GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 379 



indebted to a large number of geologists who have helped him by- 

 spoken and written criticism. Space will permit acknowledgment of 

 gratitude only to Dr. M. I. Goldman and to Dr. J. B. Woodworth, 

 whose interest and frank criticism have been especially helpful in 

 the preparation of this paper. 



THE GRADE TERMS 



It is the writer's purpose here to suggest terms which are specific 

 as regards size of piece and, at least for the larger pieces, as regards 

 shape of piece. The terms of this scheme apply to rounded 

 materials in so far as materials of the size in question become 

 rounded by transportation. Strict uniformity in this regard will 

 not fit the sediments as they occur in nature. Bowlders, cobbles, 

 and pebbles are rounded rock fragments, whereas most clay 

 particles are angular, yet geologists will recognize that they all 

 belong to a natural series. Likewise, bowlders and clay particles 

 are not commonly of the same mineral composition but in spite 

 of this fact they are the two extremes of the series of transported 

 rock fragments. By an excessive multiplication of terms it would 

 be possible to make a classification in which each term was specific 

 as to size of particle, shape of particle, lithologic character, and 

 other characteristics. Such a scheme would be highly artificial in 

 many of its categories and seems to the writer impracticable in the 

 present state of knowledge. 



The present scheme of grade terms is, accordingly, just what 

 its name implies — a series of names for clastic fragments of different 

 sizes. They apply only to rounded fragments except in the case of 

 fine sands, silts, and clays in which even prolonged transportation 

 does not always round the pieces. The names applied to the 

 different grades carry no lithologic, mineralogic, or chemical 

 significance so far as the present scheme is concerned. Sands are 

 dominantly quartzose, whereas clays are largely made up of kaolin, 

 but this fact is incidental and not necessary in the use of the 

 terms. 



FRAGMENT TERMS 



Bowlder. — This term is in common use in English-speaking 

 countries for rounded and smoothed masses of rock larger than 



