394 THE JOURNAL OF GEOLOGY. 



Niverville beaches, lay undisturbed. The loess region of the 

 Mississippi valley, having been earliest and permanently uplifted, 

 suffered no further change during the progressive elevation of 

 the Lake Agassiz basin ; and that in its turn was at rest while 

 the great area of Hudson bay has been undergoing elevation. 



Having already shown that the entire duration of Lake Agas- 

 siz was about i,000 years, we must conclude that the uplift of its 

 area, probably to heights ranging from lOO feet to mainly about 

 500 feet, occurring first at the south and later at the north, took 

 place, when in most rapid movement upward, at rates of a half a 

 foot to one foot per year. A century, therefore, would comprise 

 an elevation of 50 to lOO feet. The movement, however, was 

 evidently more or less intermittent, with pauses of slower uplift 

 or stages of rest, when the successive beach ridges were formed. 

 Nowhere else in the records of present or past epeirogenic move- 

 ments of any region have so rapid changes of level of large 

 tracts been ascertained ; and these changes seem clearly to have 

 occurred through a gradual deformation of the earth's crust by 

 quiet flexure, not by faulting and earthquakes, which would 

 break the regularity and continuity of the ascents of the beaches 

 when traced long distances. The preglacial epeirogenic uplifts 

 of drift-bearing areas, also apparently taking place without fault- 

 ing, was probably much slower ; but their final depression 

 beneath the ice-sheet may have been even considerably more 

 rapid. Very sudden and great, yet not seismic, uplifts of exten- 

 sive areas, as supposed by Prestwich for southern England and 

 Wales, to account for the " head " or " rubble drift," ^ and by 

 Shaler for the coastal border of New England, to explain the 

 origin and preservation of thekames, ^ seem, at least in my opin- 

 ion, to be physically impossible. 



• The probable nature of epeirogenic movements, in their 

 dependence on conditions of the earth's crust and interior, 



' Quart. Jour. Geol. Soc, Vol. 48, 1892, pp. 263-343, with many sections and a 

 map. 



*U. S. Geol. Survey, Seventh An. Rep., for 1885-86, pp. 310, 320, 321 ; Bulletin 

 No. 53, 1889, "The Geology of Nantucket," pp. 44, 45. 



