3. Minor Constituents: As in the coarse clastic rocks, fine elastics often 

 contain small amounts of materials which enable the subsurface geologist to 

 deduce certain facts relating to the deposit. These minor constituents are about 

 the same as those that occur in the coarse clastic rocks. Fossils are generally 

 more abundant in the fine elastics; in many sections they are limited entirely 

 to the shale portions. Whatever the minor constituents may be, they should be 

 noted in the description. When graphic symbols are provided, they should also 

 be shown in the graphic column. 



4. Luster: The lusters of shales are earthy, resinous (resinous shales are 

 usually dolomitic), waxy, soapy, oily, silky, velvety, and sooty (some very black 

 shales, such as the Chattanooga, are sooty in appearance) . 



5. Structure: Common structures of shales and siltstones are massive or 

 lumpy (called mudstones), platy, laminated, foliated, fissile, splintery, flaky, 

 jointed, and fractured. In some areas fractured shales serve as oil-reservoir 

 rocks; therefore it is important to record the presence of fractures. 



6. Induration: The degrees of induration, or hardness, are disaggregated 

 (as in dried mud in the samples), spongy (usually salty or bentonitic shales), 

 compact, brittle (dolomitic or siliceous), slaty. 



7. Inclusions: Shales and siltstones frequently contain fragments of re- 

 worked and redeposited shales, limestones, and other types of rocks. They may 

 also contain masses of gypsum; anhydrite; chert; iron oxides; nodules; and 

 pellets of barite, pyrite and mud, oolites and concretionary materials, and grains 

 of solid hydrocarbons and coals. Such extraneous grains or masses are termed, 

 collectively, inclusions. 



Carbonate Rocks 



(Plotted in sky blue) 



Because of the great variability of carbonate rocks, they are the most 

 difficult for the beginning microscopist to understand and describe. They range 

 from extremely heterogeneous to extremely homogeneous. The microscopist is 

 faced on the one hand with the problem of summarizing a vast array of detail, 

 and on the other with searching for suffcient minutiae to permit subdivision of 

 the section. The situation is further complicated by the lack of uniformity in 

 the uses of descriptive terminology. The following discussion will attempt to 

 present the subject in a manner that will enable the microscopist to standardize 

 the methods of sample determination to the extent that consistent results may be 

 expected. Many of the tests herein presented have been compared with establish- 

 ed standards and are therefore known to be reliable. However, they are not 

 entirely infallible, and a great deal depends on the ability of the sample exam- 

 iner to carry out the tests with care and to recognize conditions that might 

 interfere with the tests. 



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