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Minnesota Academy of Science 



Fig. 22. Rock Bluff of the Keweenawan. 



(22) This view, although taken from the igneous rocks of the 

 Keweenawan on the shore of lake Superior, also illustrates the 

 structures seen in the mines and other excavations of the mining 

 region df the Mesabi, and especially at the Mahoning mine. The 

 central part of this figure, showing the crumbling igneous rock, is 

 like the crumbling walls of the Mahoning mine. 



Other obvious igneous structures that have been described in 

 the Mesabi mines are amygdaloid, bomb-like balls supposed to have 

 been volcanic bombs, sheeting and jointing characteristic of lava, 

 and "purgatories," such as are seen along the Lake Superior shore. 



12. Microscopic Characters. 



The most interesting, and at the same time the most indisput- 

 able evidence of the igneous nature of the original Mesabi rocks is 

 microscopic. It is getting pretty close to the subject, when you 

 examine it microscopically, and it is not possible to go into these 

 characters, except to speak briefly of one of the most important 

 features. I mentioned that one of the new minerals that were 

 formed by the alteration of the volcanic sand is 



