Earth-Movements of Eastern America. 569 



cial epoch \ Or is the difference in gravity principally due to 

 transference of materials within the earth's crust \ In part, 

 at least, I should be inclined to the hypothesis of an earlier as 

 well as the post-Glacial deformation. 



The anomalies of gravity open up a new field for investi- 

 gating the origin of rock basins in glaciated regions, as the 

 product of erosion during several Glacial, interglacial and 

 earlier periods ; where the manner of the supposed ice erosion 

 is unexplained. Not much assistance along these lines can be 

 derived from Europe, until the gravity stations there are 

 reduced by the Hayford method. Finally, Hayford has indi- 

 cated how a region may sink as well as rise in regions of ero- 

 sion, thus becoming areas of deposition," all in conformity 

 with physical laws. 



The Fallacy of Post- Glacial Deformation being due to the 

 Melting of Ice cajys shown in the Anomalies of Gravity. 



The excess of gravity at Potsdam, X. Y., is equal to the 

 weight of about 2,000 feet of ice. Any resilience of the 

 earth's crust clue to the melting of the glaciers should not 

 raise the mass above normal gravity, as is here found. Other 

 examples of anomalies may be given in terms of ice. 



The excess of matter at Potsdam over that at Albany, 150 

 miles distant, is equivalent to 6,000 feet of ice. Both places 

 are in the same glaciated area, which is now everywhere in 

 New York state underloaded, except in the northern part and 

 near New York City. 



St. Paul is the center of an area of excessive gravity, more 

 than 400 miles in diameter. In the vicinity of that place it is 

 now overweighted by the equivalent of over 5,400 feet of ice. 

 This region was also covered with glaciers in former times. 

 Beyond the limit of the glaciated surfaces, as between Wash- 

 ington and Virginia Beach, 150 miles apart, the difference of 

 gravity is equal to a thickness of 8,200 feet of ice. The 

 former locality is overweighted while the latter is under- 

 loaded. Again a deficiency of 4,200 feet in terms of ice is 

 seen in Alabama, far south of glacial action. 



The extraordinary example of differential gravity mentioned 

 as occurring between Olympia and Seattle, Wash., is equiva- 

 lent to no less than a load of 11,000 feet of ice. 



Lastly, reference may again be made to the enormous quan- 

 tity of sediments carried down the Mississippi River, but these 

 have not yet balanced the underloading near its mouth, while 

 in contrast the country about the mouth of the Hudson Eiver, 

 which carries no load, is overweighted. 



These illustrations show great terrestrial rigidity, maintained 



* Science, vol. xxxiii, p. 6, 1911. 



