SUMMARY AND CONCLUSIONS. 1239 



which aqueous solutions are active, and while migrating they may be 

 profoundly modified. The amount of subsequent modification, while 

 migrating from the deep zone to the surface, depends upon the conditions 

 of fracture, water circulation, and all the other factors which have been 

 discussed in connection with the deposition of ore deposits by aqueous 

 solutions. The modifications in the zone of fracture of an ore deposit the 

 concentration of which was begun in the zone of flowage, may be, therefore, 

 inappreciable or of dominating importance. 



Ore deposits formed by aqueous solutions are not nearly so likely to 

 be modified by gaseous solutions as are the latter by aqueous solutions. 

 Under normal conditions an aqueous ore deposit would not be modified by 

 gaseous solutions, but if there were sufficiently powerful orogenic move- 

 ments or great igneous intrusions adjacent to an ore deposit partly formed 

 or primarily produced by aqueous solutions, it is conceivable tha/t>the water 

 might be raised above its critical temperature and thus a further concentra- 

 tion take place as a result of the work of gaseous solutions. It might be 

 supposed also that an ore deposit partly formed by aqueous solutions might 

 become so deeply buried as to pass into the zone of anamorphism and there 

 be modified by gaseous solutions. Thus, while it is admitted that modifi- 

 cation of aqueous solution deposits by gaseous solutions is possible, it is 

 believed that it is exceptional. 



Ore deposits which are precipitated almost solely by ascending waters 

 often grade into those in which descending waters have produced an 

 important effect. Thus there is transition between the ores deposited by 

 ascending waters and those deposited by ascending and descending waters. 

 Similarly there is every gradation between ore deposits formed by ascend- 

 ing waters and those produced by descending waters, and between ore 

 deposits forixied by descending waters and those precipitated from ascending 

 and descending waters. It may not infrequently happen that a single fissure 

 may fall partly in one subclass and partly in another. Thus a single ore 

 deposit may belong partly in the subclass of ores formed by ascending 

 waters, and partly in the subclass of ores produced by ascending and 

 descending waters. However, most aqueous deposits belong chiefly to one 

 of the three subclasses. 



Finally, not only are there gradations between different varieties of the 

 ore deposits among themselves, but there are gradations between the ore 



