198 GEOLOGY. 



era. It is because of the difference in the rocks, faunally considered, 

 below and above the post-Keweenawan-pre-Cambrian unconformity, 

 as much as because of the greatness of the physical changes attending 

 it, that the close of the Keweenawan is taken to mark the end of an 

 era, rather than simply the end of a period. 



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, but more abundantly 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 abun- 

 dant as to be one of the important cementing materials of the rock. 

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

 solution, and to have been concentrated in its present position by 

 ground water. The original source of the metal was probably the 

 igneous rock itself. The date of concentration is undetermined^ 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 1901, the same area yielded 76,165 tons. 

 This was 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 adequately to conceive of the 

 enormous lapse of time involved in the accumulation of such great 

 thicknesses of rock as have been referred to in the preceding pages. If 

 none of the estimates are exaggerated, there is an aggregate of more 

 than 30,000 feet of sedimentary rocks in the Proterozoic systems. 

 The accumulation of such thicknesses of sediment would in itself mean 

 a vast lapse of time, and when it is remembered that the three systems 

 are separated from one another by unconformities, each of which 

 may represent a time interval as long as that involved in the accumula- 

 tion of a system, and that at least some part of the time during which 

 the unconformities below the Huronian and above the Keweenawan 

 were developed is to be assigned to the Proterozoic, it will be readily 



1 For discussions concerning the copper, see Pumpelly, Geol. Surv. of Mich., Vol. I, 

 1869-73, pp. 19-46; Irving and Van Hise, Mono. V, U. S. Geol. Surv., and Chamberlin, 

 Vol. I, Geology of Wisconsin. 



2 Mineral Statistics for 1902, U. S. Geol. Surv. 



