1222 A TREATISE ON METAMORPHISM. 



After an ore deposit has been formed the country may pass below the 

 level of the s,ea by denudation and subsidence, may be deeply buried 

 under sedimentary rocks, and may be again uplifted and undergo a second 

 cycle of reactions which affect the nature of the ore deposits. An ore 

 deposit partly formed may be buried deep under volcanic rocks. This 

 undoubtedly has occurred on a great scale in the great region of Tertiary 

 volcanism in the Cordilleras of the West. The ore deposits there buried 

 are placed in a new environment and are undergoing a second cycle of 

 concentration or depletion. In that district it is entirely possible, indeed 

 probable, that ore deposits formed in the pre-Tertiary volcanic rocks, and 

 with a very complex history involving the work of both ascending and 

 descending waters, are now receiving additions of ores from ascending 

 waters, or, on the contrary, are being depleted, and in consequence are 

 contributing metals to waters which are rising. Such waters may deposit 

 the metals in openings within the volcanic rocks, thus making new ore 

 deposits. When in the future denudation shall have stripped off these 

 volcanics, or the upper part of them, these -ore deposits will be at the 

 surface. In this connection it is to be remembered that in the Yellowstone 

 Park and other areas there are extensive Tertiary volcanic tuffs in almost 

 every respect like formations which in the San Juan district of Colorado 

 bear important ore deposits, but the San Juan region was so elevated that 

 denudation has thoroughly dissected the volcanic material. Indeed, the 

 major streams have cut down through the volcanics to the sediments below. 

 It may be suggested that when denudation of the volcanic series shall have 

 progressed as far in various other Tertiary volcanic regions as in the San 

 Juan region ore deposits will be exposed, but this may not occur while man 

 occupies the earth. 



It is well known that fissures are places of weakness, and that move- 

 ment has again and again recurred along the old planes. Thus favorable 

 conditions for ore concentration may recur in the same places through 

 various revolutions. Physical changes of various other kinds may take 

 place. Each of the complex changes in physical history produces its 

 effect upon an ore deposit. 



