MODIFICATION OF GREAT LAKES BY EARTH MOVEMENT. 359 

 Summary of distances, time intervals, and measurements of differential earth movements. 



Pairs of stations. 



Direct 

 distance. 



Distance Interval 

 indirect ^'tween 

 tionS. j ^tes ot 

 270 \y measure- 

 ments. 



Change 

 iu rela- 

 tive 

 height. 



Change 



per 100 



miles per 



century. 



Probable 

 errors of 

 quantities 

 in last 

 column. 



Sacketts Harbor anil Charlotte 



Port Colborne and Cleveland 



Port Austin and Milwaukee 



Miles. 

 86 

 158 

 259 

 192 



Miles. Tears. 



76 22 



141 , 37 



176 1 20 



186 20 



Feet. 

 0.061 

 .239 

 .137 

 .161 



Feet. 



0.37 

 .46 

 .39 

 .43 



Feet. 



0.18 

 .11 

 .09 

 .06 





Mean 0. 41 



Weighted mean : 42 i 0. 05 



The stations of the several pairs are at different distances apart, the 

 directions of the lines connecting them make various angles with the the- 

 oretic direction of tilting, and the time intervals separating the meas- 

 urements are different. To reduce the results to common terms, I have 

 computed from each the rate of tilting it implies in the theoretic direc- 

 tion, S. 27° W. In the sixth column of the preceding table the rate is 

 expressed as the change in relative height of the ends of a line 100 miles 

 long during a century. 



Compared in this way, the results are remarkably harmonious, the 

 computed rates of tilting ranging only from 0.37 foot to 0.46 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 accuracy, that all the various 

 possible accidents were escaped, and that we have here a veritable 

 record of the slow tilting of the broad lake-bearing plain. 



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

 tury, is not entitled to the same confidence as the fact of tilting. 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. Perhaps 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 geologic 

 prediction which has thus been verified by measurement belongs to 

 Spencer, it is proper to note that the fundamental idea of modern 

 differential earth movement in the Great Lakes region was announced 

 much earlier by G. E. Stuntz, a Wisconsin surveyor. In a paper com- 

 municated 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 was 

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



The geographic effects of the tilting are of scientific and economic 

 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 the shore the water 



