PRINCIPLES CONTROLLING ORE DEPOSITS 767 



some form of sulphuret ; but they may be tellurides, silicates, or 

 carbonates. 



(£) Ores which at the place of precipitation are deposited by 

 descending waters alone. These ores are ordinarily oxides, car- 

 bonates, chlorides, etc., but silicates and metals are exception- 

 ally included. 



(c) Ores which receive a first concentration by ascending 

 waters and a reconcentration by descending waters. The con- 

 centration by ascending waters may wholly precede the concen- 

 tration by descending waters, but often the two processes are at 

 least partly contemporaneous. The materials of class (c) com- 

 prise oxides, carbonates, chlorides, and rarely metals and silicates 

 above the level of groundwater, and rich and poor sulphurets, 

 tellurides, metallic ores, etc., below the level of groundwater. 

 At or near the level of groundwater, these two kinds of products 

 are more or less intermingled, and there is frequently a transi- 

 tion belt of considerable breadth. 



How extensive are the deposits of class [a) I shall not 

 attempt to state. Indeed, I have not such familiarity with ore 

 deposits as to entitle me to an opinion upon this point. How- 

 ever a considerable number of important ore deposits belong to 

 this class. This class is illustrated by the Lake Superior copper 

 deposits. 



The ore deposits of class (b) are 'important. Of the various 

 ores here belonging probably the iron ores are of the most con- 

 sequence. A conspicuous example of deposits of this kind are 

 the iron ores of the Lake Superior region. 



It is believed that the ore deposits of class (c) are by far 

 the most numerous. I suspect that a close study of ore deposits 

 in reference to their origin will result in the conclusion that the 

 great majority of ores formed by underground water are not the 

 deposits of ascending waters alone, but have by this process 

 undergone an early concentration, and that descending waters 

 have produced a later concentration, as a result of which there 

 is placed in the upper 50 to 500 or possibly even 1000 meters 

 of an ore deposit a large portion of the metalliferous material 



