292 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



partly to a more intense movement of compression. On the eastern flanks of 

 the mountains the antidine, east of the first monocHnal slope which rests 

 directly on the Archean, is rarely seen, being concealed beneath later 

 deposits. Its slopes are probably relatively gentle, as it is not compressed 

 between two Archean masses like those of the Mosquito Range, but has a 

 broad mountainless area at its back. Could this fold be seen it would 

 probably be found to have the S character ; that is, a steeper slope to the 

 west. The fact that the monoclinal slopes on the eastern flanks are some- 

 times very steep I judge to be due to a later movement of contraction since 

 the dynamic movement, by which the upper beds have been pushed against 

 the Archean and the beds which rest directly on it, and thus brought into 

 a vertical or even an inverted position. 



As regards the correlation of folding and faulting, the geological evi- 

 dence, as I read it, is entirely opposed to the idea that the uplift of the 

 ranges was independent of and later than the flexing, and produced mainly 

 by faulting. The evidence of the Mosquito Range, which may be fairly 

 taken as a type, though perhaps an extreme one, of Rocky Mountain struct- 

 ure, certainly shows a close interdependence between folding and faulting. 

 That the rocks forming the Archean land masses were plicated and eroded 

 long before is quite evident, but that the}' were blocked out by faults and 

 lifted into a platform is purely hypothetical and incapable of proof Again, 

 while the closely appressed folds of the Appalachians are rarely found in the 

 Rocky Mountains, I consider the folding that does exist there none the less 

 a true plication. The peculiarly regular, narrow iblds of the Jura Mount- 

 ains and of the Appalachian system are simply the extreme type of closely 

 folded strata, due to peculiarly favorable conditions which it is not now 

 worth while to discuss at length, and differ from those of the Rocky Mount- 

 ains in amount rather than in kind. 



ERUPTIVE ROCKS. 



The eruptive rocks of this region fall naturally into two groups or 

 series, whether considered from the point of view of their age, of their man- 

 ner of eruption, or of their internal structure and composition. 



