MODE OF DEPOSITION OF THE LOESS. 177 



been influential. The division of opinion, therefore, is concerned with the 

 relative importance of wind and water in the distribution of the loess. The 

 question of the influence of the atmosphere as an agent of erosion, trans- 

 portation, and sedimentation has been very carefully examined by Udden, 

 with the result of showing that a large part of the loess may have been 

 deposited through this agency. 1 In a recent paper 2 Chamberlin has dis- 

 cussed the peculiarities of distribution and considered the difficulties attending 

 the application of either hypothesis to the entire deposit. The distribution 

 of the thickest and coarsest loess along the main valleys, with its o T eat 

 extent down the Mississippi, creates a strong conviction "that the deposition 

 of the loess was in some vital way connected with the great streams of the 

 region." The abrupt border of the loess at the edge of the Iowan drift sheet 

 both in Illinois and Iowa gives it a "more or less direct genetic relationship 

 with the ice." The graduation of loess into glacial clays "further tends to 

 confirm the association of the loess with glacial action." The influence of 

 glacial action is also shown in the presence of silicates which are decom- 

 posable under prolonged weathering and of calcium and magnesian car- 

 bonates, none of which can be supposed to come from the residuary clays. 

 An illustration from the Lower Mississippi Valley is given which strengthens 

 this inference: 



Above the Lafayette gravels and below the loess there is a stratum of silt which 

 does not habitually contain the characteristic silicate particles of the loess. This 

 stratum has been by most observers associated with the loess, but it is separated 

 from it by a soil horizon, as abundantly affirmed by the observations of Salisbury and 

 the writer. On the other hand, it graduates more or less freely into the Lafayette 

 sands and gravels. The stratum is, as we interpret it, the last deposit of the Lafayette 

 stage. It is a typical finishing deposit succeeding a fluvial sand and gravel. Now 

 this has special significance in this relationship in that it shows that in the stage 

 closely preceding the loess deposition the Mississippi did not lay down silts of the 

 same constitution as the loess. The inference, therefore, is that the loess is not simply 

 a fluvial silt brought down from the surface of the river basin, nor common wind drift 

 borne into it, but that it had a special origin connected with the glacial action which 

 was competent to supply precisely the kind of silt of which the loess was made. 



1 The main results of Udden's studies are presented in the following papers: "Erosion, trans- 

 portation, and sedimentation performed by the atmosphere": Jour. Geol., Vol II 1894 pp 318-331 

 '■ Loess as a laud deposit" : Bull. Geol. Soc. Anier., Vol. IX, 1897, pp. 6-9. » The mechanical composition 

 of wind deposits," Augustana Library Publications, No. 1, 1898. Lutheran Augustana Book Concern 

 Rock Island, Illinois. 



-Supplementary hypothesis respecting the origin of the loess of the Mississippi Valley by T C 

 Chamberlin : Jotirn. Geol., Vol. V, 1897, pp. 795-802. 



MON XXXVIII 12 



