382 CHESTER K. WENTWORTH 



It has long been recognized that the differences between two 

 consecutive screen size openings should be greater for the large 

 sizes than for the small. This principle is followed in the selection 

 of such limits as i, 2, 5, 10, 20 millimeters, making the limits fall 

 on convenient whole numbers in the decimal notation. This 

 series, however, is a crude approach to a geometrical series in which 

 each value bears a constant ratio to the preceding one. A geo- 

 metrical series is the ideal for such a purpose, since a change of 

 i" is of the same significance and importance in the size of 10" 

 cobbles as a change of yV" i^. the size of i" pebbles. Only by the 

 use of logarithmic or some similar graphical scheme of representa- 

 tion can the size composition data be shown adequately for great 

 size ranges. The use of a geometrical series makes the successive 

 grades fall into equal units on the graph — an arrangement much 

 easier to read and interpret than any other known to the writer. 

 The most convenient ratio for the construction of such a series is 

 the ratio 2, and the most convenient and logical starting-point, 

 I mm. A large number of mechanical analyses of sediments made 

 with screens and by microscopic measurement conforming to such 

 a series have been made.' If a more minute subdivision is needed, 

 the ratio 1/ 2 can be used, giving twice the number of grades, or in 

 exceptional cases y2. These extra subdivisions fit in with and 

 form further subdivisions of the fundamental series of the powers 

 of 2. Conformity to this geometrical series is the first consideration 

 which has guided the writer in fixing the limits between the several 

 grade terms. 



The second consideration has been the desire to make each of 

 the limits as close as possible to the common practice of the majority 

 of geologists. Figure i shows the composite opinions of twenty- 

 eight geologists of the United States Geological Survey, as reported 

 by them in response to a questionnaire on the sizes limiting several 

 of the terms. The table below shows a number of different schemes 

 of classification which have been published. There is a close agree- 

 ment between some of those shown, but, with the exception of that 

 of Udden, all lack, in the sizes of successive grades, the uniformity 



' J. A. Udden, "Mechanical Composition of Clastic Sediments," Bull. Geol. Soc. 

 Amer., Vol. XXV (1914), pp. 655-744. 



