1236 A TREATISE ON METAMORPH1SM. 



There may be complete gradations between ores deposited by processes 

 of sedimentation and those produced by processes of metamorphism, and 

 between ores apparently most widely separated. Even ores of igneous 

 origin and those deposited by aqueous solutions may grade into each other. 



It has been fully explained that ore deposits produced by processes of 

 sedimentation may grade into those produced by processes of metamor- 

 phism. Illustrating this, it may be recalled that a placer deposit formed 

 bv mechanical concentration may be profoundly modified by the circulating 

 waters of the belt of cementation. Such circulating waters may add greatly 

 to or subtract much from the amount of gold in a mechanical concentration 

 deposit It is highly probable (as explained on pp. 1042-1043), that the 

 gold conglomerates of the Rand and the deep placers of California have 

 this composite origin. It is also conceivable that ore deposits primarily 

 produced by sedimentation may also be modified by the actions of gaseous 

 solutions, but this is probably much less common than the modification by 

 aqueous solutions. 



If it be agreed that ore deposits are produced by magmatic segre- 

 gation, by gaseous solution, and by aqueous solution, it is certain that a 

 deposit may be partly segregated by one of these processes, and this work 

 supplemented by one or both of the other processes. This may be true 

 of a deposit which is the result of continuous segregation. It is hard 

 to see how an ore deposit produced by magmatic segregation can be formed 

 wholly independent of the action of gaseous and aqueous solutions. If at 

 the time a magma solidifies, magmatic segregation of some valuable metal 

 takes place to a certain extent, after solidification is complete the tempera- 

 ture is still above 365° C. for a considerable time, and the conditions are 

 almost certainly those favorable to the action of gaseous solutions. These 

 may continue the process of segregation for a long time, but finally the 

 temperature falls below 365° C, and then when the temperature is still high 

 and hot water potent, the aqueous solutions begin their work. This work 

 of aqueous sohitions may continue to the time the ores are exploited. 

 Thus an ore deposit partly formed by magmatic segregation is sure to 

 be be subsequently modified by the action of gaseous and aqueous solu- 

 tions. The modification may be by addition or subtraction of the valuable 

 material; that is, in the direction of further enrichment or in the direction of 

 depletion. For the titanic magnetites and the corundum-syenite ores it is not 



