BULLETIN OE THE GEOLOGICAL SOCIETY OF AMERICA 

 VOL. 43, PP. 293-304, PL. 45 OCTOBER 14, 1902 



ROCK BASINS OF HELEN MINE, MICHIPICOTEN, CANADA 



BY ARTHUR P. COLEMAN 



(Read before the Society January 1, 1902) 



CONTENTS 



Page 



Introduction 293 



Topography 295 



Geological conditions 297 



Probable causes of the rock basins 298 



The action of solvents 299 



Eelation of the iron ores to the basins 300 



Age of the rock basins . 303 



Introduction 



Among the thousands of lakes in Canada there are hundreds that are 

 apparently rock-rimmed, having their outlet over sills of solid rock and 

 having shores of rock in most places ; but generally there is some drift- 

 covered point in the circumference of the basin where an old channel 

 may be concealed. It is not often that one can actually trace the solid 

 rock entirety round the shore with no point hidden. This is the case, 

 however, with the two small lakes near the Helen mine which are to be 

 described in this paper. 



The causes of rock inclosed basins are various, most of them in 

 Canada being due to glacial action, the rock surface having been exca- 

 vated by the ice unequally, probably because of preglacial decay having 

 extended to unequal depths, though it was commonly believed in earlier 

 times that undecayed rocks could be deeply eroded by ice, and that 

 even basins like that of Ontario could be excavated by it. 



It is now well known that glacial ice may pass over unconsolidated 

 drift material with little or no effect in the way of excavation, especially 

 near the margin of an ice-sheet where it has grown thin, and that exca- 

 vation into solid rock is to be expected only under special conditions — 

 for instance, where the ice is thick and its action directed toward certain 

 points by the shape of its bed. 



XLIV— Bum,. Geot,. Soc. Am., Vol. 13, 1901 (293) 



