ditions, either present or past, the parameters may well be different. 

 Their true effectiveness for characterizing environments can de deter- 

 mined only after similar compilations have been made for other regions 

 and all are compared. 



Size analyses and the resulting statistical parameters alone cannot identify 

 the agent or environment responsible for the origin of a sediment. However, 

 there is little doubt that data obtained from size analyses will improve the 

 classification and description of clastic sediments. This is especially true of 

 the fine-grained sediments where many so-called very fine-grained sandstones 

 are actually siltstones. Shepard's proposed system (1954) of nomenclature 

 based upon the ratio of sand, silt, and clay demonstrates that more attention 

 must be given to the finer clastic particles if a standard system of nomen- 

 clature for clastic rocks is to be established. Too often the particles below 1/16 

 millimeter are ignored because of the time-consuming procedures required in 

 the analysis of these small particles. A rapid and easy method of determining 

 relative percentages of silt-and-clay size particles in the so-called pan fraction 

 is needed. 



In summary, it may be stated that size analyses have a definite place in the 

 study of clastic sediments. More attention needs to be given to the silt-and-clay 

 size particles, especially in the classification of the sediments and in lithofacies 

 studies. Statistical methods are useful in the study of size analyses, but we 

 should guard against becoming so involved in statistics that we neglect the 

 geological objectives of the anaylses. 



HEAVY MINERALS Minerals with a specific gravity greater than 



about 2.86 are termed heavy minerals. Most 

 clastic sediments are composed of minerals having a specific gravity less than 

 2.86. 



The less common heavy minerals must be separated from the bulk of the 

 sediment before they can be studied. Various separating techniques have been 

 used, but the most common and widely used techniques involve liquids having 

 a specific gravity ranging from 2.8 to 2.96. 



Some of the more common heavy minerals include tourmaline, zircon, 

 garnet, staurolite, and various pyroxenes and amphiboles. Heavy-mineral 

 studies aid in determining the source area or provenance of sediments and in 

 some instances, are useful in correlation problems. The literature on heavy- 

 mineral techniques and interpretations is voluminous, and those interested in 

 the details of the subject are referred to the standard textbooks of Krumbein 

 and Pettijohn (1938) and Twenhofel and Tyler (1941). 



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