122 Subsurface Geologic Methods 



no general agreement exists as to what sizes these terms include. Recently 

 the tentative size scales shown in figure 51 have been proposed by De 

 Ford ^^ and Hills'^" (West Texas Geological Society) for carbonate rocks. 

 These are recommended for consideration and trial. 



For rocks that are combinations of clastic grains and crystals, the 

 dominant constituents should determine the main name, and the minor 

 constituent should determine the modifier. To each, the size terms in figure 

 51 may be applied. A fine sandy, medium-paurograined dolomite would 

 be a dolomite with crystals between .056 and .032 mm. in diameter that 

 contained less than 50 percent of fine sand. In those combinations in 

 which this terminology may be cumbersome, especially when the composi- 

 tion of the sand is indicated, the modifier may be added as a qualifying 

 phrase. In those cases where more accurate designation of the proportion 

 of the constituents is desirable, the estimated percentages of them may be 

 indicated. 



As Hills has noted, experience has shown that full descriptions, of 

 subsurface samples have saved much time and money, whereas meager 

 descriptions have necessitated one or more re-examinations of the cuttings. 

 Hills also recommends that wildcat wells and pay sections be described 

 in more detail than field wells or long sections of comparatively insig- 

 nificant beds between key beds. 



Shape and Roundness 



Shape (sphericity) and roundness of sedimentary grains are inde- 

 pendent properties and should not be confused. Sphericity is a measure 

 of the approach to spherical form and may be expressed roughly as a 

 ratio between the diameters of the grains. In contrast, roundness is a 

 measure of the angularity of the corners and edges of the grains. 



In most subsurface samples the particles or crystals are monomineral, 

 and their shape is largely dependent on their composition. Therefore, 

 separate-shape terms usually are not necessary when the mineral compo- 

 sition and texture are indicated. For rock fragments, the shape terms sug- 

 gested by Krynine,^^ as slightly modified by the writer, are indicated. 

 The methods used for more detailed measurements of shape and roundness 

 than ordinarily needed are outlined elsewhere in this section. 



Equant — Length of grain is less than 1^ times its width and thickness. 



Prismatic — Length of grain is 1^ to 3 times its width and thick- 

 ness. 



Tabular — Length and width of grain are 1^ to 3 times its thickness. 



Acicular — Length of grain is more than 3 times its width and thick- 

 ness. 



Platy — Length and width of grain are more than 3 times its thick- 

 ness. 



2" DeFord, R. K., op. cit. 

 '- Hills, J. M., op. cit. 



^^ Krynine, P. D., The Megascopic Study and Field Classification of Sedimentary Rocks: Jour. Geol. 

 ogy, vol. 56, no. 1, pp. 130- 165, Jan. 1948. 



