GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 385 



recognizes this difficulty and the grade terms here defined are 

 appHcable strictly and without modification to but few natural 

 sediments. They are proposed as the foundation of, and in the 

 course of development of, the several class terms and adjectives 

 proposed in the last part of this paper. Photographs of grade 

 aggregates from 64 mm. to yV mm. are shown in the accompanying 

 Figures 2 and 3. 



Gravel. — Among some geologists the term gravel has been used 

 only for material composed of small pebbles and granules, but more 

 commonly and especially in America and in reference to glacial 

 gravels, the term has been used to include material containing 

 great bowlders up to a meter or more in diameter, and has recently 

 been so defined by J. S. Flett^ who considers the term to be the 

 equivalent of conglomerate as applied to the consolidated rocks. 

 It is in this latter and prevailing sense that the term is here used 

 and combined with quahfying words in the terms bowlder gravel, 

 cobble gravel, pebble gravel, and granule gravel. 



Sand. — The term sand is in common use among all English- 

 speaking geologists for mineral grains smaller than i or 2 mm. 

 and larger than silt. By some writers, sand is applied only to 

 rounded mineral grains, but others use the term more generally. 

 Sand is from the Anglo-Saxon word of the same spelling and 

 meaning. 



Silt. — The term silt is considered by soine geologists to apply 

 properly to deposits containing organic matter in addition to the 

 mineral particles. The writer was unable to find any considerable 

 support of this view by either past or present authorities, and has 

 here used the term for the grades designated in the table on page 

 384. The word silt is probably akin to a number of Germanic 

 roots meaning to sift or filter, compare German seihen, to strain. 



Clay. — After consideration of a number of alternative terms, 

 the term clay has been selected as most likely to be acceptable to 

 geologists for the finest clastic sediments. A few geologists 

 objected to the term on the ground that it impHed plasticity or 

 that it referred to a definite chemical composition. It is the view 



^ J. S. Flett, Encyclopaedia Britannica, nth ed. (1911), Vol. XII, p. 382, and 

 Vol. VI, p. 913. 



