REVIEWS i6i 



The book contains a fund of interesting and useful information and 

 is elaborately illustrated with shaded relief maps, halftone plates, diagrams 

 and sections. To the geologist it will prove of value either while travel- 

 ing by the Denver and Rio Grande, or as a convenient reference book 

 wherein is summarized much of the geology of these portions of Colorado 

 and Utah. 



It may be obtained for one dollar from the Superintendent of Docu- 

 ments, Washington, D.C. 



R. T. C. 



The Shapes of Pebbles. By Chester K. Wentworth. Bulletin 



730 C, U.S. Geological Survey, 1922. Pp. 24, pis. 2, figs. 17. 



Contains two separate papers entitled "A Method of Measuring 



and Plotting the Shapes of Pebbles" and "A Field Study of 



the Shapes of River Pebbles." 



The writer of the above papers shows clear appreciation of the 



importance of the quantitative elements in geology. Such efiforts as 



these are gradually lifting the ''study of earth features" from a purely 



descriptive account to a truly measurable science. 



The first of these two papers is essentially an account of the methods 

 employed in determining curvatures of pebbles. Two terms are intro- 

 duced, (i) the "roundness ratio," or the ratio of the radius of curvature 

 of the sharpest "developed" or secondary edge to the main radius of 

 the pebble, and (2) the "flatness ratio," or the ratio of the radius of 

 curvature in the most convex direction of the flattest "developed" 

 face to the mean radius of the pebble. The measurements are made 

 with a cleverly modified optical convexity gage. The results for each 

 pebble are plotted on logarithmic co-ordinates, applying the "roundness 

 ratio" as ordinate and the "flatness ratio" as abscissa; perfect spheres 

 would thus lie near the upper left corner, angular pieces with reduced 

 facets toward the lower left corner, and pebbles with nearly plane facets 

 toward the lower right corner. 



A number of pebbles of different origins (glacial, sandblasted, and 

 river-worn) were thus plotted and the writer was enabled to establish 

 clearly a distinctive grouping with boundary lines for each of these types. 

 There is a slight overlap of glacial and river pebbles, but glacial and 

 sandblasted pebbles are quite distinct. It thus appears that a fairly 

 ready method is available for classifying sediments of large-diameter 

 particles and of uncertain origin. A large number of measurements are 



