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



The temperature being supposed to remain the same as at present, 

 during each year %-"-g--f- of 38-26 feet of ice, or -615 foot, would be 

 melted annually. At the same rate there would be melted in 

 12,000 years, 7,380 feet, or 14 miles. Though we do not know 

 how much ice was actually melted from the frigid zone at the close 

 of the glacial period, or within any inter-glacial era, it may be 

 mentioned that, in calculating the continental submergence supposed 

 to have been brought about by the last glacier, Dr. Groll estimates 

 that at least two miles of ice were removed from the southern 

 hemisphere. 1 The inadequacy of the inter-glacial periods to ac- 

 complish this is obvious in view of the above calculation. 



Eegarding the second proposition but little need be said, though 

 the subject is a broad one, and worthy of the study of a Hooker or 

 a Darwin. We know that crude soils, such as fresh glacial clays 

 from some yards beneath the surface, are not adapted to the support 

 of a luxuriant vegetation. Thistles, yuccas, cacti, and other hardy 

 plants may spread over barren clays, sterile sands, and tufaceous 

 wastes, and are sometimes planted with the object of reclaiming 

 such areas : but even when other germs are not lacking, it is only 

 after considerable periods that a richer flora supersedes these plant- 

 pioneers. 2 After the retreat of a glacier, however, there would be 

 a dearth of seeds and germs ; and their spread over the glaciated 

 wastes would be slow. This is substantiated by the observations 

 of Professor Alphonse de Candolle in the Alps, and of Professor 

 Blytt, of Christiana, in Scandinavia, who find that " of the valleys 

 laid bare at the glacial period, those whose glaciers retreated first 

 present a richer and more varied vegetation than those which re- 

 mained a long time covered with ice." 3 Dr. Hayden also observed 

 an analogous phenomenon during the season of 1877. The western 

 slope of the Wind Eiver Mountains exhibits abundant traces of 

 glaciation evidently quite recent, geologically speaking, though it 

 may well be doubted whether the glaciers have existed within many 

 centuries ; yet " scarcely any vegetation has sprung up on the light 

 glacial soil." 4 



While freely admitting that any conclusions, drawn from so 

 imperfect a record as that afforded by our surface deposits, and from 

 ulterior speculations, must be only provisional and of little weight, 

 the writer feels justified, in view of the foregoing considerations, in 

 rejecting the hypothesis of inter-glacial vegetation. He may be 

 permitted, however, to express the high estimation in which he is 

 inclined to hold the general principles of Dr. Croll's most ingenious 

 and plausible theory. 



1 " Climate and Time," Am. ed., pp. 377 and 388. For difference in amount of 

 heat received by the two hemispheres, see ibid., table iv. p. 320. 



2 The well-known fact that the productiveness of soils and the richness and 

 variety of the flora they support, is directly proportional (other things being equal) 

 to the amount of humus in tbe soil, is strongly emphasized by the analyses given in 

 the annual report of the Wisconsin Geological Survey for 1878, which has just been 

 received. 



3 " Eeport on the Transactions of the Geneva Society of Physics and Natural 

 History," by Dr. J. Miiller, translated for " Smithsonian Eeport," 1877, p. 221. 



i Resume for 1877 in Smith. Eep. 1877, p. 58. 



