MECHANICAL CONSTITUTION OP THE LOESS. 471 



east. This stratum is in places differentiated into two parts, separated by a 

 soil-like horizon. This differentiation is not common to the entire valley. 

 The silt mantle may be traced almost in unbroken continuity northward to 

 the border of the glacial drift, whence it spreads itself over the drift, reach- 

 ing over the drift surface some hundreds of miles to the northward. In this 

 northern stretch the silt mantle is correlated with a second episode of the 

 earlier glacial epoch. It graduates down into a stratum of bowlder clay 

 that overlies a bed of vegetable material, which in turn overlies another till. 

 Both of these tills I have been accustomed to correlate with the earlier glacial 

 epoch. I do not wish, however, to raise differences of opinion on that point 

 here. It is unimportant to the main conclusions which we desire to reach. 



Besides this continuity, there is a further reason for regarding these silt 

 deposits as contemporaneous with the ice invasions. 



They are made up in part of glacial particles — that is, particles derived 

 from the mechanical abrasion of the glacier. These particles consist of de- 

 composable silicates, dolomites, and limestoues and were rasped from rocks 

 of these varities lying further north. Such decomposable particles do not 

 abound in residuary clays but are abundant constituents of glacial clays. 



It seems necessary to suppose that this mantle of loess and loess-like silt 

 was originally deposited as a horizontal stratum across the entire Missis- 

 sippi bottom and border land. At the present time it undulates over the 

 hills. At first thought it would seem that the depositing waters might have 

 been deep and the silt laid down as an undulatory mantle, but it would 

 seem necessary to extend the same hypothesis to the deposition of the gravels, 

 where its application is manifestly excluded by the nature of the deposit. 

 I feel sure from observation in certain cases that full investigation will show 

 this seeming mantling to be the result of the gradual degradation of the hills 

 accompanied by creep of the pliant and plastic material. This phenomenon 

 of creep has a wide expression, entirely independent of the area under con- 

 sideration ; but upon that I cannot dwell. 



During the first glacial episode, the altitude and slope of the lower Mis- 

 sissippi basin were so low as to permit the deposit of this silt on bluffs which 

 are now 200 feet, more or less, above the present Mississippi bottom. Before 

 the second glacial epoch, according to the division I make, there was an 

 elevation sufficient to permit the erosion of the great trench of the lower 

 Mississippi by the predecessor of the present river. This erosion amounts 

 in round numbers to a trench about three hundred feet in depth and about 

 sixty miles in width. Some of the bluffs that are crowned by these silts are 

 200 to 250 feet in height ; and Professor Call's recent investigations show 

 80 to 100 feet of silt in the bottom. It is, therefore, I think, safe to say that 

 in round numbers there was an erosion of the magnitude named reaching 

 from Cairo south to the Gulf, with corresponding erosion trenches along the 



