1238 A TREATISE ON METAMORPHISM. 



mechanical processes. Thus one might consider that the residual deposits 

 which are classified as mechanical concentrates are partly chemical concen- 

 trates, and on this basis chemical deposits might be divided into chemical 

 precipitates and residual concentrates. However, residual deposits are 

 placed with the mechanical concentrates mainly because of their close 

 alliance with stream and beach deposits, which are plainly mechanical 

 concentrates. 



This leads to the transition between residual, stream, and beach 

 deposits. It is well known that a residual deposit on a hill or slope may 

 pass by imperceptible stages into the concentrates at the bottom of a stream. 

 It is known also that stream deposits may extend along the stream until 

 the ocean is reached, where a beach deposit may exist. It is therefore clear 

 that there is every possible gradation between residual, stream, and beach 

 deposits. 



An ore deposit which is formed mainly by gaseous solutions may be 

 modified by aqueous solutions, for when the temperature has fallen so that 

 the water is below its critical temperature, it is little short of certain that 

 the processes of segregation may and usually will be continued by aqueous 

 solutions, just as in the case of magmatic ores. Thus ore deposits are 

 probably seldom formed solely by gaseous solutions without any subse- 

 quent modification by aqueous solutions. Probably very often ore 

 deposits, perhaps inaugurated by precipitation from gaseous solutions, have 

 been profoundly modified by aqueous solutions. Like the ore deposits 

 first segregated by magmatic segregation, the later water action may be in 

 the direction of addition or subtraction. As to the importance of subse- 

 quent action by aqueous solutions upon ore deposits partly formed by 

 gaseous solutions, there may be difference of opinion. It is possible that 

 rocks which do not freely permit the circulation of underground water may 

 be more pervious to gaseous solutions, and that in very dense rocks deep 

 within the earth gaseous solutions may segregate ore deposits — such as those 

 associated with heavy anhydrous silicates or gangue minerals — which while 

 in that position are not materially modified by the subsequent action of 

 aqueous solutions. But ore deposits which are formed in the zone of 

 anamorphism must pass well up into the zone of fracture, if not to the 

 surface, before they are accessible to man. They are now in the zone in 



