THE LAKE SUPERIOR PROTEROZOIC 337 



fossiliferous formation is a part of the Cambrian system, the oldest 

 system of the Paleozoic group. 



Copper. The Keweenawan system contains the most extensive 

 deposits of native copper known. The metal occurs in pores and cracks 

 of the igneous rock, and between the pebbles and grains of some 

 parts of the sedimentary beds. In the conglomerate at some of the 

 richer mines, the copper is so abundant as to be an important 

 cementing material of the rock. The copper is believed to have 

 been deposited by magmatic waters (i. e., waters of lavas), and to a 

 lesser extent by thermal ground waters which had dissolved the 

 nu-tul from the igneous and sedimentary rocks. 1 



In 1875 the Keweenawan formation of northern Michigan 

 yielded 16,089 tons of copper, about 90 per cent of all that was 

 produced in the United States. In 1911 the same area yielded 

 109,093 tons, but this was only about 20 per cent of the copper 

 produced in the country that year. 



The ores of silver, cobalt and nickel in the Huronian formations 

 are to a large extent at least associated with basic igneous rocks, 

 perhaps of Keweenawan age, intruded into Huronian rocks. 



General Considerations Concerning the Lake Superior Proterozoic 



Duration of time. It is difficult to conceive of the great lapse 

 of time involved in the history of the Proterozoic era. The esti- 

 mates give an aggregate thickness of more than 30,000 feet for the 

 sedimentary rocks of the Proterozoic systems. The accumulation 

 of so much sediment would in itself mean a vast lapse of time, and 

 when it is remembered that the four systems are separated from 

 one another by unconformities, each of which may represent as 

 much time as that involved in the accumulation of a system, it will 

 be seen that the duration of the Proterozoic era was exceedingly long, 

 possibly comparable to all succeeding time. It would appear that 

 it should be spoken of in terms of tens (at least) of millions of 

 years, rather than in terms of a lesser denomination. 



Destruction of rock implied. Thick beds of sediment mean the 

 destruction of a still larger volume of older rock, for much of the 

 more soluble part of the rock destroyed does not appear in the 

 sedimentary formations. Had the Archean lands in the vicinity of 



1 Van Hise and Leith, Mono. LIT, U. S. Geol. Surv. For earlier discussions, 

 see Irving and Van Hise, Mono. V, U. S. Geol. Surv., Chamberlin, Vol. I, Geol- 

 ogy of Wisconsin. 



