1242 A TREATISE ON METAMOKPHISM. 



may grade into the first class, and after the transition the deposit may be 

 rich enough to warrant exploitation at depth. If such work be undertaken, 

 it must be understood that the rich upper products peculiar to the belt of 

 weathering and to the belt of sulphide and telluride enrichment will not be 

 duplicated at depth. It therefore appears to me that the determination 

 to which of the classes of ore deposits produced by underground waters a 

 given deposit belongs has a direct practical bearing upon its exploration 

 and exploitation. 



Again it has been seen that the ores produced by magmatic segrega- 

 tion and those which are produced by gaseous solutions form a relatively 

 small class., and not only so, but that the ore deposits themselves are 

 usually rather circumscribed. When, therefore, deposits have either of these 

 origins, experience does not justify the belief that they will be of great 

 importance. For instance, if a copper deposit is associated with heavy 

 anhydrous gangue minerals, and has irregular boundaries, it can not be 

 expected to prove comparable in richness and extent to the great copper 

 deposits produced by aqueous solutions. 



It is my hope that mining engineers and geologists will study ore 

 deposits in various regions with reference to the principles discussed in this 

 treatise. It appears to me that he who does this will be capable of inter- 

 preting better than before the phenomena which he finds in the ore body 

 or bodies with which he is particularly concerned. 



In addition to the points specially emphasized in this volume, accurate 

 descriptions should be made of the relations of the different minerals of ore 

 deposits, of the occurrence of each ore mineral with reference to the wall 

 rocks, and of the variations of the ores in composition and richness at 

 various depths, reckoning both above and below the level of ground water. 

 Moreover, such a study should include close observation of the g-angue 

 minerals in their relations to one another and to the valuable minerals, for 

 in many instances they give important testimony as to the origin of the ore 

 deposit. A study should be made of the stratigraphy, including the changes 

 of country rocks and wall rocks, the pervious and impervious formations, 

 the phenomena of deformation, such as folding, faulting, jointing, breccia- 

 tion, secondary structures, etc. The volcanism of the district should be 

 fully investigated. Finally, the general metamorphism should be worked 

 out. In short, my point of view is that the principles controlling the 



