AGE OF THE BASINS 303 



terial is now being dissolved from the basins than is deposited in them , 

 there is no means of deciding. 



Age of the Rock Basins 



The age of these two remarkable rock basins can not be very precisely 

 determined, all that is certain being that they are post-Huronian and 

 pre-Glacial. No rocks later than the Keweenawan eruptives and con- 

 glomerates and the immediately overlying Lake Superior sandstones are 

 known in the region. If the latter once covered the Helen mine, the age 

 of the basins must be post-Cambrian ; but the horizontal sandstones 

 often found around the shores of Superior have never been reported at 

 a level so high as 650 feet above the lake, at least on the north shore, so 

 that the erosion of the region probably began before the Cambrian. It 

 is probable that the Archean mountains, whose stumps are no doubt 

 represented bty the Laurentian granites and gneisses, with Huronian 

 synclines sharply nipped in between, have never been buried under later 

 rocks, in which case the shaping of the topography may have begun 

 with the Paleozoic. 



The formation of the million tons of iron ore at the mine toward the 

 eastern end of Boyer lake must have been a very slow process, and if 

 the basin was complete before its deposit was commenced it may even 

 be Paleozoic in age ; but if the ore was there before the basin, as seems 

 more probable, and was partly dissolved and removed in its formation, 

 the date assigned will be very much later. When the mine is worked 

 at lower depths than now we shall probably learn with more certainty 

 the relationships of the ore body to the lake basin and be able to solve 

 some problems now left open. 



One naturally asks if the rest of the basin of Boyer lake was once 

 filled with ore, since dissolved away, and if the basin of Sayers lake also 

 once contained ore ; but this question can hardly be answered at pres- 

 ent. No doubt much iron ore has gone down the valley in solution, for 

 the beds of sand forming lake terraces near Michipicoten harbor are 

 often cemented with limonite over wide areas ; but this is of course a 

 comparatively modern deposit, certainly post-Glacial, and has little to 

 do with the early history of the rock basins. 



We may safely say that the two deep ponds at the Helen mine occupy 

 basins very much more ancient than those of most other lakes in Canada, 

 to be compared in age with such profound cavities as the one occupied 

 by lake Superior rather than with the tens of thousands of large and 

 small basins produced by the Glacial period. 



