BIBLIOGRAPHY. 49 



Wright, G. F. Man and the Glacial period. New York, 1892, 364 pages. 



Preservation of the glacial grooves on Kelly's Island: Science, Vol. XVII, 



1891, pp. 358-369. 



Additional evidence bearing on the glacial history of the Ohio Valley: Am. 



Geologist, Vol. XI, 1893, pp. 195-199. 



Unity of the Glacial epoch: Am. Jour. Sci., 3d series. Vol. XLIV, 1893, 



pp. 351-373. 



Continuitj^ of the Gla,cial period: Am. Jour. Sci., 3d series, Vol. XLVII 



1894, pp. 161-187. 



Cincinnati ice dam: Pop. Sci. Monthly, Vol. XLV, 1894, pp. 188-198. 



The age of the second terrace on the Ohio, at Brilliant, near Steubenville: 



Jour. Geol., Vol. IV, 1896, pp. 218-219. 



The age of Niagai-a Falls, as indicated by erosion at the mouth of the gorge: 



Science, new series. Vol. VIII, 1898, p. 502; also Proc. Am. Assoc. Adv. Sci., 

 Vol. XLVII, 1898, pp. 299-300. 



New method of estimating the age of Niagara Falls: Pop. Sci. Monthly, Vol. 



LV, 1899, pp. 145-154. 



OUTLiIKE OV TIMK RELATIONS, OR GLACIAL SUCCESSION. 



This outline is essentially tlie same as the writer's outline presented 

 in Monograph XXXVIII, the only important modification being in the 

 subdivisions of the late Wisconsin glacial stage. These are more complete 

 in the Ohio district than in the Illiiiois, and require a corresponding elabo- 

 ration of the outline. 



In the progress of the studj^ of the glacial deposits the complexity of 

 the glacial history has been gradually unfolded. After the abandonment 

 of the iceberg hypothesis, the early students approached the study writh the 

 hypothesis of a single and practically continuous period of drift deposition, 

 in which the ice sheet at one time covered the entire glaciated area. This 

 period was supposed to have terminated with a single higli stage of water, 

 attending the melting of the ice, which was termed the Champlain epoch. 

 But it soon became apparent tliat this simple hypothesis could not be made 

 to cover the complex glacial history. Evidences of a complicated succes- 

 sion of recessions and advances of the ice sheet were recognized, and a 

 sharp controversy arose concerning the importance of these oscillations. 

 It was held by some students that they are of minor importance, and mark 

 short or partial retreats and advances in a single epoch of glaciation; while 

 others contended for the necessity of recognizing two or more ice invasions, 

 between which there were extensive and prolonged deglaciation intervals. 



