418 W. J. McGee — Geology of the Mississippi Valley. 



place before the final retreat of the ice. It may be mentioned in 

 passing that some careful observations recently made in a neigh- 

 bouring State point to a yet more sweeping conclusion. During 

 last season, Prof. IS. H. Winchell, the State Geologist of Minnesota, 

 found that the unmodified glacial drift of that State graduates 

 bodily into the loss of the Missouri Valley. 1 



As intimated at the outset, the general conclusion reached hj the 

 writer is that three different glacial epochs have succeeded each 

 other at long intervals ; and from analogy with the present, as well 

 as from direct evidence, that a characteristic fauna and flora spread 

 over the land during each of these intervals, whose counterparts are 

 to be sought for in the more recent marine formations. Though 

 purely hypothetical, and warranted only by the known uniformity 

 of the natural processes, and the apparent inadequacy of catastrophic 

 agencies, 2 it may also be suggested that earlier glacial eras have 

 probably occurred at long intervals, as argued by Dr. Croll, and 

 that these cataclysms may have inaugurated the disturbances 

 separating the successive geological eons. 



It would seem, however, to be palpably unjust to the talented 

 author of "Climate and Time," and to the able geologists who have 

 fallen in with his conclusions, to reject the hypothesis of inter-glacial 

 periods, in the sense in which the term is used by the Scottish 

 geologists, without offering any reason therefor. Hence, if the 

 introduction of arguments not strictly geological may be permitted, 

 the chief reasons for not adopting that view may be briefly outlined. 



Premising that it may be laid down as an axiom that inter-glacial 

 eras of mild climate could only exist when the whole of the ice of 

 the preceding epoch had been removed from the Polar regions, the 

 leading objections to the assumption of such eras may be summed 

 up in two propositions. First, it would probably require all, or 

 nearly all, of such an era to melt the ice accumulated around the 

 pole ; and second, it would probably require a much longer period 

 to effect the fertilization of the morainic debris, and the introduction 

 and acclimatization of a luxuriant flora, such as that represented in 

 the North American forest bed. 



In discussing the first proposition, the leading principles of the 

 ingenious theory explaining secular variations in climate, which was 

 first developed and elaborated by Dr. Croll, will be adopted. 



For our present purpose it may be assumed that the melting of 

 ice and the elevation of the earth's temperature is due to solar heat 

 alone ; and both stellar and proper terrestrial heat may be disre- 

 garded. The amount of solar heat reaching the earth's surface is 

 known ; and the proportion cut off by the earth's atmosphere has 

 been determined by Pouillet and others. The amount reaching the 

 outer surface of the terrestrial atmosphere is thus known also. It is 

 sufficient, as we are told by Dr. Tyndall, 3 to melt a layer of ice 100 



1 " Ann. Rep. Nat. Hist. Surv. of Minn.," 1878. See also Ann. Journ. Sci., Feb. 

 1879, p. 168. 



- Vide " Cataclysmic Theories of Geological Climate," Croll — in Geol. Mag. 

 for Sept. 1878. 



3 "Heat as a Mode of Motion," Amer. ed., par. 684. 



