tions. Technical names for the different grain sizes have been avoided in favor 

 of every-day adjectives which everyone will understand. 



TABLE 3-11 



Grain-Size Classification 



Millimeters 



Coarse over 2.00 



Medium 2.00 to 0.25 



Fine 0.25 to 0.05 



Very Fine 0.05 to invisible 



Below the range of 12 X power are two classes: 



Sublithographic dull luster, earthy, opaque 



Lithographic porcelaneous, semi-translucent 



The relative proportions or distribution of grain sizes can be recorded 

 as follows: 



"0.01 to 0.15" indicates a gradation of grain sizes within the stated range. 

 "0.01, max. 0.15" indicates a preponderance of fine material with occasional 



large grains. 

 "0.05" means an even-grained rock with the stated average grain size. 



Character of Grains: The origins of carbonate grains, as indicated before, 

 are widely different, and a description of such a rock is not complete unless 

 the character of the constituents is included. It is usually not sufficient to state 

 "fragmental limestone" or "oolitic limestone." The fragments and oolites should 

 be described. Oolites may be classed as spheroidal, spherical, elliptical, irregular, 

 flattened, etc. Fragments are described in much the same way as sand grains: 

 sharp, angular, subangular, rounded, and globular (or spheroidal). When 

 discernible, the origin of the fragments should be given also; i.e., coral, shell, 

 or limestone fragments. 



Texture: The textures of many types of limestones will be indicated if 

 grain shapes and sizes are stated as suggested above. However, certain textural 

 terms are widely used in the descriptions of crystalline varieties and are well 

 established in the vernacular of the oil geologist; and in view of this fact, they 

 are given in Table 3-III. It has been observed that certain types of porosity 

 are frequently associated with specific textures. While the textural and porosity 

 types are not always associated, the frequency is sufficient to warrant attention 



Structure: Structural features of carbonate rocks include stylolites, fractures 

 microfractures, laminae, banding, crinkling, concretions, whorls, and brecciation 

 Many of the structural characters can only be inferred from the small chips 



Color: The "normal" colors of limestones and dolomites are gray, white 

 buff, and brown. Less frequently they are red, orange, various hues of green 

 purple, and black. These colors may occur in combination in a variety of pat 

 terns, such as mottling, banding, speckling, and grading. The manner of color 



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