PRINCIPLES CONTROLLING ORE DEPOSITS 769 



or even a third concentration may take place. The first concen- 

 tration by igneous or sedimentary processes may be the more 

 important or dominant process, or the additional concentration 

 or concentrations by underground waters may be the more 

 important or dominant process. In some cases therefore the 

 ores may be referred to as produced by igneous agencies, in 

 others as produced by processes of sedimentation, in others as 

 produced by these in conjunction with underground waters, and 

 in still others as produced mainly by underground waters. 



Ore deposits which are precipitated almost solely by ascend- 

 ing waters will grade into those in which descending waters have 

 produced an important effect, and thus there will be transition 

 between classes (#) and (c). Similarly there will be every grada- 

 tion between classes [a) and [b) and between classes (£) and (<:). 

 If this be so it will not infrequently happen that a single fissure 

 may fall partly in one class and partly in another. Thus a single 

 ore deposit may belong partly in class [a) and partly in class (<r). 

 However, in most cases the workable part of a deposit will 

 largely belong to one of the three classes. 



Not only are there gradations between different varieties of 

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

 and the rocks ; for the ore deposits in many cases are not sharplv 

 separated from the country rocks, but grade into them in various 

 ways. 



In answer to the above objection concerning gradations, it 

 may be said that I know of no classification of ore deposits 

 which has yet been proposed to which the same objection may 

 not be urged with equal or greater force. 



However this retort does not give any criterion by which the 

 usefulness of the above classification may be tested. The test 

 is, does this classification give a more satisfactory method of 

 studying ore deposits than has heretofore been possible ? Will 

 an attempt to apply this classification assist mining engineers 

 and geologists in accurately describing ore deposits ? Will the 

 classification to a greater extent than any previous one give 

 engineers rules to guide them in their expenditure in exploration 



