(') GENERAL STATEMENT. 



in each sample has been determined. It appears that all 

 of these samples and presumably the greater part of such 

 materials as owe their present position and arrangement 

 to the action of the atmosphere may be referred to some 

 one of four categories. These may be characterized as 

 1) lag gravels, or coarse residual deposits in the rear of 



bpen made, particles so minute constituting only a very small proportiou of even 

 the finest atmospheric sediments. In general, v:hen the finest grades have been 

 found in a quantit.y amounting to only a small fraction of a per cent of any 

 sample, they have been neglected. For the sake of convenience the following de- 

 signations of the different grades will be used iu this paper : 



Coarse gravel diameter from 8 to 4 millimeters. 



Gravel " "4 "2 



Fine gravel " "2 "1 " 



Coarse sand " " 1 " \ " 



Medium sand " " i- " j " 



Fine sand " "4 " •? '' 



Very Jine sand " " 1 " ^-^ " 



Coarse dust •' " ^L " _i^ '• 



Medium dust " " J^ "^L 



Finedust •■ " Jj " ^\.g 



Very Jine dust " " y|-g- " -ji^ '' 



Down to the particles measuring % of a millimeter all the separations were 

 made by sieves, and beloAv this the per cent of the weight of each gi-ade was de- 

 termined by microscopic measurements and by calculation from the number of 

 grains counted in each grade. In nearly all samples which have been examined, 

 there is a medium grade, -which is i)resent in greatest quantity, while the otlier 

 grades diminish in bulk the more the size of their particles differ from the medium 

 gi-ade. The latter will here be called the c/ii>//)?pi)-erfi>»/, or the »ma-/»»»H(, and 

 the two decreassiiig series on either side vsill be referred to as the coarse and the 

 fine admixtures. 



In this connection I desire to state that I am under great obligations to Pro- 

 fessor MiltonWhitney, of the United States Department of .\gricnlture, from whom 

 I received much valuable information regarding the inechanical analysis of sujier- 

 flcial deposits, in examining a series of Illinois soils some years ago. Down to 

 medium sand, the grades here adopted correspond to the coai-ser grades in the 

 scale which he has adopted for soil analysis. Below this size I have found it ne- 

 cessary to make u.se of another .scale. In the analysis of soils it is of particular 

 importauce to determine the quantity of "clay" consisting of particles below the 

 size of -5-^ mm., while in an investigation of the nature of the sorting effected by 

 different mechanical forces, all the grades xjresent in any considerable quantity are 

 of equal significance. In the separations here made the sizes of the fragments in 

 the successive grades increase uniformly in a geometric ratio. 



