GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 389 



above. The term claystone is here proposed as most satisfactory 

 for the indurated equivalent of clay, as defined above. 



THE CLASS TERMS 



The grade terms defined above for aggregates and for rocks are 

 applicable, as the writer has pointed out in another part of this 

 paper, to very few sediments of natural origin. Most such sedi- 

 ments are composed of particles of several or many grades and the 

 names suggested above with definite numerical limits cannot 

 properly be applied to them. This difficulty has been long recog- 

 nized by students of sedimentary rocks and several schemes for 

 meeting the difficulty have been proposed. Notable among these, 

 because of the large amount of data published in accordance with 

 it, is that of the United States Bureau of Soils which is given below. 



CLASSIFICATION OF SOIL MATERIAL 



United States Bureau of Soils* 



Soils containing 20% silt and clay: 



Coarse sand more than 25% very coarse sand and coarse sand and 



less than 50% any other grade 

 Sand more than 25% very coarse sand, coarse and medium 



sand, and less than 50% fine sand 

 Fine sand more than 50% fine sand, or less than 25% very 



coarse sand, coarse and medium sand 

 Very fine sand more than 50% very fine sand 



Soils containing 20-50% silt and clay: 



Sandy loam more than 25% very coarse sand, coarse and medium 



sand 

 Fine sandy loam .... more than 50% fine sand or less than 25% very 



coarse sand, coarse and medium sand 

 Sandy clay less than 20% silt 



Soils containing more than 50% silt and clay: 



Loam less than 20% clay, less than 50% silt 



Silt loam less than 20% clay, more than 50% silt 



Clay loam 20-30% clay, less than 50% silt 



Silty clay loam 20-30% clay, more than 50% silt 



Clay more than 30% clay 



* C. C. Fletcher and H. Bryan, "Modification of the Method of Mechanical Soil Analysis," U.S. 

 Depi. of Agric, Bur. Soils, Bull. No. 84 (1912). 



