ORE SHOOTS. 1231 



tliem. To explain an ore shoot of an individual mine, ordinarily a number 

 of the above causes need to be combined, and in some cases doubtless, 

 other causes which have not been treated. No study is more important 

 economically, more fascinating, or more difficult than to ascertain the par- 

 ticular combination of factors which produced the ore shoots in a given 

 district or mine. 



From the foregoing it is plain that no general statement can be made 

 in explanation of ore shoots. In each district, in each mine, in each part of 

 a mine, all the phenomena must be studied closely in the light of correct 

 theories of ore deposition in order to reach an explanation applicable to 

 the particular case. 



It is well known that in the districts which are mineralized the work- 

 able ore deposits are ordinarily confined to relatively small areas, although, 

 so far as one can see, the amount of metalliferous material furnishing ore 

 deposits may have been the same throughout the districts. The absence of 

 workable ore deposits for the larger parts of the districts ordinarily is due 

 to lack of favorable combination of the various special factors mentioned, 

 and doubtless many others which have not been considered. As better 

 illustrating my meaning, I may again mention the iron and copper ores of 

 the Lake Superior region. The iron-bearing formation has an extensive 

 occurrence through the Lake Superior region, but the workable iron ores 

 are confined to small areas which have been subject to ancient and recent 

 metamorphism, and in which there are favorable structural features. The 

 Lake Superior copper deposits equally well illustrate the principle. All of 

 the mines now being exploited are confined to an exceedingly narrow area 

 on Keweenaw Point, while the copper-bearing rocks occupy an extensive 

 belt about nearly the entire Lake Superior basin. Moreover, these 

 copper-bearing rocks are mineralized in many places, as is shown by the 

 widely disseminated copper. But, unfortunately, in many areas a little 

 copper is concentrated in many amygdaloid or sandstone belts, rather than 

 in a single amygdaloid or sandstone. For instance, in certain districts 

 scores of amygdaloidal flows lie upon one another. In each of these beds 

 the scoriaceous upper surface bears metallic copper, but in none is it in 

 sufficient amount to make the copper a workable deposit. Had the copper 

 deposited in a number of these amygdaloid formations been concentrated 

 in one of them, a workable ore deposit would have been produced. 



