A STUDY IN THE PETROLOGY OF SEDIMENTARY 
ROCKS’ 
G. SHERBURNE ROGERS 
U.S. Geological Survey, Washington, D.C. 
INTRODUCTION 
Since the introduction of the petrographic microscope about the 
middle of the last century a great mass of detailed information con- 
cerning the composition of rocks has been accumulated, but by far 
the greater part of this concerns igneous rocks alone. Meta- 
morphic rocks have received their due share of attention, but 
sedimentary rocks have been almost entirely neglected. Thus, 
most university courses in petrology touch on the last-named class 
only in a more or less perfunctory way, and similarly in practical 
work the geologist generally confines the use of his microscope 
chiefly or entirely to the igneous rocks. This disparity is the more 
notable because of the fact that sedimentary rocks preponderate at 
the earth’s surface, and while the study of their petrology does not 
seem to possess the fascination which attracts the students of 
igneous rocks, their areal importance is certainly such as to com- 
mand attention. This paper is a preliminary and more or less 
tentative discussion of the value of microscopic work in the corre- 
lation of stratified rocks, with an example of an application recently 
made by the writer. Mr. D. F. Hewett, of the United States 
Geological Survey, has been working along similar lines, and the 
writer takes this opportunity of expressing his appreciation of Mr. 
Hewett’s many valuable suggestions. 
Sediments, by their very nature and the manner of their 
derivation, do not lend themselves readily to the taxonomic prin- 
ciples which we apply to igneous rocks, nor can their almost infinite 
variations be traced and interpreted as clearly. It is perhaps for 
this latter reason that geologists have in general been content to 
classify a rock as sandstone, or shale, or arkose, without attempting 
t Published by permission of the Director of the U.S. Geological Survey. 
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