8 A. P. COLEMAN 



parallel with it runs a band of crystalline limestone 30 feet thick 

 and more than a mile long. 



In the cases just mentioned sediments such as clay, limestone, 

 and carbon were deposited with silica and iron oxide or sulphide. 

 The carbon makes it altogether probable that sea weeds lived on 

 the muddy bottom, so that the waters must have been cool enough 

 for life and free from poisonous substances. 



In a number of places near Lake Nipigon the iron ranges include 

 large amounts of arkose as well as the slaty rocks mentioned above. 

 Thin sections present the usual angular or subangular fragments of 

 quartz and feldspar imbedded in a finer grained matrix. The forma- 

 tion of these greenish gray arkoses suggests a land surface of granite 

 or gneissoid rocks exposed to weathering in a cool and moist 

 climate, as shown by Professor Barrell, in his excellent study of 

 Climates and Terrestrial Deposits. These rocks cover in all many 

 square miles and must have a thickness of a thousand feet or more, 

 unless greatly reduplicated by folding. Near Poplar Lodge they 

 have a width of a quarter of a mile with dips of from 6o° to 8o°, 

 though banded jasper and hematite and also a little green schist 

 are interleaved with the arkose, making up perhaps one-tenth of 

 the whole. 



THE COUCHICHING PHASE OF THE KEEWATIN 



Associated with the iron formation at a number of points on 

 Rainy Lake, Rainy River, near Dry den, etc., one finds gray fine- 

 grained schists and gneisses having the character of the Couchich- 

 ing as described by Lawson; but these rocks occur in larger areas 

 apart from known iron ranges. They are composed of quartz, 

 biotite, sometimes muscovite, and often some orthoclase or pla- 

 gioclase; and they frequently contain sillimanite, garnet, and 

 staurolite, or pseudomorphs after staurolite. They are evidently 

 sandy or clayey sediments recrystallized, and may be compared 

 with the sedimentary gneisses and quartzites of the Grenville series 

 of eastern Canada so well described by Adams. 



The materials of which they were formed must have been 

 derived from granite or gneiss and not from the basic eruptives 

 with which they are associated. In the decay of the original rocks 

 much of the feldspar must have been decomposed, the alkalies 



