SOME PRE-CAM BRIAN UTERATURIi OF NORTH AMERICA 473 



Huronian, Kcweenawan, Pleistocene, and Recent rock systems are 

 represented. The Keewatin consists of greenstone and greenstone 

 schists, rhyolite, rhyolite porphyry, and feldspar porphyry, rhyolite 

 tuff, agglomerate, and conglomerate, and iron formation. The 

 Laurentian consists of granite and granite-gneiss, bathoHths, 

 intrusives into the Keewatin. The Keweenawan and Huronian are 

 represented by intrusives. Pleistocene drift and Recent alluvial 

 and travertine deposits cover a considerable porti<m of the district. 



The Keewatin iron formation occurs in two nearly parallel, 

 synclinorial belts running nearly east and west. Folding was accom- 

 plished by fracture and flow, predominantly the latter, although 

 locally the formation is intensely broken an<I even faulted. 



The various iron formation phases consist of cherty iron carbo- 

 nate, ferruginous cherts, actinolite magnetite (|uartz rocks, inter- 

 bedded with elastics whose grain suggests that they are of pyro- 

 clastic origin. The cherty iron carbonate and some of the iron oxide 

 phases are probably original. The alterations of the iron formation 

 have been chiefly anamor])hic. Among the secondary minerals is 

 dumortierite, a unique basic aluminium silicate, never before 

 reported from iron formation. 



Moore finds difficulty in accounting for the clean-cut banding of 

 the iron formation as well as its sharj) contact with the interbedded 

 elastics on the hypothesis that the iron salts were contributed by 

 the weathering of basic igneous rocks. These characters he finds in 

 perfect accord with Leith's theory of direct igneous contribution. 

 However, direct evidence for the latter he believes is lacking, since 

 the iron formation is nowhere in direct contact with basic igneous 

 rocks excepting at faults, and is also separated from rhyolite by 

 slates, and what appear to be water-deposited })yrocIastics, contain- 

 ing no iron formation. If the elastics in associati(m with the iron 

 formation are pyroclastic in origin, considerable difficulty in the way 

 of the direct-contribution hypothesis would be removed. The iron 

 and silica could have been delivered as well by weathering as by 

 direct contribution in Moore's opinion. He cites the solubility of 

 the silica in the iron formation as shown by Van Hise and Leith, the 

 silica content of bog ores described by Hunt and himself as evidence 

 of the adequacy of weathering in causing the deposition of silica. 



