124 Subsurface Geologic Methods 



most megascopic and binocular examinations of subsurface samples, the 

 following definitions as proposed by Russel and Taylor ^^ are recom- 

 mended: 



Angular — Showing very little or no evidence of wear. Edges and corners 

 are sharp. 



Subangular — Showing definite effects of wear. The grains still have their 

 original form, and the faces are practically untouched, but the edges and cor- 

 ners have been rounded off to some extent though the angles between the faces 

 may still be sharp. 



Subrounded — Showing considerable wear. The edges and corners are 

 rounded off to smooth curves, and the area of the original faces is considerably 

 reduced, but the original shape of the grain is still distinct. 



Rounded — Original faces almost completely destroyed, but some compara- 

 tively flat faces may be present. There may be broad re-entrant angles between 

 remnant faces. All original edges and corners have been smoothed off to rather 

 broad curves. 



Well-rounded — No original faces, edges, or corners remain. The entire 

 surface consists of broad curves; flat areas are absent. However, the original 

 shape of the grain may be suggested by its present form. 



It is particularly important to note whether the sedimentary particles 

 have about the same rounding, or whether there is a mixture of angular 

 and well-rounded grains. "Mixed" rounding of the grains of a sand 

 generally indicates that the grains have not had the same past history 

 and may have been derived from different sources. Because rounding of 

 quartz sand grains is a very slow process, particularly for grains of fine 

 or very fine size, rounded or well-rounded grains probably indicate deriva- 

 tion from a pre-existing sediment. 



The deposition of quartz cement in a quartzose sandstone usually 

 modifies the original shape and roundness of the grains. Shape and 

 roundness measurements must be made on the original grains to be signifi- 

 cant. Any sandstone that sparkles in the sunlight or is seen to reflect 

 light from flat faces when viewed under the binocular microscope usually 

 has had the original shape and roundness of the grains modified by cement. 



Surface Texture 



The surface textures of sand grains may be seen megascopically or 

 inferred from the general appearance of the rock but usually can be 

 seen more clearly through a binocular microscope. A classification of 

 surface textures prepared by Williams ^^ and recommended here is as 

 follows: 



Luster (grains also may be smooth or rough). 



A. Dull 



B. Polished 



Relief (grains may be dull or polished). 

 A. Smooth 



'* Russell, R. D., and Taylor, R. E., Roundness and Shape of Mississippi River Sands: Jour. Geology 

 vol. 45, no. 2, pp. 225-267, Apr. -May 1937. 



^ Williams, Lou, Classification and Selected Bibliography of the Surface Textures of Sedimentary 

 Fragments: Nat. Research Council Comm. on Sedimentation Rept. 1936-1937, pp. 114-128, 1937. 



