Geological Papers. 103 



eral applicability which shall distinguish the two and assign 

 to each its appropriate part are wanting." 



Mr. Udden says'" that "the wind has been very important 

 and perhaps more potent and far-reaching in its influence in 

 the forming of the loess than water deposition," his opinion 

 being based upon his study of the wind as a geological agent. 

 "It is necessary," he continues, "to speak of the objections to 

 aqueous deposition based upon topographic relations of the 

 loess. This deposit not only borders valleys, but blankets 

 interfluvial tracts as well, often resting on an eroded surface 

 like snow. It is found on the highest as well as the lowest parts 

 of previously eroded tracts. It is not rare to find it occurring 

 at elevations differing several hundred feet within the dis- 

 tance of a few miles. The occurrence of terrestrial shells in 

 the loess is also a serious objection to the aqueous deposition 

 theory, especially of deposition in a large body of water. 



"Adequacy of wind, as shown in the Mississippi valley, to 

 form seolian deposits, is shown as follows: (a) The universal 

 presence of mineral dust in the atmosphere, and its constant 

 settling, necessitates its accumulation in places where erosion 

 is at a standstill or where it does not exceed the rate of at- 

 mospheric sedimentation, (b) Erosion of the flat, loess-cov- 

 ered uplands is at the present time exceedingly slow as com- 

 pared with the average rate of denudation of the whole Mis- 

 sissippi valley. 



"Rain-water on a level surface appears to soak into the 

 ground as rapidly as it falls, even in the heaviest rains. This 

 is especially true where the surface is covered with vegetation. 

 By far the greater part of the land area in the region of the 

 loess consists of such flat land. And where the drainage of the 

 area is so sluggish as not to equal the secular accumulations of 

 atmospheric dust on the land surface, loess dust would of 

 course accumulate. 



"In this mechanical composition fine wind-sediments and 

 loess are largely identical. The bulk of each consists of par- 

 ticles l-16th to l-64th of a millimeter in diameter, with two 

 nearly symmetrically decreasing series of admixtures above 

 and below their size. No aqueous deposit with such a range of 

 altitude in so few miles could be so uniform in its mechanical 

 composition as the loess is. Again, were the deposits composed 

 of as fine material as it is and laid down by water, it would 



10. Bull. Geol. Soc. Amer., vol. IX, p. 6. 



