6 A. P. COLEMAN 



In reality the iron formation is found in practically every Kee- 

 watin area, always near the top of the series, and sometimes with 

 a thickness of 1,000 or 1,500 feet. 



The iron formation differs so much from later sediments that 

 some geologists regard it as something peculiar and apart, belong- 

 ing perhaps to the earth's earliest times and produced only under 

 conditions very different from those of the present. It has been 

 described, for instance, as a chemical sediment deposited in a hot 

 sea where volcanic eruptions were taking place. So many specu- 

 lations have been indulged in on this fascinating subject that too 

 much space would be required to recapitulate them. 



In many places in Ontario, however, the iron formation is so 

 closely associated with commonplace sedimentary materials, slate 

 charged with carbon, arkose, and crystalline limestone, that one 

 can hardly believe it to have been formed under peculiar condi- 

 tions not repeated in later times. 



In any case the other sedimentary rocks, often covering large 

 areas and with considerable thickness, must be looked on as normal 

 products of conditions which have persisted ever since. 



In the following pages descriptions will be given of the chief 

 Keewatin sedimentary rocks, and their distribution will be out- 

 lined. As the iron formation, because of its economic importance, 

 has been most carefully studied, it will be taken up first. 



THE IRON FORMATION IN ONTARIO 



In the states near Lake Superior the Keewatin iron formation 

 consists mainly of jasper of varying colors closely interbanded with 

 hematite, less often magnetite. Iron formation of a very similar 

 kind has been found between the Vermilion range in Minnesota 

 and Fort William on Lake Superior, and in smaller areas near 

 Batchawana Bay, Lake Temagami, and in a number of other places 

 in northern Ontario. More commonly in Ontario, however, the 

 silica is in the form of chert, quartzite, or a sandstone-like aggre- 

 gation of grains, while the interbanded iron ore is mostly mag- 

 netite. Probably the differences are largely due to more exten- 

 sive metamorphism in the latter as compared with the former type. 



In most of the regions of Ontario where the iron formation has 



