184 GEOLOGY. 



precipitated from solution also seem to have entered into the system, 

 as in the case of the iron-bearing formations. The character of the 

 beds as first laid down was much the same as that of the Huronian. 

 The fact that the succession of clastic formations is not the same in 

 different parts of the region shows that the conditions of sedimentation 

 varied from point to point about the ancient lands, even within the 

 Lake Superior region. 



Certain characteristics of some of the rocks of this system deserve 

 special mention. Within the system there are considerable thick- 

 nesses of black shale, the color of which is due to the presence of carbon, 

 often in considerable quantity. Where the rocks are highly metamor- 

 phic, the black shale has been changed to graphitic schist. The con- 

 tent of carbon is thought to imply the existence of life, though no 

 fossils have been found in the Lake Superior region. 



Another peculiar and important part of the system is the iron 

 ore. Here belong the iron ores of the Mesabi (Minn.), Penokee-Goge- 

 bic (Wis. and Mich.), and Menominee (chiefly Mich.) regions. As in 

 the Archean and Huronian, the ore is in the form of ferric oxide (chiefly 

 hematite, Fe 2 03), but, as in those systems, the ferruginous formations 

 originally consisted of iron-bearing cherty carbonates, and sediments 

 containing ferrous silicate. In so far as the iron was a carbonate, 

 both its original form and the changes through which it has passed 

 seem to imply the existence of life during the period of deposition. 



Igneous rocks. — There is reason to believe that igneous forces were 

 active during the period, and that lavas were sometimes intruded into 

 the beds, and into the Archean and Huronian beneatn. The total 

 amount of igneous rock associated with the system is great, frequently 

 predominating (as at the east end of the Penokee-Gogebic range, Mich.) 

 over the rocks of sedimentary origin. 



Thickness of sediments and duration of the period. — The period of 

 Animikean sedimentation continued until sediments many thousands 

 of feet in thickness had accumulated. In the Penokee region, that 

 portion of the system which still remains after notable reduction by 

 erosion, has an estimated thickness of about 14,000 feet. In other 

 regions the thickness of what now remains is 5000 to 6000 feet, 

 though the average, where definitely known, is considerably less. It 

 is certain, however, that existing thicknesses often fall far short of the 

 original, for the portions now remaining sometimes (Marquette region) 



