290 E. A. Smith — Post-Eocene Formations of Alabama. 



stage of this emergence and a pause in it being recorded in 

 the yellow surface loam of the Second Bottom of the larger 

 streams. 



If we compare these Pleistocene deposits of Alabama with 

 those of the Mississippi River, we shall mark the following : 



1st. The Port Hudson clay of the Mississippi finds its paral- 

 lel in our blue clay stratum with its plant remains. The Port 

 Hudson clay, however, is far richer in lime, effervesces with 

 acids, and contains concretions of lime carbonate, something 

 not yet observed in Alabama. 2d. The sands present no mate- 

 rial points of difference in the two states : in both occur masses 

 of pebbles, and both are found as second terraces. 3d. The 

 loess is not found in Alabama. 4th. The yellow loam of 

 Mississippi finds its counterpart in the uppermost layer of our 

 second bottom ; the published analyses of these loams, given 

 in volumes Y and YI of the Tenth Census Reports, show very 

 closely similar chemical composition. The main difference is 

 in its distribution in the two States, the yellow loam of Missis- 

 sippi being spread over an area of thirty miles or more from 

 the river bluff, while with us the area rarely exceeds a mile. 

 The explanation of the variations in the character of the mate- 

 rials and the distribution of the Pleistocene of Mississippi and 

 Alabama, is to be found, I think, in the following considera- 

 tions : (a) Dr. Hilgard has shown that the axis of the Missis- 

 sippi River has been the region of greatest oscillation in this 

 part of the continent during Post-Eocene time, the aggregate 

 of depression and elevation being 1200 feet.* A depression 

 of 450 to 500 feet would allow the waters of the great river to 

 spread beyond its banks as far as we now find the yellow loam. 

 Moreover, the valley of the Mississippi beyond its alluvial 

 plain, is much wider than the corresponding valleys of the 

 Alabama rivers, and the same amount of depression would on 

 this account cause the flooding of very unequal areas in the 

 two cases, (b) The Mississippi drains a region which was once 

 covered by glaciers, and its sediments naturally contain much 

 material of Glacial origin. The loess is generally, in this 

 country at least, admitted to be of this character, and the high 

 percentage of carbonate of lime in the clay and its lime con- 

 cretions, are probably due to the same cause. Our Alabama 

 rivers had no glaciated areas to draw upon for their materials, 

 and they have deposited no loess, no highly calcareous clays. 

 (c) In both States, the second bottoms of the minor streams are, 

 as a rule, sandy throughout, and lack the yellow loam capping. 

 This at least seems to be the character of the second terraces 

 of all the rivers of Mississippi that are tributary to the great 

 river, while Pearl and Leaf Rivers that flow into the Gulf 



* Hilgard. this Journal, vol. xliii, p. 399. 



