horizontally for distances varying from a few 



miles 



Their original width , for they 



from 



to fifty feet. But when we enquire into the proofs of the mass 

 having risen or fallen suddenly on the one side of these great 

 rents, several hundreds or thousands of feet above or below the 

 rock with which it was once continuous on the other side, we 

 find the evidence defective. There are grooves, it is said, and 

 scratches on the rubbed and polished walls, which have often 

 one common direction, favouring the theory that the move- 

 ment was accomplished by a single stroke, and not by a 



series 



movements 



But, in fact, the striae are 

 not always parallel in such cases, but often irregular, and 

 sometimes the stones and earth which are in the middle of 

 the fault, or fissure, have been polished and striated by 

 friction in different directions, showing that there have been 

 shclings subsequent to the first introduction of the fragment- 

 ary matter. Nor should we forget that the last movement 

 must always tend to obliterate the signs of previous tritura- 











