VII. 



The Genesis of Mountain Ranges. 



THE Origin of Mountain Ranges is one of the most interesting 

 J- problems in Geology. It is only of late years that anything 

 worthy of being called a theory of their origin has been formulated- 

 This is not surprising, because their internal structure as well as their 

 external form had to be first known. American Geologists have had, 

 perhaps, the best opportunities of engaging in this enticing study, 

 hence the interest it has evoked has been greater on the other side of 

 the Atlantic than here. 



Among those who have given form to our ideas on the subject 

 Professor Joseph Le Conte has been one of the foremost. It is 

 therefore an important event when, as President of the American 

 Association for the Advancement of Science, he chooses for his theme 

 a review of what is already known of mountain structure, and an 

 analysis and criticism of the leading theories that have been advanced 

 to account for the origin of mountain ranges. 



Professor Le Conte commences by drawing a distinction between 

 what he calls a Formal Theory and a Causal or Physical Theory. The 

 first must precede the latter, and while the formal theory according 

 to his view is well advanced, the physical is in a very " chaotic state." 

 The Formal Theory is summarised as follows: — (i) "Mountain 

 ranges while in preparation for future birth, were marginal sea- 

 bottoms receiving abundant sediment from an adjacent land-mass 

 and slowly subsiding under the increasing weight. (2) They were at 

 first formed, and continued for a time to grow, by lateral pressure 

 crushing and folding the strata together horizontally and swelling 

 them up vertically along a certain line of easiest yielding. (3) That 

 this line of easiest yielding is determined by the hydrothermal soften- 

 ing of the earth's crust along the line of thickest sedimentation. (4) 

 That this line, by softening, becomes also the line of greatest metamor- 

 phism, and by yielding, the line of greatest folding and greatest 

 elevation ; but (5) when the softening is very great, sometimes the 

 harder lateral strata are jammed in under the crest, giving rise to 

 fan-structure, in which case the most complex foldings may be near, 

 but not at the crest. Finally (6), the mountains thus formed will be 

 asymmetric, because the sedimentary cylinder-lenses from which they 

 originated were asymmetric." 



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