THE PLEISTOCENE OR GLACIAL PERIOD 891 



several glacial stages are found in the central basin of the Mississippi, 

 for here only are all members of the series present. The criteria 

 that have been used in estimating relative duration embrace (1) 

 the amount of erosion of the drift, (2) the depth of leaching, weather- 

 ing, and decomposition of its materials, (3) the amount of vegetable 

 growth in interglacial intervals, (4) the climatic changes indicated 

 by interglacial and glacial floras and faunas, (5) the times needful 

 for the migration of faunas and floras, particularly certain plants 

 whose means of migration are very limited, (6) the time required 

 for advances and retreats of the ice, and some others. A few of 

 these, as the first, are subject to direct measurement; but most of 

 them are matters of judgment. 



The average of the estimates of five glacial geologists who have 

 most studied the data, is shown in the following table: 



From the Late Wisconsin 1 to the present, 1 time-unit. 



From the lowan to the present, 3 to 5 



From the Illinoian to the present, 7 to 9 



From the Kansan to the present, 15 to 17 



From the sub-Aftonian to the present, x 



2. Of the efforts that have been made to measure in years the 

 post-glacial interval, those based upon the recession of Niagara and 

 St. Anthony Falls are the most significant. 2 It is important, how- 

 ever, to note precisely what is being measured. In both these in- 

 stances, the measurement attempted is the time occupied in the 

 recession of the falls from their starting point to their present posi- 

 tions. It is as important to know when they began their gorge cut- 

 ting, as to know how long they have been occupied in it. The 



1 See IX, p. 874. 



2 References on Niagara: Gilbert, Am. Jour. Sci., 3d ser., Vol. XXXII, 

 1886; Science, Vol. VIII, 1886; Proc. Am. Assoc. Adv. Sci., Vol. XXXV, 

 1887; Kept. N. Y. Com. State Res. at Niagara, 6th Kept., 1890, Chapter in 

 Physiography of the United States, and Bull. U. S. Geol. Surv. Upham, Am. 

 Jour. Sci., 3d ser., Vol. XLV; Jour. Geol., Vol. I, 1893; Am. Geol., Vol. XI, 

 1893, and XVIII, 1896, and Pop. Sci. Mo., Vol. XLIX, 1896. Spencer, Am. 

 Jour. Sci., 3d ser., Vol. XLVIII, 1894, Am. Geol., Vol. XIV, 1894, and Evo- 

 lution of the Falls of Niagara; and Taylor, Bull. Geol. Soc. Am., Vol. IX, 

 p. 84. 



St. Anthony Fatts: Winchell, N. H. Fifth Ann. Kept. Natl. Hist, and 

 Geol. Surv. of Minn., 1876; Geol. of Minn., Vol. II, 1888, 23d Ann. Kept., 

 1894; Southall, Tb? Epoch of the Mammoth, p. 373. 



