890 GEOLOGY 



their lake-like expansions were high enough to cover the areas 

 overspread by loess, it is not clear that there could have been 

 appropriate habitat for the land fauna of the time. 



Under the eolian hypothesis, or at least one phase of it, tl 

 river flats are supposed to have supplied the material of the lc 

 which was whipped up by the winds and re-deposited on the ad- 

 jacent uplands. The rivers are thus made essential factors in 

 distribution, though not the direct agents of deposition. ThL< 

 hypothesis seems on the whole to best fit the phenomena of tl 

 larger part of the upland loess of the Mississippi basin. The cor 

 stituents of the loess, which appear to have come from the glacial 

 drift, were derived largely from the deposits made by glacial waters, 

 or from later flood plain silts derived from the glacial formations; 

 but it is probable that some of the loess was derived from glacial 

 drift directly, before it became clothed with vegetation. 1 



The Duration of the Glacial Period 



The desire to measure the great events of geological history in 

 terms of years increases as the events approach our own time. The 

 uncertainties attending such measurements are, however, so great 

 that the results have an uncertain value, and do little more than 

 indicate the order of magnitude of the time involved. Attempts 

 to determine the date and duration of the glacial period fall mainly 

 into two categories: (1) Estimates of the relative duration of the 

 several glacial and interglacial epochs, and (2) estimates in year> 

 of the time since the close of the glacial period. 



1. The best data for estimating the relative duration of the 



1 References. Loess is described in the geological reports of the following 

 States: Iowa, Illinois, Missouri, Arkansas (Report on (Yowley's Ridge), 

 Kentucky (Report on Jackson Purchase Region), Tennessee, Loui>i.m.i 

 (1899 and 1902), Mississippi (1854 and 1860), Minnesota (Vol. I, and Report 

 for 1880), South Dakota (Bull. I), and Nebraska (Vol. I). Other references 

 are Pumpelly, Am. Jour. Sci., Vol. XVII, 1879; McGee, Eleventh Ann. Kept.. 

 U. S. Geol. Surv.; Chamberlin and Salisbury, Sixth Ann. Rept., V. S. (l.-ol. 

 Surv.; Todd, Bull. Phil. Soc. of Wash., Vol. IV, Bull. Geol. Soc. of Am., Vol. 

 V and Science, N. S., Vol. V; Shimek, Am. Geol., Vols. XXVI 1 1 :md X \ \ 

 Bull. la. Lab. Nat. Hist., Vols. I, II, and V, Proc. la. Acad. Sci., Vols. I 

 V, VI, and VII; Leverett, Am. Geol., Vol. XXXIII, and M<m<>-. XXXVII 

 Calvin, Bull. Geol. Soc. Am., Vol. X, p. 119; Chamberlin, Jour. Gen I.. Vol. 

 1897; Davis, Explorations in Turkestan, 1905; and Willis, Itcsearches in Chin* 

 Vol. I, Carnegie Institution. 



