324 



HISTORICAL GEOLOGY 



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tioned as a constituent of the Huronian. In 

 the Animikean the largest and richest deposits 

 of that indispensable ore that are yet known 

 have been found (Fig. 323). They occur in 

 the form of thick beds in the sedimentary 

 rocks. Some of the Animikean formations 

 originally contained a large amount of iron 

 minerals, together with quartz and other im- 

 purities ; this was further enriched in certain 

 spots where the underground waters dissolved 

 out everything except the iron minerals and, 

 in some cases, even filled the pores thus left 

 with still more oxide of iron. 



The mines of the Lake Superior region 

 supply more than 80 per cent of the ore 

 from which is derived the iron used in the 

 great industries of this country. This is 

 equal to more than one third of the world's 

 output. More ore is now taken from a single 

 mine in the Mesabi district of Minnesota 

 each year than was mined in the entire 

 United States before the Civil War. 



Keweenawan system. Still a third great 

 system lies upon the eroded edges of the 

 Animikean strata and occasionally laps over 

 upon even older formations. In this we have 

 the record of one of the greatest episodes of 

 local volcanic activity known in geologic time. 

 The eruptions seem not to have come from 

 definite craters, but the fluid lava simply 

 welled up through cracks in the surface and 

 spread over wide areas. A series of these 

 flows accumulated one above the other to 

 a depth estimated at more than 6 miles. 

 The great number of the flows may be ap- 

 preciated when we consider that most of 



