CLIMATE AND PHYSICAL CONDITIONS OF KEEWATIN 7 



been carefully mapped and studied it includes also more or less 

 siderite, or pyrite, or pyrrhotite, so that not the whole of the iron 

 is contained in the oxides. 



There is, however, another variety of the formation which has 

 received less attention, consisting of granular silica with little or 

 no iron, but sometimes interbanded with gray or green schistose 

 materials. This appears to be the common form in the far west, 

 near Fort Frances on Rainy River, and near Kenora on the Lake- 

 of- the- Woods. In these localities sandstone-like rocks are found 

 quite extensively with the gray schists described by Lawson as 

 Couchiching. It may be that the sources of iron ran out toward 

 the west, leaving only the silica. 



The sandstone-like variety of iron formation, when first found 

 by the present writer, was thought to be an ordinary sediment. 

 It resembles a white or gray or brownish sandstone of even grain, 

 and is often so loosely cemented that the rock may be crumbled 

 in the fingers. Thin sections, however, show little or no clastic 

 structure. The quartz grains are polyhedral individuals which 

 have grown from centers until they met. Every transition may 

 be found between these relatively coarse-textured varieties and 

 the very fine-grained silica, often chalcedonic, of the jaspers. The 

 quartzitic variety occurs in or near the eruptive granite of the 

 Lauren tian. In it the anhedra of quartz are firmly cemented 

 together. 



As mentioned before, in most places in Ontario the silica and 

 iron ore are accompanied by ordinary sedimentary material. In 

 a number of thin sections sillimanite occurs, a silicate of alumina 

 that must have been recrystallized from clay. On the east shore 

 of Lake Nipigon, and in other places, the banded silica and mag- 

 netite are interbedded with gray slate or phyllite and often pass 

 gradually into this rock, which is, of course, a metamorphosed 

 clay. Frequently also a few feet of black carbonaceous slate 

 underlie the iron formation, as at the Helen mine, Michipicoten, 

 and at Grassy Portage on Rainy Lake. 



At Goudreau Lake southwest of Missanabie, the iron formation 

 contains a small amount of granular silica with magnetite, and a 

 large amount of pyrite, the sulphide replacing the oxide; and 



