Subsurface Laboratory Methods 121 



Color 



In the past color description of sedimentary rocks has been a more 

 or less "every man for himself" proposition. Not only have different men 

 described the same rock differently, but the same man often cannot dupli- 

 cate his own descriptions. Recently, DeFord ^^ discussed previous work 

 and made suggestions that have been partly responsible for the prepara- 

 tion and publication of a "Rock Color Chart." ^'^ Although the writer has 

 not yet had the opportunity of using this chart, it or one of the other 

 color standards recommended by DeFord should be used in describing 

 color. 



Grain Size 



Most sediments are composed of clastic particles that have been 

 transported to their place of deposition, of crystals that have grown in 

 situ, or a combination of clastic particles and crystals. Rocks containing 

 fossils that have not been transported might be considered as another 

 group. No size terms for either clastic or crystalline sedimentary rocks 

 have been generally accepted for subsurface work. Thus, it is important, 

 whatever method of size classification is adopted, to indicate what that 

 scale of measurement is. 



For clastic rocks, exclusive of clastic carbonates, the terms "conglom- 

 erate," "sand" (sandstone), "silt" (siltstone), "clay" (claystone), and 

 "shale" are widely used, but different workers define and use them differ- 

 ently. The Wentworth grade scale, shown in figure 51, has probably been 

 used most widely, and has the advantage of separating sand and silt at the 

 one-sixteenth-millimeter size. Above this size the individual grains are 

 clearly visible to the naked eye; below this size they are indistinct or in- 

 visible. This size also is approximately the point of separation between 

 wash-load and bed material in some present-day streams and may have 

 rather widespread genetic significance in ancient deposits. One-sixteenth 

 millimeter also is approximately the size that separates the productive 

 from nonproductive sands in some oil and gas fields. 



For most subsurface work, comparison with a standard set of samples 

 or sieve separates mounted in glass vials or on microscope slides (fig. 52) 

 will permit adequate description of size. The description should indicate 

 the average size of grain and the spread or "sorting" about that average. 

 In combinations of sand with pebbles, silt, or clay, the main constituent 

 should determine the rock name, and the minor constituent should be 

 used as a modifier, i.e., silty fine sand, fine sandy silt, etc. 



The size of crystalline sediments is commonly recorded as "coarse," 

 "medium," "fine," "very fine," "cryptocrystalline," or "lithographic," but 



" DeFord, R. K., Rock Colors (review) : Ami. Assoc. Petroleum Geologists Bull., vol. 28, no. 1 

 pp. 128-137, Jan. 1944. 



^ Goddard, E. N., Chairman Rock-Color Chart Coram., Rock Color Chart: U. S. Geol. Survey Spec. 

 Pub., 1948. (For sale by Nat. Research Council, Div. of Geology and Geography, 2101 Constitution Ave., 

 Washington 25, D. C, $5.50.) 



