68 GENERAL, CONCLUSIONS. 



The following statements, which were made in a letter 

 written by Professor Dana just before his death, set forth 

 certain objections to the eolian hypothesis. "With I'egard 

 to the eolian work along- valley plains, I think great 

 caution is necessary because eolian work is of a fitful 

 kind. The more powerful winds blow in gusts or rather a 

 succession of them, and each of the gusts is of a rather 

 narrow limit; and in each gust great velocity is succeeded 

 by a decline in which the depositions vary accordingly as 

 to fine and coarse and limit. Making loess — unstratified 

 — by the winds would require a steady breeze sufficient 

 to move the light earth or sand long in a common direc- 

 tion, but too near unvarying in force or velocity to pro- 

 duce alternations from coarse to fine. It is an even kind 

 of work that winds are not often fit for."*) In the last 

 edition of Dana's classic Manual the correctness of Kicht- 

 hofen's theory of the Chinese loess is regarded as improb- 

 able owing to the absence of winddrift structure (lamina- 

 tion)**). Possibly the absence of such structure was 

 Dana's chief objection to an eolian hypothesis of the 

 origin of the American loess. His argument that the 

 deposit from every changing gust of wind must vary in 

 coarseness according to the velocity, expresses a. general 

 law which certainly is true, but it seems that there are 

 some special conditions which supervene, as explained 

 above, and that these will necessarily modify the results 

 of the operation of this law and limit its application to 

 such deposits as are accumulating rapidly near places 

 of atmospheric erosion. 



*) See Journal of Geology, Vol. 3, p. 342. 

 **) Manual of Geology, p. 195. 



