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APPENDIX. 



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CO 



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Mr. Forbes (Fig. 26), we have a form ot mountain structure different from the foregoing 



examples. The section is indeed truncated, the 

 eastern flanks of the chain not having been explored, 

 yet even this incomplete view seems to suggest a 

 very complicated origin for the whole. The eastern 

 and most lofty range is formed of the older sedi- 

 mentary formations with granitic rocks, while to the 

 west of it, and connected with it by lofty plateaux, 

 there rises an apparently independent mountain 

 range, in which volcanic phenomena are enormously 

 developed. 



From his observations of the Andes in Chili, and 



from his study of the 

 ^Darwin on the Andes of yo]canic phenomena in 



the same regions, Mr. 

 Darwin has made some very instructive remarks upon 

 the phenomena of elevation and of mountain forma- 

 tion (Trans. Geol. Soc., Lond., 1838, Second Series, 

 Vol. VI.) He first establishes the fact of a coincidence, 

 and hence infers a common cause for earthquakes, 

 volcanic eruptions, and the permanent elevation of 

 large areas (continental elevation). He even asserts 

 that no theoiy of the cause of volcanoes which is 

 not applicable to such elevations can be considered 

 as well-grounded. From the proved recurrence of 

 these phenomena, resulting in an upheaval of several 

 hundred feet within the recent geologic period, he 

 asserts the adequacy of the cause to produce, and 

 explain continental elevation. Observation thus leads 

 him to the same conclusion as Mr. Hopkins, that 

 the fracture of strata and the formation of mountain 

 chains are only subsidiary phenomena attendant on 

 continental elevation. Mr. Darwin makes some very 

 reasonable suppositions to remove Mr. Hopkins' 

 objections to the successive formation of parallel 

 fissures ; the process is such a slow one that a long- 

 established fissure might well become clogged up by 

 solidified intruded rock ; and thus lead to the pro- 

 duction of a new line of fracture, He believes that 

 such successive formation can be proved in- the case 

 of the several axes of the Andes, of which he dis- 

 tinguishes eight or more. He states his opinion that 

 a chain of which the axis is volcanic only differs 

 from one in which the axis is formed of plutonic 



.H O 



