562 J. W. Spencer — Terrestrial Gravity and observed 



evolution of our physiography whether geographical or geo- 

 logical. 



With additional data obtained at intermediate points, the 

 contours of anomalies of gravity will be somewhat modified. 

 The points for new determinations which promise the greatest 

 results would seem to be along the lines of greatest tilting 

 measured in the raised beaches, and others crossing the 

 mountains and extending to the coast ; also along other lines 

 prolonged northward to beyond the Canadian highlands. 



Submarine Valleys and Canons Indenting the Border of the 



Continent. 



Having made extensive studies of these features,* and also 

 published a recent summary of them in Hull's u Suboceanic 

 Physiography of the North Atlantic,*' which is a monograph in 

 atlas forin,t only a general reference need here be made to 

 them. The submarine canon of the Hudson is the one most 

 completely surveyed. It has a length of fifty miles and has 

 been found to reach a depth of over 7,000 feet below sea level. 

 The winding trench itself has a depth in one locality of 4,000 

 feet. It is also characterized by a succession of steps. 



The St. Lawrence River has a depth of over 1000 feet where 

 the estuary widens out into the Gulf, through which extends a 

 broad valley (to a depth of 2000 feet, with high banks) 

 reaching to the border of the Continental Shelf, which is 

 here incised by a canon to an undetermined depth, similar to 

 that of the Hudson. Like features occur off the Gulf of 

 Maine, beyond Chesapeake Bay, between Florida and the 

 Bahamas, and in extensions of the Mississippi and probably of 

 all other rivers entering both the Atlantic and the Gulf of 

 Mexico. 



The submarine continental slope is also characterized by 

 platforms or terraces, notably between 2400 and 3000 feet. 



All of these valleys are pure topographic forms of erosion 

 in plateau regions. They are not of exceptional occurrence, 

 but have already been noticed in scores if not hundreds of 

 instances. There is no other geological or geographical explana- 

 tion of their origin, supported by any evidence, than that they 

 were formed by stream erosion at a time when the land was at 

 a higher level, or the sea lower than now. 



Such changes of level in modern geological times seem stag- 

 gering to those who ignore these great features, or, as I have 

 heard it said, that such an explanation is discredited by the 

 studies of Prof. John F. Hay ford, relating to isostasy, in show- 

 ing that the continent is now nearly balanced in its relation to 

 ocean areas. But Hayford says : " There is abundant geologi- 



* These have appeared in the Bull. Geol. Soc. Am., this Journal, etc. 



f Published by Edward Stanford, No. 11 Long Acre, London, W. C, 1912. 



