A SCALE OF GRADE AND CLASS TERMS FOR 

 CLASTIC SEDIMENTS^ 



CHESTER K. WENTWORTH 



State University of Iowa 



CONTENTS 



Introduction 



The Grade Terms 

 Fragment Terms 

 Aggregate Terms 

 Rock Terms 



The Class Terms 



INTRODUCTION 



In no other science does the problem of terminology present 

 so many difficulties as in geology. With the growth of knowledge 

 in any field of investigation, men devise new terms or redefine old 

 ones in the attempt to convey more precise and definite ideas. 

 In all the branches of science much confusion has followed the 

 redefinition of old terms because of the indiscriminate use of the 

 terms both in the old and the new senses. But in geology, dif- 

 ficulties of this kind are pecuharly great. 



Because geology is a field science and has followed in the 

 footsteps of exploration, it has acquired terms from all parts of the 

 world. Many of the names for the less common special features 

 have come from the dialect or colloquial speech of that part of the 

 world where they are best developed. With the use of these 

 terms of geologists of other regions, much irregularity of usage 

 and hence much confusion has arisen. 



Since 191 7, the writer had been engaged in the study of abrasion 

 and shaping of cobbles and pebbles by the action of running water. 

 In the course of this study the loose usage of cobble, pebble, and 

 related terms (in which his own practice was no exception) has 

 impressed him with the need of greater uniformity of usage and 



^ Published by permission of the Director of the United States Geological Survey. 



377 



