262 Dr. J. W. Spencer — Subsidence versus Qlacial Dams. 



sea-water of which we have knowledge, must have contained 

 free oxygen, carbonic acid, and other potent chemical reagents. 

 Are we to believe that these active reagents sat still and did nothing 

 for thousands upon thousands of years until at the end of ^ons 

 dynamic metamorphism, like a muscular pedagogue with a long 

 birch, came to warm them into action ? 



VI. — Post-Pliocenje Continental Subsidence (in America) versus 

 Glacial Dams. 



By J. W. Spencer, M.A., Ph.D., F.G.S. L. and A. 



THE growing interest in the evolution of the Continent now calls 

 for more accurate information than formerly regarding the 

 changes of level of land and sea in recent geological times. The 

 amount of these oscillations was one of the most important factors 

 in the investigation of the " Building of the Great Lakes." Hence 

 the study of the history of the lakes has contributed to our know- 

 ledge of the changing relations of the continent and the sea. 



From a study of the submerged channels along the American 

 coast, it has been shown that the continent was greatly elevated 

 during some epoch or epochs intervening between the middle Mio- 

 cene and the early Pleistocene periods.^ The elevation of the land 

 was over three thousand feet higher than now, and probably reached 

 for a short time to over five thousand feet. 



The elevated condition of the continent was followed by a depres- 

 sion of the land to far below the present altitude, before the upward 

 movement restored the now existing conditions. There may have 

 been more than one episode of elevation and depression ; but the 

 problem that we seek to answer is ; "What was the maximum de- 

 pression of the later Pleistocene times, after the great beds of 

 boulder clay were formed? for the great elevation was shortly 

 before that epoch. 



' Most geologists are ready to accept the high continental elevation, 

 but there are differences of opinion respecting the amount of the 

 subsidence. Although many have their own views upon this 

 subject, few serious attempts have been made to solve the problem 

 unbiassed by theory. 



We must seek for the evidence of the late regional elevation in 

 the remains of old shore-lines, such as beaches, terraces and sea- 

 clifi"s, which are more or less disturbed and obliterated. Isolated 

 remnants of beaches are not accepted by all as proof of a recent 

 elevation, although found at high altitudes, but the beaches often 

 contain the direct proof of their own elevation. 



No better example is found than the Iroquois Beach of the 

 Ontario Basin. This elevated shore-line is one of the youngest and 

 best preserved in the Great Lake region. It rests upon the youngest 

 " till " deposits. Since its formation it has been warped towards the 

 north-east, and thus at Fine, north of the Adirondack Mountains, it 



^ "High Continental Elevation preceding the Pleistocene Period (in America)," 

 by J. W. fcjpencer, Geol. Mag. Decade III. Vol. YII. 1890, p. 208. 



