THE PROTEROZOIC ERA 



459 



must have affected the first, while those which affected the third 

 must have affected both the older systems. The oldest system is, 

 on the average, most metamorphosed, and the youngest least. 



Carbonaceous slates. One of the significant formations of this 

 region is black shale or slate, the color of which is due to the presence 

 of carbon, often in considerable quantity. The content of carbon 

 is thought to imply the existence of life when the sediments were 

 deposited. Where the rocks are highly metamorphic, the black 

 shale has been changed to graphitic schist. 



Iron ore. Another important formation is iron ore. Here 

 belong the iron ores of the Mesabi (Minn.), Penokee-Gogebic (Wis. 

 and Mich.), and Menominee (chiefly Mich.) regions (Fig. 352). The 

 ore is in the form of ferric oxide (chiefly hematite, Fe 2 O 3 ), but in 

 this form it represents an alteration from iron-bearing cherty car- 

 bonates, and sediments containing ferrous silicate. The alteration 

 was brought about by ground- water, circulating through the rocks. 



The region about Lake Superior yields more iron ore than any 

 other area of equal size in the world. In 1907, the aggregate pro- 

 duction of this region was 41,526,579 long tons, 1 which was more 

 than 78 per cent of all that was produced in the United States that 

 year; of this, the Mesabi region produced nearly 27,245,441 tons. 

 The iron ores of this region are partly in the Archean (about Ver- 

 milion, Minn.), partly in the older divisions of the Huronian group 

 (about Marquette, Mich.), but most largely in the Animikean 

 (Menominee and Gogebic regions, Mich, and Wis., the Mesabi region 

 of Minn., and some of the ores about Marquette). The following 

 table gives the production in tons for these several regions for 

 certain years preceding 1906: 



(Long tons; maxima for each region in Italics.) 



Mineral Resources of the United States. 



