Studies for Students 



A CLASSIFICATION OF COMMON SEDIMENTS AND SOME CRI- 

 TERIA FOR IDENTIFICATION OF THE VARIOUS CLASSES 



ARTHUR C. TROWBRIDGE 



State University of Iowa 



Sedimentary rocks are useful to the geologist chiefly through 

 the record of the history of the earth which they contain, and it 

 is desirable that classifications of these rocks should be so arranged 

 as to bring out as much of that history as possible. The common 

 classification of sedimentary rocks, on the basis of texture and 

 mineral composition, into conglomerates, sandstones, shales, and 

 limestones is hardly adequate, because the origin of the rock is 

 only shghtly implied, if at all, in the definition. For instance, 

 under the present classification, sandstone is sand cemented; but 

 the discovery of sandstone in a locality may record either a shallow 

 sea not far from shore, dry, windy conditions and the piling-up 

 of sand by wind, the previous existence of lakes receiving sand from 

 near-by lands, or fluvial conditions under which the sand was 

 deposited by streams. Similarly, conglomerate may be marine, 

 fluvial, pluvial, glacial, or lacustrine in its origin. Even limestone 

 is being laid in inland lakes today, as well as in the sea, and by 

 wind in the Bermuda Islands. 



If sedimentary rocks are to perform their function as true 

 recorders of past events, their classification must be based on the 

 origin of those rocks. The agents and processes by which sedi- 

 mentary rocks are made lie chiefly in the realm of physiographic 

 geology. This paper places a physiographic interpretation on the 

 origin of sedimentary rocks. 



It is of course understood that there is a wide gap between 

 sediments and sedimentary rocks, but it is also believed that a 

 classification of the various sorts of sediments will be at least a step 

 toward a more effective classification of sedimentary rocks, and 



420 



