THE PROTEROZOIC ERA 



325 



them were less than one hundred feet in thickness. Later in 



this period the eruptions apparently came at wider intervals, 



and meanwhile 



coarse sandstones 



were deposited hi 



the same region. 



Finally the lava 



ceased to flow out 



and so, toward the 



close of the period, 



only sedimentary 



rocks were made. 



These lavas and 



sandstones /form 



the Keweenawan 



system. Since 



they were laid 



down, they have 



been moderately 



tilted but not 



much altered. 



It appears that 

 the lava originally 

 contained minute 

 quantities of cop- 

 per. Part of this 

 copper, furnished in solution to the active underground waters, 

 was deposited in certain porous layers in the sandstones and 

 gravels, as well as in the cindery portions of the lava flows 

 themselves. From these enriched bands vast quantities of 

 pure copper have been mined during the last few decades. 



FIG. 323. Distribution of ore deposits in the 

 Proterozoic rocks of eastern United States ; iron 

 districts are shown by the black patches, and 

 copper deposits by the crosses. 



PROTEROZOIC ROCKS IN OTHER REGIONS 



Rocks of Proterozoic age are found in many parts of this 

 and other continents, but the formations cannot be matched 



