GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 381 



Silt particle. — The term silt particle is here applied to individual 

 particles smaller than very fine sand grains but larger than clay 

 particles. The term silt from which it was derived was objected 

 to by some geologists on grounds that are stated under the heading 

 of silt. These grounds were not sustained even by a minor part 

 of the data available to the writer and the term is here used as the 

 most satisfactory one. 



TABLE I 

 The Grade Terms 



The Pieces 



The Aggregate 



The Indurated Rock 



Bowlder 

 256 mm. — 



Cobble 

 64 mm. — 



Pebble 

 4 mm. 



Granule 

 2 mm. 



Bowlder gravel 



Cobble gravel 



Pebble gravel 



Granule gravel 



Bowlder conglomerate 



Cobble conglomerate 



Pebble conglomerate 



Granule conglomerate 



Very coarse sand grain 

 I mm. 



Very coarse sand 



Very coarse sandstone 



Coarse sand grain 

 1/2 mm. 



Medium sand grain 

 1/4 mm. 



Fine sand grain 

 mm. 



Very fine sand grain 

 1/16 mm. — — — — - 



Silt particle 

 1/256 mm. 



Clay particle 



Coarse sand 



Medium sand 



Fine sand 



Very fine sand 



Silt 



Clay 



Coarse sandstone 



Medium sandstone 



Fine sandstone 



Very fine sandstone 



Siltstone 



Claystone 



Clay particle. — After consideration of several other terms for 

 the materials finer than silt, the term clay was finally adopted 

 as most likely to meet with general approval. Clay particle is 

 therefore used for the individual pieces. 



The size limits. — In fixing the limiting sizes of the several 

 grades of the scheme shown in the table, the writer has been 

 governed by two considerations. First, there is a growing accept- 

 ance among geologists and engineers of a series of sieves for the 

 classification of natural clastic materials in which the openings of 

 consecutive size stand to one another in the ratio 2 or 1/2 starting 

 with I mm. as the standard. 



