334 PROTEROZOIC ERA 



Other ores. 1 Silver, nickel, and cobalt occur in workable quan- 

 tities in the Huronian rocks at various points, especially in Canada. 

 Rich ores of silver and cobalt (largely Lower Huronian) are found 

 at Cobalt, Ontario, and ores of nickel at Sudbury. 



Thickness. The thickness of the Huronian systems is hard to 

 measure, because of their deformation; but if the maximum thick- 

 ness of the individual formations of different localities is taken, their 

 aggregate is several miles. 



The following section from the Marquette region may be regard- 

 ed as fairly typical for the region: 



f Michigamme slate and schist. Several thousand feet 

 .1 (maximum) in thickness. 



1 Ishpeming formation, largely quartzite. 1,500 feet (max- 

 imum) thick. 



[ Negaunee formation or series (slate, schist, jaspilite, iron 



ore, etc.). 1,500 feet (maximum) thick. 

 Middle Huronian 4 Siamo slate. 1,200 feet (maximum) thick. 



| Ajibik quartzite (in places schistose). Nearly 1,000 feet 

 [ (maximum) thick. 



r Wewe slate (including some other sorts of rock). More 

 than 1,000 feet (maximum) thick. 



Kona dolomite (some clastic beds). More than 1,300 feet 

 Lower Huronian < , . s , , . , 

 (maximum) thick. 



Mesnard quartzite. Several hundred feet (maximum) 

 thick. 



The Keweenawan System 



Constitution and thickness. In some parts of the Lake Superior 

 region a fourth system of pre-Cambrian rocks, the Keweenawan, 

 overlies the Upper Huronian. Unlike the Huronian systems, it is 

 composed more largely of lava-flows than of sedimentary strata. 



The lava beds of the Keweenawan constitute its lower and 

 larger part. The earlier flows of lava seem to have occurred on 

 land, and to have followed one another at short intervals, for the 

 surface of one flow was not eroded much before the next overspread 

 it. Later, the intervals between flows appear to have been longer, 

 and thin beds of sediment were deposited between successive sheets 

 of igneous rock. The sedimentary beds increase in importance 

 upward until, in the upper part of the system, lava beds fail alto- 

 gether. In the valley of the St. Croix River, in northwestern Wis- 

 consin and the adjacent parts of Minnesota, there are said to be 65 

 a Van Hise and Leith. Monograph LII, U. S. G. S., pp. 591-6. 



