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Wentworth, 1922. A Scale of Grade and Class Terms for Clastic Sediments.pdf

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A Scale of Grade and Class Terms for Clastic Sediments Author(s): Chester K. Wentworth Source: The Journal of Geology, Vol. 30, No. 5 (Jul. - Aug., 1922), pp. 377-392 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30063207 . Accessed: 30/09/2013 17:05 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps
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  A Scale of Grade and Class Terms for Clastic SedimentsAuthor(s): Chester K. WentworthSource: The Journal of Geology, Vol. 30, No. 5 (Jul. - Aug., 1922), pp. 377-392Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30063207 . Accessed: 30/09/2013 17:05 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at  . http://www.jstor.org/page/info/about/policies/terms.jsp  . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.  . The University of Chicago Press  is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. http://www.jstor.org This content downloaded from 128.163.8.40 on Mon, 30 Sep 2013 17:05:32 PMAll use subject to JSTOR Terms and Conditions  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 peculiarly 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 '917, 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 This content downloaded from 128.163.8.40 on Mon, 30 Sep 2013 17:05:32 PMAll use subject to JSTOR Terms and Conditions  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 replies from this smaller 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 378 This content downloaded from 128.163.8.40 on Mon, 30 Sep 2013 17:05:32 PMAll use subject to JSTOR Terms and Conditions  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 This content downloaded from 128.163.8.40 on Mon, 30 Sep 2013 17:05:32 PMAll use subject to JSTOR Terms and Conditions

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