GENERAL STATEMENT. 



The great lightness of the air, when compared with 

 water, and the comparatively high velocity of its cui-rents 

 will necessarily render any materials it may carry and 

 deposit somewhat different in composition and structure 

 from those which are laid down in water. They are as a 

 rule finer, they exhibit a different bedding and are more 

 capriciously placed. These characteristics are familiar in 

 a general way. It is here desired to present more exact 

 information on this subject, particularly as to the me- 

 chanical composition of these deposits; and to show how 

 this changes under varying circumstances of deposition. 

 It is hoped that this may lead to a more certain identifi- 

 cation of wind sediments, wherever the}' may be found. 

 The inquiry seems to be of special importance in connec- 

 tion with the study of superficial deposits. 



Samples of different kinds of materials moved by the 

 wind have been collected from different places of desposi- 

 tion and from the atmosphere directly for this stud,v- 

 Each of these has been separated into grades of different 

 coarseness and the per cent of the weight for each grade *) 



*) The particles ranging' between two successive separations ■will here be re- 

 ferred to as a grade. In tliR analyses these separations were made in a uniformly 

 decreasing series of diametrical dimensions, the diameter of the largest particles in 

 one grade having twice the lengi-h of the diameter of the coarsest particles in the 

 next finer grade. The coarsest grade consists of rock fragments with diameters 

 ranging from IG to 8 millimeters, the next from 8 to 4 mm., and so on, down to 

 particles measuring from ^l-^ to ^-\,^ mm. Below this size no separations have 



