MODIFICATION OF THE (.RICAT LAKFS l'4,} 



The stations of tlie several pairs are at diftcront distances apart, 

 the directions of the lines connecting tiiein make various angles 

 with the theoretic direction of tilting, and the time intervals 

 separating the measurements are ditVerent. To reduce the results 

 to common terms I have computed from each the rate of tilting it 

 implies in the theoretic direction, S. 27° W. In the sixth column 

 of the preceding table the rate is cxi>rest as the ciiange in relative 

 height of the ends of a line 100 miles lon^ during a centurv. 



Compared in this way, the results are remarkal)ly harmonious, 

 the computed rates of tilting ranging only from 0.87 foot to 0.4() 

 foot per 100 miles per century ; and in view of this harmony it is 

 not easy to avoid the conviction that the buildings are firm and 

 stable, that the engineers ran their level lines with accuracv, that 

 all the various possible accidents were escaped, and that we have 

 here a veritable record of the slow tilting of the broad lake-bear- 

 ing plain. 



The computed mean rate of tilting, 0.42 foot per 100 miles per 

 century, is not entitled to the same confidence as the fact of tilt- 

 ing. Its probable error, the mathematical measure of precision 

 derived from the discordance of the observational data, is rather 

 large, being one-ninth of the whole quantity measured. Perhajis 

 it would be safe to say that the general rate of tilting, which may 

 or may not be uniform for the whole region, falls between 0.30 

 and 0.55 foot. 



While the credit of formulating the working hypothesis or geo- 

 logic prediction which has thus been verified by measurement 

 belongs to Spencer, it is proi)er to note that the fundamental idea 

 of modern diflferential earth movement in the Cireat Lakes region 

 was announced much earlier by G. R. Stuntz, a Wisconsin sur- 

 veyor. In a paper communicated to the American Association 

 for the Advancement of Science in 1869, he cites observations 

 tending to show that in 1852-'53 the water of Lake Superior 

 stood abnormally high at the west end while it wius unusually 

 low at the east, and he infers that the land is not stable. 



The geographic effects of the tilting are of scientific and eco- 

 nomic importance. Evidently the height of lake water at a lake's 

 outlet is regulated by the discharge and is not affected by slow 

 changes in the attitude of the basin ; but at other points of tiie 

 shore the water advances or retreats as the l)asin is tipt. Con- 

 sider, for example, Lake Superior. On the map (Fig. 7) a line 

 has been drawn through the outlet at the head of St Marys 

 river in a direction at right angles to the direction of tilting. All 



