36 THE MARQUETTE lEOX-BEAEING DISTKICT. 



Some of the smaller ones "apjjroacii much nearer to segregated veins, and 

 might be classed with them, were they not developed on so large a scale 

 as to render it difficult to conceive of segregation as a siifficient cause for 

 their production " (p. 212). Hence the author declares that there is only 

 one hypothesis that will explain their occurrence. We quote as follows 

 (pp.212, 213-215): 



They are simply parts of the rocky crust of the earth, and, like other igueous 

 rocks, have been i)oured forth from the interior iu the molteu or plastic state. No 

 other origin can be assigned to the dome-shaped and conical masses of Lake Superior 

 and Missouri, or to the elongated ridges of the first-named region. 



The eruptive origin of the great Lake Superior ore-masses seems also well sus- 

 tained by the phenomena which they exhibit. They alternate with trappeau ridges 

 whose eruptive origin can not be doubted, and which themselves contain so much 

 magnetic oxide disseminated through their mass, as one of their essential ingredients, 

 that they might almost be called ores. These eruptive masses include the largest 

 and purest deposits of ore which are known in the Lake Superior or the Missouri 

 iron regions; but there are other localities in both these districts where the mode of 

 occurrence of the ore is somewhat difierent and where the evidences of a direct 

 igneous origin are less marked. This class comprehends those lenticular masses 

 of ore which are usually included within gueissoidal rocks, and whose dip and strike 

 coincide with that of the gneiss itself, but whose dimensions are limited. * * * 

 Such beds of ore as these may in some cases be the result of segregating action; but 

 the facts seem rather to indicate that they are made up of the ruins of preexisting 

 igneous masses which have been broken and worn down during the turbulent action 

 which we may suppose to have been preeminently manifested during the Azoic epoch, 

 and then swept away by currents and deposited in the depressions of the sedimentary 

 strata then in process of formation. In confirmation of this hypothesis * * * it 

 may be noticed that the ores occurring in this form and position are less pure than 

 those of decidedly igueous origin, as if they had become more or less mixed with 

 sand during the in-ocess of reconstruction, so that they not unfrequently require to be 

 separated from their earthy impurities by washing before they can be advantageously 

 used. Again, it may be observed in the case of some of the ore beds of this class 

 that the bed rock or foot wall is considerably rougher or more irregular in its outline 

 than the hanging wall or roof, as if depositions had taken place upon a surface 

 originally rough and uneven, the upper surface of the ore being considerably smoother 

 and more regular than the lower one, and sometimes separated from the rock by a 

 thin seam of calcareous matter. 



There is still another form of deposit which is not unfrequently met with iu the 

 Lake Superior region. * » • This consists of a series of quartzose beds of great 



