572 J. W. Sjyencer — Terrestrial Gravity and observed 



is in a district of overweighting as if the zone extends seaward, 

 while New England appears to be underloaded, as at certain 

 points. 



Between Lake Ontario and Lake Cham plain, a terrestrial 

 bulge occurs, rising to 660 feet or more, and forming part of* 

 the barrier to the Ontario basin (the barrier is partly dissected 

 and overlaid by drift). I have measured this amount of tilting 

 in the abandoned shore lines. 



This region is now found to be overloaded to the equivalent 

 of 700 feet of rocks, while the country to the east, south and 

 west is almost everywhere underloaded. Thus the anomalies 

 of gravity conhrm the results derived from the survey of the 

 amount of deformation. 



At Ithaca and Albany the deficiency of weight is even 

 greater than the excess in the St. Lawrence bulge. 



No pendulum measurements have been made in Canada, but 

 there is little doubt that excess would be found extending to the 

 focus of post-Glacial elevation located in the Canadian high- 

 lands. 



The St. Lawrence bulge is due to terrestrial movements, 

 which have occurred mostly in post-Glacial days, since the 

 birth of Niagara Falls, whether due to squeezing up of 

 materials or to sinking of adjacent regions, which everywhere 

 to the south are now underweighted. 



The deficiency of gravity at Ithaca and Albany suggest that 

 the pre-Glacial channels in these regions may owe part of 

 their great depth to slopes, different from the modern ones, 

 deformed in earlier Glacial and inter-Glacial epochs. 



The anomalies of gravity show that the St. Lawrence bulge 

 is now overweighted to the equivalent of 2,000 feet of ice, 

 above the normal, or to 6,000 feet above the underloaded 

 region of Albany. At St. Paul the excess of gravity equals 

 5,400 feet of ice. Both localities are inside the glaciated area. 

 Elsewhere, differential loads have been observed equivalent to 

 even 7,500 and 11,000 feet of ice. The anomalies of gravity, 

 here shown, confirm the fallacy of any hypothesis that the 

 post-Glacial warping described is due to the melting of the ice 

 sheets, which view is also contrary to the physiographic devel- 

 opment of the St. Lawrence and Great Lake regions. 



A considerable amount of the great changes of level have 

 occurred sometime since the beginning of the Glacial period, 

 which, I believe, may have been extended over a million of 

 years or perhaps much more. 



The cause of the great changes of level of land and sea are 

 believed to be due, primarily, to differential contraction of the 

 earth's interior, under the land and ocean areas; that recent 

 deformation of the region of the Great Lakes and Appalachian 



