350 PRACTICAL LESSONS IN SCIENCE. 



which make up a group of rocks known as the Upper Huronian. 

 The exposed area of these rocks is much greater than that of the 

 Lower Huronian, and they include the most of the rich iron ore 

 deposits of the Lake Superior region. 



These rocks in some places are as much as 13,000 feet in thick- 

 ness. In general these rocks are not as highly metamorphic as 

 the Lower Huronian. At the close of Upper Huronian time 

 the surface was raised above the sea and exposed to erosive 

 agencies again. After some time great floods of volcanic mate- 

 rials were poured out over this region. Later the region was cov- 

 ered by the sea and sediments were deposited which afterwards 

 became conglomerates and sandstones. These volcanic and 

 sedimentary rocks make up the Keweenawan group or series. In 

 some cases the volcanic rocks are in terst ratified with the sediment- 

 ary rocks, which seems to indicate a subaqueous flow of lava. This 

 series is very extensive, and in some places is estimated to have 

 a thickness of 50,000 feet. These rocks include most of the rich 

 copper deposits of the Lake Superior region. Rocks to the east ot 

 Lake Superior belonging to this or an earlier series contain thick 

 beds of graphite as well as iron. 



The reactions between the air, water, and land so purified the 

 air and water that low forms of aquatic life began to appear, 

 and soon became abundant, even acquiring new characters and 

 spreading out over the rocks. Great quantities of insoluble iron 

 ore were disseminated throughout the slag-like crust. Water 

 containing carbon dioxide derived from decaying organic matter 

 changed this insoluble peroxide into the soluble protoxide and 

 carried it down to the ponds, lakes and marshes, where taking 

 oxygen from the air, it again became insoluble, and settling to the 

 bottom helped to form sediments which afterward became thick 

 beds of iron ore. The immense beds of iron ore and graphite, 

 indicate that some forms of life must have been very abundant ; 

 but there is no positive evidence as to its character, as all 

 organic remains were destroyed when the sediments were changed 

 to rocks. 



Granitic, gneissoid, schistose and slate rocks make up the 



