TYPES OF SAND GRAINS 775 



CRITERIA FOR THE RECOGNITION OF VARIOUS TYPES OF SAND GRAINS 



BY W. H. SHEEZER 



Published as pages 625-662 of this volume. 



Discussion 



Prof. J. Barbell : I have listened with great interest and instruction to 

 Professor Sherzer's paper, since the determination of the geographic and cli- 

 matic conditions under which ancient sandstones were deposited, as indicated 

 by the character of the grains, is a problem which in the progress of more 

 exact knowledge must become of great importance. Several queries arise, 

 however, at the present time in regard to the degree of security of conclusions 

 based on the studies concerning modern sands which have thus far been re- 

 corded and which are used as a basis for interpreting the past. 



First, the character of sands accumulated through wind, current, and wave 

 action vary widely within themselves, depending on the duration and intensity 

 of the several forces of accumulation. Complexities also enter into the prob- 

 lem through the concurrence of more than one agency at work and the previ- 

 ous nature of the material which has been acted on. To learn the limits of 

 these modifying conditions hundreds of examples of modern sands should be 

 studied, sands collected from type localities where the conditions of accumula- 

 tion can be well determined. The distinctions now drawn from modern sands 

 and applied to those of ancient origin are based on relatively few examples. 

 May not therefore the distinctions appear sharper and safer than they really 

 are? 



Second, in such studies as are needed of typical sands efforts should be 

 made toward quantitative determinations of various characteristics, so as to 

 minimize as much as possible the personal equation in deciding on such in- 

 definite qualities as roundness and smoothness of grains, especially as these 

 qualities vary widely among the individual grains of any one sample. Meas- 

 urements of the minimum size of rounded grains have been made, but these 

 are not enough. A suggestion in this direction may be taken from a method 

 of separating slate from coal devised, I believe, by the late Eckley B. Coxe, of 

 Drifton, Pennsylvania. The coefficient of friction depends on the shape of the 

 fragment and the smoothness of the surface. Fragments of a uniform size 

 coming from the screen are allowed to slide down a sheet-iron trough with a 

 slot at the bottom. The coal fragments, sliding and rolling faster, are able to 

 jump this slot; the slate, sliding more slowly, drops through. Similarly, a 

 quartz sand could be separated into sizes by sieves and each size subjected to 

 various processes to determine the percentage of different forms. In applying 

 such methods to ancient sands, however, such adaptations would have to be 

 made as would allow for secondary growths and permit microscopic determi- 

 nations, but decomposed portions of beds might give sands which could be 

 directly compared. 



Third, some emphasis is given to the preceding points by the consideration 

 of a little-known paper by A. R. Hunt, published in 1887 in the "Report and 

 Transactions of the Devonshire Association," pages 498-515. He finds on the 

 Skerries Shoal that many of the grains of quartz sand are well rounded down 



