568 J. W. Spencer — Terrestrial Gravity and observed 



Thus, I find a remarkable agreement between the results of 

 my own physiographic researches and the geophysical investi- 

 gations of Hayford and Bowie, so that no other explanation of 

 the deformation of our great drainage seems possible, than that 

 the tilting is due, either to an excess of matter, squeezed in at 

 a considerable depth (thus raising the land as appears in the 

 deformed shore-lines), or to a differential sinking of a great 

 elevated area increasing in rate, on receding from the zone of 

 more rigid rocks, which last hypothesis I have been approach- 

 ing for some time, and here propose. The surface effects 

 would be the same in both cases. This problem will require 

 another chapter. 



As no gravity determinations have been made in Canada, 

 one must depend alone on the physiographic investigations. 

 By these I have shown that the focus of maximum post-Glacial 

 elevation is located about 300 miles north of the outlet of Lake 

 Ontario (near latitude 49° N., longitude 76° W.) or adjacent 

 to the axis of the most southern lobe of the Lauren tian High- 

 lands.* It is most desirable that a line of gravity stations 

 should be extended to this point. I should expect to find an 

 excess to the mountain axis, but this overloading is hardly 

 likely to extend to Hudson Bay. 



From Lake Superior and the states to the west, it would 

 almost seem that the excess of gravity should be found through- 

 out the country all the way to the Lauren tian belt of north- 

 western Canada. 



Deficiency of Gravity and the Hudson- Champlai?i Valley. 



, The northeastern extension of the Delaware Yalley, between 

 the mountain ridges, joins that of the Hudson at Kingston, 

 and appears to have once belonged to the same drainage sys- 

 tem flowing northward. But the flattening out of the post- 

 Glacial rise (three feet, as found in the terraces by Wood- 

 worth) would not completely explain the deep trench 

 (750+ feet) in hard crystalline rocks at the aqueduct tunnel of 

 Storm King, four miles north of Westpoint. 



The anomalies of gravity may throw some light on the sub- 

 ject. At Albany the deficiency is equal to the weight of 

 1,435 feet of rock, as if the region had sunk 1,465 feet below 

 that of North Hero, 160 miles to the north. Wood worth 

 shows a deformation of 650 feetf between these points, or 

 about half of the gravity anomaly, with the depression south- 

 ward. May not there have been additional deformation of the 

 difference in amount before or during some stage of the Gla- 



* Trans. Royal Soc. Canada, 1888, cited before. Also Jour. Geol., vol. 

 xix, pp. 57-60, 1911. Bull. Geol. Soc. Am., vol. xxiv, 1913. 

 f Report N. Y. State Museum, Bull. 84, chart opp. p. 226, 1905. 



