THE ICE AGE 307 



that much copper is mined on the south shore of Lake 

 Superior, especially on the Keweenaw Peninsula. Large 

 pieces of almost pure copper are common here, and have no 

 doubt been formed where we find them to-day. If, how- 

 ever, you should now and then find large pieces of metallic 

 copper in the glacial soil at St. Paul and Minneapolis, and 

 even as far south as Illinois and Iowa, you would find this 

 metal far from its natural source or origin, and as a think- 

 ing person you would ask, How did it come to be there ? 

 As a matter of fact, such pieces of copper are frequently 

 found in the regions mentioned, and generally show that 

 they have been pressed or rolled by some heavy mass. The 

 only reasonable explanation is that they were dropped by 

 the melting ice sheet. Geologists have calculated from re- 

 ceding river gorges, like that of Niagara below its falls, and 

 that of the Mississippi between Fort Snelling and Minne- 

 apolis, that the ice probably disappeared between ten and 

 twenty thousand years ago ; but nothing is known about 

 the length of the glacial epoch itself. There is good evi- 

 dence that northern Europe had its glacial epoch as well as 

 North America. 



For information about the probable causes of the glacial epoch and 

 for further proofs of it, the teacher is referred to some text-book on 

 geology. On the recession of the Mississippi gorge, see Geology of 

 Hennepin County in the Final Report of the Minnesota State 

 Geologist. 



82. Formations consisting of vegetable debris. When 

 we studied the mosses, we learned that thick layers of peat 

 have been formed, and are still being formed, by the decay 

 of mosses and other vegetation. The coal beds, which are 

 from less than an inch to forty feet thick, were also formed by 

 plants. This is proved by the many impressions of plants 

 we find in them, and also by roots and tree-trunks which 

 have been discovered in them. The coal plants were mostly 



