THE PROTEROZOIC ERA 



323 



FIG. 321. Stereogram showing down-folded rem- 

 nants of Proterozoic rocks surrounded by the 

 Archeozoic complex. 



proof could be desired that the Archaean rocks had been folded, 

 metamorphosed, laid bare as land, and profoundly eroded 

 before the Proterozoic rocks were deposited upon them. 



Huronian system. The Huronian rocks are quartzite, 

 limestone, and slate, with the addition of beds of iron ore and 

 jasper. Where metamorphosed the predominating rocks are 

 schists. The beds 

 are usually much 

 folded and they 

 are exposed at the 

 surface as long 

 down-folded bands 

 within the out- 

 crops of the Ar- 

 chaean (Fig. 321). 

 In at least one dis- 

 trict a well-marked 

 unconformity di- 

 vides the Huronian strata into two systems, the lower of which 

 is evidently much older than the upper. 



Igneous intrusions of different ages cut through them here 

 and there, and lava flows are sometimes found interbedded 

 with the sediments themselves. Around the batholiths of 

 granite and the other large intrusions, the rocks may be altered 

 to schists ; and it then becomes difficult to discriminate them 

 from those other schists which belong to the Archaeozoic 

 group. 



Animikean system. Unconformably above the Huronian 

 rests the Animikean, 1 another system of sedimentary rocks 

 and lava flows, which is in general much like the Huronian 

 (Fig. 322). On the average, however, the rocks are less 

 folded and less metamorphosed, in some places not at 

 all. The quartzites and slates are traversed by a few dikes 

 and larger intrusions of later age. Iron ore has been men- 



1 The geologists of the U.S. Geological Survey class the Animikean as 

 Upper Huronian. 



