378 CHESTER K. WENTWORTH 



more careful definition of such terms. With this need in mind, 

 he sent to about sixty of his colleagues of the United States Geologi- 

 cal Survey a questionnaire asking them to give the limiting dimen- 

 sions in their conception or usage of the terms bowlder, cobble, 

 pebble, sand grain, and clay particle. Replies were received from 

 about thirty of the men. These were studied and compared and 

 the composite results presented in preliminary unpublished form 

 which was distributed to more than one hundred geologists through- 

 out the country in the hope of receiving additional comment and 

 criticism. A small number of very helpful replies were received 

 and utilized in modifying, to some extent, the size limits and the 

 terms used. 



Early in 1921, mimeographed copies of this modified scheme 

 of terms were sent to about a dozen geologists in this country and 

 England who were known as workers in the field of sediments and 

 sedimentary rocks, and deemed competent to criticize the usages 

 proposed. They were asked to reply to specific questions in regard 

 to the terms which had been subject to the most criticism and to 

 comment in general upon the plan. The rephes from this smaher 

 group were most gratifying, since nearly every geologist addressed 

 sent a reply which the writer found useful in the preparation of 

 the classification here presented. 



In addition to the studies mentioned above, the writer com- 

 menced in 1920 the collection of definitions of sedimentary rock 

 terms. These definitions are taken verbatim from textbooks, 

 dictionaries, encyclopedias, and glossaries. They are typewritten 

 on cards with the proper references and filed under the name of 

 the term defined. Many of the definitions collected are from 

 sources seventy-five to one hundred years old and represent the 

 former usage of certain terms as understood by the compiler. 

 The definitions collected in this way vary greatly in value and none 

 is to be regarded as of absolute authority. They constitute, how- 

 ever, part of the data of the problem. 



As will appear from the foregoing, the writer has compiled the 

 present scheme of classification in part from a specific study of the 

 terms here presented and in part from the results of a general 

 consideration of terms in the field of sedimentary rocks. He is 



