1240 A TREATISE ON METAMORPHISM. 



deposits and the rocks, for the ore deposits in many cases are not sharply 

 separated from the country rocks, but grade into them in various ways. 



Therefore it appears that not only are there no hard and fast lines 

 between ore deposits of different modes of origin, but that a single ore 

 deposit may be produced by various combinations of most widely diverse 

 processes. Indeed, it is certain that many ore deposits are not produced 

 by a single process, but by several processes, one following- another, or 

 even by more than one cycle of processes. Thus, ore deposits which 

 receive the first concentration by magmatic segregation may be concen- 

 trated further by gaseous solutions, still further by aqueous solutions, still 

 further by sedimentary processes, and again by aqueous solution. It is 

 clear that there may be every gradation between ore deposits produced 

 solely by magmatic segregation and solely by aqueous solutions. 



For the production of many ore deposits in their present condition 

 there have been many stages of concentration by various processes. One 

 cycle of concentration may have followed another through a large part of 

 geological time in accordance with the general law for the segregation of each 

 of the metals in certain formations. (See pp. 947-948.) Many iron-ore 

 deposits have certainly, and many deposits of other metals have probably, 

 been produced as the result of the work of several cycles. 



While a part of the metals for many and perhaps most of the ore 

 deposits have an exceedingly composite history, in the majority of cases a 

 final process is dominant, and ore deposits may therefore be fairly classified 

 on the basis of the last permanent process. For instance, there is no ques- 

 tion that the Lake Superior iron ores were deposited by descending aqueous 

 solutions; the copper deposits of the same region by ascending aqueous 

 solutions, and the lead and zinc deposits of the Mississippi Valley by 

 ascending and descending aqueous solutions. 



Looked at broadly from the above point of view, the total mass of 

 existing ore deposits of all kinds taken together represents the accumulation 

 of the valuable metals of all previous processes of concentration. The 

 processes of segregation are continuous. We catch the series of cycles 

 at a certain stage, and upon the basis of the existing facts we classify 

 them. In coming geological periods existing ore deposits will be sources 

 from which, by various processes of segregation, the ore deposits of that 

 distant time will derive a portion of their material. Another source of 



