





L 



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the* 



toltf- 



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Cir. VII.] 



OF IGNEOUS FORGES. 



121 



m 



•oni the parallelism 



again. 



striae that have been last produced. 



When rocks have been once fractured, and freedom of mo- 

 tion communicated to detached portions of them, these will 

 naturally continue to yield in Jthe same direction, if the pro- 

 cess of upheaval or of undermining be repeated again and 



The incumbent mass will always give way along the 

 lines of least resistance, and therefore usually in the places 

 where it was formerly rent asunder. Probably, the effects of 

 reiterated movement, whether upward or downward, in a 

 fault, may be undistinguishable from those of a single and 

 instantaneous rise or subsidence ; and the same may be said 

 of the rising or falling of continental masses, such as Sweden 

 or Greenland, which we know to take place slowly and in- 

 sensibly. 



Doctrine of 



the 



sudden upheaval of parallel mountain- 



1833 



chains. — The doctrine of the suddenness of many former 

 revolutions in the physical geography of the globe has been 

 thought by some to derive additional confirmation from a 

 theory respecting the origin of mountain-chains, advanced in 



by a distinguished geologist, M. Elie de Beaumont. 

 In several essays on this subject, the last published in 1852, 

 he has attempted to establish two points ; first, that a variety 

 of independent chains of mountains have been thrown up 

 suddenly at particular periods ; and, secondly, that the con- 

 temporaneous chains thus thrown up, preserve a parallelism 

 the one to the other. 



These opinions, and others by which they are accompanied, 

 are so adverse to the method of interpreting the history of 

 geological changes which I have recommended in this work, 

 that I am desirous of explaining the grounds of my dissent, 



mvseli the more 



the generalisations alluded to are those of a skilful writer, and 

 an original observer of great talent and experience. I shall 

 begin, therefore, by giving a brief summary of the principal 

 propositions laid down in the works above referred to.* 



* Ann. des Sci. Nat., Septembre, No- Fran 9 aise, No. 15. May, 1830. Bulletin 

 vembre, et Decembre, 1829. Eevue de la Societe Geol. de France, p. 864. 



