Subsurface Laboratory Methods 



201 



grain ^zes in any natural deposit, because of differences in the mineralogical 

 composition of different grades. ... It is difficult to determine the effect of 

 shape of grain on porosity, because of the difficulty of obtaining angular par- 

 ticles of the same size. . . . Angularity may either increase or decrease 

 porosity; most often it increases porosity. The only type of "angularity" found 

 to cause a decrease in porosity is that in which the grains are mildly and uni- 

 formly disk-shaped. 



Factors affecting permeability (in addition to temperature, hydraulic 

 gradient, and coefficient of permeability) include uniformity and range 

 of grain-size, shape of grain, nature and uniformity of packing, surface 

 conditions of the grains, stratification, consolidation, and cementation of 

 the material. ^^ 



Krumbein ^^ has given an interesting discussion on determining spher- 



fi5 



1 

 .9 



.8 

 .7 

 .6 



.5 



I 



.4 

 • 3 

 .2 



.1 



.1 



.2 .3 



.6 ,7 



.8 



.9 1 



.4 .5 

 c/b 



Figure 84. Chart for determining sphericity of pebbles; a, b, and c, values represent 

 the long, intermediate, and short dimensions of the pebble. (From Krumbein, 

 Jour. Sedimentary Petrology.) 



"^ Fraser, H. J., op. cit., p. 959. 



Krumbein, W. C, Measurement and Geological Significance of Shape and Roundness of Sedimen- 

 tary Particles: Jour. Sedimentary Petrology, vol. 11, no. 2, pp. 64-72, Aug. 1941. 



