SUMMARY AND CONCLUSIONS. 1237 



supposed that the action of the gaseous and aqueous solutions is usually of 

 great consequence; but for the sulphide ores and the gold ores which have 

 been held to be produced by magmatic segregation alone, I hold the work 

 of the solutions to be a necessary and in most instances the dominant part 

 of the process. 



Whether ore deposits produced by magmatic segregation are likely to 

 be altered mainly by gaseous solutions or by aqueous solutions will depend 

 largely on the zone in which the igneous rocks are intruded. If the rocks 

 are batholiths in the zone of anamorphism the magmatic segregations which 

 there form are likely to be modified by gaseous solutions, and so long as 

 they remain in that zone aqueous solutions are likely to be unimportant. 

 Where the igneous rocks are intruded into the zone of fracture the con- 

 ditions are not favorable for action of gaseous solutions for any length 

 of time, but are especially favorable for the long-continued action of 

 aqueous solutions. Thus we would expect that magmatic-segregation ores 

 would be especially likely to be modified by gaseous solutions in the zone 

 of anamorphism, and by aqueous solutions in the zone of fracture. 



Metamorphic ore deposits formed mainly by gaseous solutions or by 

 aqueous solutions, or by the two combined, may form without any implica- 

 tion that they are partly due to magmatic processes. Often after igneous 

 rocks are intruded ores do not begin to form until gaseous solutions are 

 active, or much later, when aqueous solutions are at work. 



Not only are there transitions between ore deposits produced by 

 sedimentary, igneous, and metamorphic processes, but there are transitions 

 between the classes and subclasses. To illustrate, there are transitions 

 between chemical precipitates and mechanical concentrates; between 

 residual, stream, and beach deposits; between the deposits produced by 

 gaseous and aqueous solutions; between deposits produced by ascending- 

 waters, descending waters, and ascending and descending waters. 



The chemical precipitates and mechanical concentrates are connected 

 by the residual deposits. Very frequently chemical solution of a rock in 

 which the ore was disseminated is of equal or greater importance than the 

 mechanical concentration. The residual iron ore mantling Iron Mountain 

 Missouri, is quite as much due to the solution of the porphyry as to its 

 mechanical disintegration. The tin ore in the alluvium of the Malay 

 Peninsula is probably concentrated more extensively by chemical than by 



