Earth-Movements of Eastern America. 571 



continents should be raised (perhaps for relatively short 

 periods) with the formation of canons along their borders, 

 such as actually occur, and subsequently subside to renewed 

 isostatic conditions. Mountain-making movements, block and 

 *other faulting, subsidence of deep basins, transportation of 

 eroded materials (with subsequent expansion) and deposition 

 are secondary features. This means also changes of ocean 

 level, in relationship to the continental border, as well as 

 deformation on the land. 



I am not aware of the announcement of any explanation of 

 the cause of the deformation of the lake regions other than a 

 hypothetical readjustment after the removal of the ice, which 

 I have here shown to be untenable from the evidence found 

 in the variations of terrestrial gravity, as well as from the 

 physiographic evolution of the lake regions and from the 

 occurrence of similar tilting in the Appalachian belt, south of 

 the glaciated region. My hypothesis is now that this unequal 

 warping of the surface, without thrusts, results from differen- 

 tial sinking of areas, resting on foundations of unequal rigidity, 

 during epochs of subsidence of greater regional, or perhaps 

 continental extent, to which the deformed areas are subordi- 

 nate, with superficial effects. The explanation of the features 

 of deformation is inseparable from the investigations of the 

 great changes of level found in the submarine valleys. 



Summary of Discussio?i. 



The submarine valleys or canons, dissecting the platforms 

 or terraces on the edge of the continental mass, demonstrate 

 great subsidence of the land, or rise of the ocean level, in very 

 late geological times. Correspondingly, almost everywhere a 

 deficiency of gravity characterizes the continental border, and 

 often extends far inland among the mountains themselves. 

 South of ISTew York, the mountain region is generally over- 

 weighted, but bordered on both sides with underweighted 

 zones. Even Florida shows excess of matter, as in the moun- 

 tain zone. This may be taken as additional evidence of a late 

 union of the West Indies with the continent. 



The differential gravity between the Appalachian zone (at 

 Washington) and the coast is equivalent to 2,855 feet of rock 

 strata. Thus the proof of recent changes of level, as observed 

 in the submarine canons, is supported by investigations of ter- 

 restrial gravity, to at least the extent mentioned. 



The deficiency of gravity in the lower Mississippi region 

 seems very remarkable, when the river has been carrying down 

 its enormous load for so long a time. In contrast, the mouth 

 of the Hudson River, which transports scarcely any materials, 



