He Gee — Gulf' of Mexico as a measure of Isostasy. 191 



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altitude, though the relative lifting of the Appalachian axis 

 persisted ; then came the Columbia inundation, which was 

 coeval with the first known ice-invasion, during which the 

 Atlantic and Gulf again united and flowed inland 20 to 500 

 miles, rising 100 to 700 feet above their present 

 level ; finally the land rose so high as to permit 

 the excavation of the submarine channels of the 

 Hudson, the Delaware, the Susquehanna, the 

 Potomac and the Mississippi, but soon subsided ; 

 and this last subsidence of the southern land 

 seems to have been little affected by the later ice 

 invasions, and is perhaps, even probably, yet in 

 progress. These oscillations are represented 

 graphically in the accompanying diagram (figure 

 3) in which the ord mates represent time, the 

 abscissas continental altitude.* 



On comparing, or rather contrasting, these 

 great continental oscillations with the gentle 

 modern movement along the shores of the Gulf, 

 they are found to differ widely : The modern 

 subsidence appears to be a gentle warping in such 

 direction as to deepen the basin and gradually 

 submerge its perimeter ; the old oscillations were 

 widespread and involved both sea bottom and 

 continent. The modern movement is slight and 

 commensurate with the simple and uniform- 

 processes of degradation and sedimentation ; the 

 old movements were cataclysmic, and utterly 

 transcended the influence of rain and rivers — 

 they were indeed of greater amplitude than any 

 other continent movements of southeastern North 

 America since the close of the Cretaceous. 



Now the modern movements not only yield a 

 quantitative measure of isostacy (with a "prob- 

 able error " whose value is indeterminate) but 

 give a rude measure of the efficiency of degrada- 

 tional transfer of matter on the surface of the 

 earth's crust in producing deformation ; while the 

 movements recorded in the Columbia and Lafay- 

 ette formations were of so much greater ampli- 

 tude that they may not be referred to a similar 

 cause. In this province as in others, therefore, 

 it becomes necessary to discriminate the two 

 classes of earth movements elsewhere called, 



* They are represented physiographically in a series of tectonic maps illustrat- 

 ing a paper on die Lafayette formation now in press as a part of the 12th Ann. 

 Rep. U. S. G-eol. Survey (pis. xxxix-xli). 



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