202 Subsurface Geologic Methods 



icity values of pebbles. Three-dimensional values are recorded, maxi- 

 mum (a), intermediate (6), and minimum (c) (fig. 83). The ratios b:a 

 and c:b are calculated. From these ratios the sphericity index is obtained 

 from a control chart (fig. 84). 



Roundness values of grains and pebbles as determined by Wadell ^^ 

 are computed from a maximum-plane image (projected cross section) in 

 which the summation of the radius of the individual corners is divided 

 by the number of corners and this value divided by the radius of the max- 

 imum-inscribed circle. Roundness is expressed by the formula: 



where r is the radius of each corner, R is the radius of the maximum 

 inscribed circle, A^ is the number of corners, and P is the degree of round- 

 ness. Roundness values may also be obtained from a chart. Grains may 

 have the same roundness but varying sphericities, whereas other grains 

 may have the same sphericity but varying roundness. 



The flatness ratio ^^ of pebbles and grains is expressed by the form- 

 ula: 



It 



F=(length+width-^twice the thickness) =- - 



The combination of sphericity, roundness, and flatness values permits 

 quantitative expression of the shape characteristics of grains and pebbles. 



ELECTRON-MICROSCOPIC ANALYSIS— SOME GEOLOGIC 

 APPLICATIONS IN CORRELATION WORK 



CARL A. MOORE 



Ability to discriminate among minute objects that lie very close to- 

 gether is described as the resolving power of a microscope. In spite of 

 various methods that may be employed to increase the resolving power of 

 a light microscope, objects separated by less than 0.1 micron (0.0001 milli- 

 meter) cannot be resolved. Thus it is that the limits of the light micro- 

 scope are not the lack of skill on the part of the designer but rather are 

 due to the light — ^the media used for observation. 



This limitation of the light microscope is an important factor in 

 microscopy; for example, the study of viruses must be conducted with par- 

 ticles and separations much smaller than this, and the study of colloids 

 necessitates greater resolving power than that possible with the light micro- 

 scope. 



With the introduction of the electron microscope, this limit on resolv- 

 ing power has been greatly decreased, since the magnification is no longer 



®' Wadell, Hakon, Sphericity and Roundness oj Rock Particles: Jour. Geology, vol. 41, pp. 310-331, 

 1933. 



"S Wentworth, C. K., The Shapes of Beach Pebbles: U. S. Geol. Survey Frof. Paper 121-C, pp. 

 75-83, 1922. 



