332 PROTEROZOIC ERA 



Since none of the limestones are known to contain fossils, their 

 organic origin cannot be affirmed. Each of the three periods of 

 sedimentation was long, though their duration is unmeasured. 

 Each system contains much igneous rock, some of which was 

 extruded while sedimentation was in progress, and some intruded 

 later. Locally, igneous rock is more abundant than sedimentary. 

 The unconformable relations of the three Huronian systems, and 

 the unconformity of the third below the Keweenawan, show that 

 after the deposition of the first, second, and third systems respective- 

 ly, geographic changes occurred, resulting in erosion where sedi- 

 mentation had been in progress. 



The material for the sedimentary part of the first of these 

 systems doubtless came from the exposed part of the Archean, 

 while the sedimentary parts of the second and third systems came 

 from the exposed parts of all older formations. 



In places, the sedimentary rocks still remain in the condition 

 of conglomerate, sandstone, and shale, though more commonly the 

 sandstone has been changed to quartzite or quartz schist, and the 

 shale to slate or schist. Some of the igneous rock is massive, while 

 some of it has been changed to schist. The rocks which are least 

 altered are, as a rule, those which have been least deformed, and 

 in places they are still nearly horizontal, as when first deposited. 

 The oldest system is, on the average, most metamorphosed, and the 

 youngest least. 



Carbonaceous slates. One of the significant formations of this 

 region is black shale of slate, whose color is due to carbon. The 

 carbon is thought to imply the existence of life when the sediments 

 were deposited. Where the rocks are highly metamorphic, the 

 black shale has been changed to graphitic schist. 



Iron ore. Another important formation is iron ore. Here 

 belong the iron ores of the Mesabi (Minn.), Penokee-Gogebic (Wis. 

 and Mich.), Menominee (chiefly Mich.) and other regions (Fig. 306). 

 The ore is in the form of ferric oxide (chiefly hematite, Fe 2 3 ), but 

 in this form it represents an alteration from an iron-bearing forma- 

 tion, originally deposited as chemical sediments, composed largely of 

 iron carbonate and iron silicate, with some ferric oxides. These 

 materials are believed to have been derived, directly or indirectly, 

 from basic igneous rocks, extruded into the sea. 1 The alteration 

 to ore was brought about at a later time, by ground- water circulat- 

 ing through the rocks. 



1 Van Hise and Leith. Mono. LII, U. S. Geol. Surv. 



