INTERPRETATION OF LAND FORMS 143 



inadequacy of the current disintegration as a source of material. 

 When we consider the immense area covered by loess to depths 

 varying from 50 to 2,000 feet, and the fact that this is only the 

 very finest portion of the product of rock-destruction, and again 

 that the accumulation represents only a very short period of 

 time, geologically speaking, surely we must seek a more fertile 

 source of supply than is furnished by the current decomposition 

 of rock surface. 



It seems to me that there are two important sources : I. The 

 silt brought by rivers, many of them fed by the products of 

 glacial attrition flowing from the mountains into the central 

 region. Where the streams sink away, or where the lakes which 

 receive them have dried up, the finer products of the erosion 

 of a large territory are left to be removed in dust storms. 



II. The second . . . source is the residuary products of a 

 secular disintegration. ' ' 



The evidence presented by Pumpelly for the eolian 

 origin of loess structure, texture, composition, fossil 

 content and topographic position is complete, and to him 

 belongs the credit for the correct interpretation of the 

 Mississippi valley deposits. Unfortunately his contribu- 

 tion came at a time when the geologists of the central 

 States were intent on tracing the paths and explaining 

 the work of Pleistocene glaciers, and the belief was 

 strong that loess was some phase of glacial work. Its 

 position at the border of the lowan drift so obviously 

 suggests a genetic relation that the fossil evidence of 

 steppe climate suggested by Binney in 1848 65 was mini- 

 mized. Students of Pleistocene geology in Minnesota, 

 Iowa, Nebraska, Missouri, although less vigorous in 

 expression, were substantially in agreement with Hilgard 

 (1879). 66 "The sum total of anomalous conditions 

 required to sustain the eolian hypothesis partakes 

 strongly' of the marvellous." The last edition of Dana's 

 Manual, 1894, and of LeConte's Geology, 1896, the two 

 most widely used text books of their time, oppose the 

 eolian theory, and Chamberlin, in 1897, 67 states: "the 

 aqueous hypothesis seems best supported so far as con- 

 cerns the deposits of the Mississippi Valley and western 

 Europe" (p. 795). Shimek, in papers published since 

 1896 has shown that aquatic and glacial conditions can 

 not account for the loess fossils, and the return to the 

 views of Pumpelly that the loess was deposited on land 



