PRE-CAMBRIAN NOMENCLATURE 6i 



The locality visited for a study of the Rainy Lake' Couchiching 

 was, however, decidedly unfortunate, since characteristic Couchiching 

 is not displayed at Shoal Lake. 



It must be remembered that Lawson's admirable work in the region 

 was devoted to mapping the rocks on a lithological basis, since the 

 time relationships were obscure. Finding a series of sediments dis- 

 tinct from the prevailingly eruptive or pyroclastic rocks which he had 

 previously studied on the Lake-of-the-Woods, he gave them the name 

 Couchiching, and mapped all the sedimentary gneisses and schists 

 under the same color. He considered the Couchiching to be lower 

 than the Keewatin, and in general he was correct in this; but the 

 significance of the conglomerates occurring in various parts of the 

 region was evidently not clear to him. The mica schists which overlie 

 the basal conglomerate at Shoal Lake should really be excluded from 

 the Couchiching, on account of their much later age, though litho- 

 logically very like the typical rocks. The greater part of the rocks 

 mapped as Couchiching lie far below the conglomerate, often at the 

 base of the series of schists, through which, as Lawson proved, the 

 granite and gneiss have pushed eruptively, though similar bands 

 occur at higher levels among the Keewatin ash rocks and volcanics. 

 The later schists above the conglomerate clearly belong to the Huron- 

 ian and not to the Keewatin. 



If the committee had examined the region about Rice Bay, a few 

 miles west of Shoal Lake, they would have found a thick series of 

 Couchiching schists and gneisses associated with banded sihca and 

 graphitic slate of the iron formation, undoubtedly more ancient than 

 the Shoal Lake conglomerate since the latter incloses many pebbles of 

 the same banded rocks. Similar associations occur at Fair's farm, 

 about two miles west of Fort Frances, where the iron formation is inti- 

 mately connected with typical Couchiching. A still better example 

 of the same relation is found at various places to the north ; for instance, 

 along the Canadian Pacific Railway near Dryden, where an iron range 

 has been traced for several miles, everywhere parallel to and inter- 

 folded with characteristic Couchiching schists. 



In the American pre- Cambrian regions south of Lake Superior 

 the amount of sedimentary material associated with the Keewatin 

 pyroclastics and eruptives is insignificant, so that, not unnaturally, 



