466 GEOLOGY 



deposited were brought into such an attitude as to be subject to 

 erosion, but the changes did not, as a rule, involve great folding 

 of the strata. They are generally tilted, but only locally folded 

 and faulted. In keeping with their structure, neither the sedi- 

 mentary rocks nor the igneous rocks are greatly metamorphosed. 



After the warping which followed the deposition of the Kewee- 

 nawan system, the exposed surfaces of this and older systems suf- 

 fered protracted erosion. Ultimately the land about Lake Superior 

 sank again, and when the sea came back, a new series of sedimentary 

 beds were deposited unconformably on the eroded surface of those 

 which had preceded. The waters of the returning sea teemed with 

 life, for the formation then deposited contains abundant fossils. 

 This abundantly fossiliferous formation, resting unconformably 

 on those which had preceded, 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 the pores 

 and cracks of the igneous rock, and in the interstices 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. 

 In its present form, it is believed to be a precipitate from aqueous 

 solution, and to have been concentrated by ground-water. The 

 original source of the metal was probably the igneous rock itself. 1 



In 1875 the Keweenawan formation of northern Michigan 

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

 produced in the United States. In 1905, the same area yielded 

 102,807 tons, but this was only about 26 per cent of the copper 

 produced in the country that year. 2 



General Considerations Concerning the Lake Superior Proterozoic 

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

 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 



1 For discussions of the copper, see Irving and Van Hisc, Mono. V, U. S. 

 Geol. Surv.. :ind Ch:iiiil)crliii, Vol. I, Geology of Wisconsin. 



2 Mineral Statistics for 1905, U. S. Geol. Surv. 



