Earth- Movements of Eastern America. 567 



miles southeast of Potsdam, it is about 903 feet above sea-level. 

 Consequently the post-glacial tilting is 540 feet, occurring 

 mostly about the eastern end of the lake and beyond. This de- 

 formation has raised a rocky barrier across the St. Lawrence 

 valley under a drift covering, which is deep in the buried 

 channel. The obstruction to the Ontario basin extends to 

 Montreal, where the last rapids are located. There is no fur- 

 ther impediment to the river below this point. In the vicinity 

 of the uppermost rapid the maximum warping is computed 

 to be 750 feet. If the tilting be carried back to Cleveland the 

 measured rise thence to the vicinity of Potsdam is found to 

 reach 660 feet. The pendulum measurements have been made 

 at these points. 



From the map of this new chapter in geodesy by Mr. ¥m. 

 Bowie, the excess of weight at Potsdam" represents an over- 

 loading of 700 feet of rock (corrections for elevation above the 

 sea having been made in gravity reduction). The probable error 

 of each station is about 100 feet. At Cleveland the underload- 

 ing is equal to 100 feet of rock. These total 800 feet. Is the 

 closeness of these figures a coincidence or a confirmation of 

 both methods % Perhaps both. 



East of the mouth of the Niagara Kiver (at Wilson) a defi- 

 ciency of gravity occurs equal to 335 feet of rock. This 

 increases the differences in the two measurements of the St. 

 Lawrence bulge, part of which may be in the figures, or be 

 clue to earlier deformation not recorded in the beaches, or per- 

 haps to the denudation in the Ontario valley, before or during 

 the earlier Glacial period. The underloading is also shown 

 at Ithaca, situated on the highlands, where it is equal to 765 

 feet of rocks removed. * 



The underweighting in the Finger Lake region suggests a 

 greater deformation than is found in the beaches of later date, 

 thus further explaining the great depth of their pre-glacial 

 channels, and discrediting the hypothesis of their glacial origin. 



All of the anomalies of gravity limit the overloading to the 

 Saint Lawrence bulge of northern New York, while the remain- 

 der of the state (except New York City), Pennsylvania to near 

 its southern boundary, and most of Ohio, are underloaded. On 

 the eastern side of the bulge the excess of gravity is reduced 

 to 30 feet of rock at North Hero, on the peninsula at the north- 

 ern end of Lake Champlain, and beyond, to the northern point 

 of Maine, this region south of the Saint Lawrence is supposed 

 to be underloaded. This corresponds with the absence of 

 rocky barriers across the river below Montreal. 



* The excess observed here seems to be independent of the density of the 

 rocks, for at Lake Placid, among crystalline rocks, to the south, it is only 

 + 200 feet. The deficiency of 1,600 feet at Virginia Beach is not due to the 

 lighter Cretaceous and Tertiary strata, which are probably about 2,200 feet 

 thick over crystallines, as this is their thickness at Norfolk, a few miles away. 



