658 



J. A. UDDEN COMPOSITION OF CLASTIC SEDIMENTS 



Diameters in millimeters. 



From— To— 



Fine sand 1/4 1/8 



Very fine sand 1/8 1/16 



Coarse silt or dust 1/16 1/32 



Medium silt or dust 1/32 1/64 



Fine silt or dust 1/64 1/128 



Very fine silt or dust 1/128 1/256 



Coarse clay 1/256 1/512 



Medium clay 1/512 1/1024 



Fine clay 1/1024 1/2048 



In most samples which have been examined there is one medium 

 grade, which is present in greater quantity than any other, while the 

 other grades diminish in mass the more the size of their particles differ 

 from the medium grade. The latter will here be called the chief ingre- 

 dient, or the maximum, and the two decreasing series on either side will 

 be referred to as the coarse and the fine admixtures. The ingredients in 

 the grades nearest in size of its elements to the size of the elements in 

 the maximum grade will be called the proximate admixtures, and these 

 grades will be called the proximate grades. The grades most different 

 in size of its elements from the size of the elements in the maximum 

 ingredient will be called the distant admixture, and the same grades will 

 be called the distant grades. In many samples the decrease in quantity 

 of the admixtures away from the maximum is not continuous, but there 

 is an increase in quantities from the proximate to the distant grades. 

 In such case we may call one or more grades secondary maxima. It will 

 also be convenient to refer to the grades of admixtures by numbers, des- 

 ignating them in pairs as the grades of the first, second, and third, 

 etcetera, order from the maximum. 



The separations of the materials measuring more than 16 millimeters 

 in diameter were made by counting the pebbles and boulders of each 

 grade and calculating the bulk or weight in each grade from the number 

 of the pebbles or boulders, regarding them as perfect spheres of mean 

 sizes in each grade ; thus : ' 



Sizes of boulders in millimeters. ^o™ders°^ 



1 



Divisors used. 



Percentages of 



grades 

 to whole sample. 



512-256 



256-128 



128- 64 



1 



432 



2,624 



1,638 



1 



8 



64 



512 



1.0 

 54.0 

 41.0 



64- 32 



3.0 







