THE PLEISTOCENE OR GLACIAL PERIOD. 383 



estimate to be put upon the importance of the interglacial intervals, 

 the above statements are fully justified by the data now accumulated. 

 Besides the greater advances and retreats, there were numerous halts 

 or oscillations which probably affected the oncomings as well as the 

 retreats of the ice. 



The proofs of the. interglacial intervals and the evidences of their 

 duration are found in the surface changes which were wrought by 

 drainage after the deposition of one sheet of drift, and before the depo- 

 sition of the next, in the depths to which earlier sheets of drift were 

 leached and oxidized by weathering before the deposition of later 

 ones upon them, in the accumulations of peat, soil, etc., now found 

 between different sheets of drift, and in some cases in the changes 

 of topographic attitude which intervened between the deployment 

 of successive ice-sheets. 1 



The following are the American stages of the glacial period now 

 recognized in the interior of North America, numbered in the order 

 of their age: 



XIII. The Champlain sub-stage (marine). 



XII. The glacio-lacustrine sub-stage. 



XL The Later Wisconsin, the sixth advance. 



X. The fifth interval of deglaciation, as yet unnamed. 



IX. The Earlier Wisconsin, the fifth invasion. 



VIII. The Peorian, the fourth interglacial interval. 



VII. The Iowan, the fourth invasion. 



VI. The Sangamon, the third interglacial interval. 



V. The Illinoian, the third invasion. 



IV. The Yarmouth, or Buchanan, 2 the second interglacial interval. 



III. The Kansan, or second invasion now recognized. 



II. The Aftonian, the first known interglacial interval. 



I. The sub-Aftonian, or Jerseyan, the earliest known invasion. 



These stages were by no means equal, the earlier being markedly 

 longer than the later. There was something like a geometrical grada- 

 tion from the earliest and longest to the latest and shortest. 



1 Distinct glacial epochs and the criteria for their recognition, Jour, of Geol., Vol. I, 

 pp. 61-84. 



2 The Buchanan gravels lie between the Kansan and Iowan drift-sheets, in locali- 

 ties where the Illinoian is not present, and hence it is not quite certain what intei- 

 val is represented by their deposition. 



