ORIGIN OF ROCK BASINS 295 



Our largest rock basin, that of Superior, is explained as a synclinal 

 depression, probably "of very ancient date, and a number of other large 

 lakes, like Ontario, are supposed to have been formed by differential 

 elevation, the lower part of a river valley being raised so as to pond 

 back the water. Still others are explained as produced by faulting, 

 blocks having slipped down during readjustments of equilibrium, as 

 some geologists hold regarding the deep lake expansions of the Ottawa 

 chain of waters. 



None of these explanations can apply to the two small basins at the 

 Helen mine, and the theory of solution seems the only one that is sat- 

 isfactory. 



Topography 



Lakes Bo}^er and Sayers occupy most of the floor of a narrow valley, 

 at the east end of which is the Helen iron mine. The region is near 

 Michipicoten bay, on the northeast side of lake Superior, and has the 

 usual rugged character of that shore, but is easily accessible by the rail- 

 way, twelve miles long, connecting the mine with the ore clocks. This 

 and the mining operations provide good opportunities to study the rela- 

 tionships of the lakes. 



The outlet of the valley is 624 feet above lake Superior, according to 

 the railway levels, though only 7 miles in a straight line from the shore, 

 and there is a fall of 126 feet in less than 300 yards to Talbot lake, 

 which represents the general level of the neighboring valleys. 



The Helen Mine valley is a narrow trough, three-quarters of a mile 

 long and less than a quarter of a mile wide, running about east and 

 west and opening northwest toward lake Talbot. The walls of the valley 

 rise steeply to a height of 200 feet or more on the north and south, while 

 the east end is closed by Hematite mountain, much the highest point in 

 the region, rising 450 feet above the valley and 1,100 feet above Supe- 

 rior. Climbing up from lake Talbot, which is 498 feet above lake 

 Superior, the small stream draining the valley above becomes more and 

 more rapid till, after plunging over the few feet of rock which hold in 

 Sayers lake, it is a succession of short falls. This small lake or pond is 

 624 feet above Superior, 1,200 feet long, and 600 feet wide, and is fol- 

 lowed toward the east, after a dam. of rock 200 feet across, by Boyer 

 lake, 650 feet above Superior, about 1,700 feet long, and half as broad. 

 To the east of Boyer lake is the ore body, which rose about 100 feet 

 above it before mining operations began, and behind it is Hematite 

 mountain. Boyer lake has been carefully sounded, and before it was 

 partially drained to facilitate mining was 133 feet deep, with, however, 



