1052 A TREATISE ON METAMORPHISM. 



solutions; and still later, when the rocks have farther cooled, further 

 additions may be made by aqueous solutions. Indeed, I suspect that a 

 considerable number of ores have had such a composite history, viz, first, 

 segregation by magmatic processes, further segregations by gaseous 

 solutions, and finally segregations by aqueous solutions. I hold this latter 

 stage to be commonly the one in which occurs the final concentration 

 necessary to make a deposit of value, and therefore entitled to the name of 

 ore. In the field the general terms "contact-action," to cover all of these 

 three kinds of work, and "contact metamorphic ores," to cover the total 

 result, may be useful; but one can not make a classification of ore deposits 

 in which he introduces at the same time contact metamorphic ores and the 

 classes of igneous ores, gaseous solution ores, and aqueous solution ores. 



DIVISION C. ORES PRODUCED BY PROCESSES OF METAMORPHISM. 



It is believed that ore deposits produced by processes of metamorphism 

 are of dominant importance. While ore deposits produced by processes of 

 sedimentation and by igneous processes are of great consequence, they 

 together are subordinate in importance to those produced by processes of 

 metamorphism. 



Ores produced by processes of metamorphism may theoretically be 

 divided into two groups — those deposited in their present positions by 

 gaseous solutions and those deposited in their present positions by aqueous 

 solutions. 



The second group is believed to be more important than all other ore 

 deposits. I hold, therefore, that the dominant agents which deposited ores 

 in their present positions are aqueous solutions. How important is the 

 group of ores deposited by gaseous solutions is uncertain, as the criteria 

 have not been fully worked out by which these deposits can certainly be 

 discriminated from those precipitated from aqueous solutions. 



GROUP A. ORES DEPOSITED BY GASEOUS SOLUTIONS. 



Ores deposited from gaseous solutions comprise all those ores which 

 are precipitated from gaseous mixtures of all kinds. They therefore include 

 all deposits actually formed by pneumatolytic, fumarolic, and solfataric 

 action. That ore deposits have been precipitated from gaseous solutions 



