460 GEOLOGY 



Thickness of rocks. The thickness of the Huronian systems is 

 difficult of measurement, because of their wide-spread deformation, 

 but if the maximum thickness of the individual formations of 

 different localities is taken, the aggregate thickness is not less than 

 three miles. Such thicknesses, however, are rarely attained in any 

 one locality. 



The following section from the Marquette region gives an idea 

 of the sequence of formations in one place, and may be regarded 

 as fairly typical for the region: 



Upper Huronian 



Middle Huronian 



Lower Huronian 



Michigamme slate and schist. Several thousand feet 



(maximum) in thickness. 



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. 

 Siamo slate. 1,200 feet (maximum) thick. 

 Ajibik quartzite (sometimes schistose). Nearly 1,000 



feet (maximum) thick. 

 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 



(maximum) thick. 

 Mesnard quartzite. Several hundred feet (maximum) 



thick. 



The Keweenawan System 



Constitution, thickness, and relations. In some parts of the 

 Lake Superior region, a fourth system of pre-Cambrian rocks, the 

 Keweenawan, overlies the Upper Huronian. This system is 

 unlike the Huronian systems in being 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 seem to have occurred on the land, 

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

 of one flow was not sensibly eroded before the next overspread it. 

 Later in the period, the intervals between the flows of lava appear 

 to have become longer, and thin beds of sediment were deposited 

 between successive sheets of igneous rock. Still higher in the sys- 

 tem, the sedimentary, beds increase in importance until, in the upper 

 part of the system, the lava beds fail altogether, and there follows 

 a succession of sandstones and conglomerates of great thickness. 



