and adjacent land-mass conditions that may supply various types of source 

 materials. These factors may change independently over short or long periods of 

 time. If the source of material and the conditions of deposition or precipitation 

 of calcareous matter remain fairly constant for a long time, no significant litho- 

 logic variations would result that might serve to identify a stratum. A slight 

 change involving the source, type, or amount of clastic furnished to a lime- 

 depositing environment might not affect greatly the lithologic appearance of a 

 sediment, but such material when left as a residue would be diagnostic and 

 would serve for correlation and identification. The amount of silica, iron, or salts 

 in the sedimentary basin might change and give pyrite, siliceous limestone, vari- 

 ous types of chert, and other minerals or constituents of diagnostic value — all 

 of which might be independent of clastic material or changes in land-mass 

 conditions or source material. 



Circulating water and replacement and alteration of constituents before 

 and after lithification would change the original residues. These changes, if of 

 sufficient magnitude, might be observed in a lithologic examination of samples, 

 but only the study of residues would show the small changes that might be 

 useful in a detailed subdivision of beds. Correlation using residues of secondary 

 origin could only be used locally or as far as the effect of the modifying con- 

 ditions could be traced. 



Correlations for distances greater than 50 miles are risky, unless some 

 significant wide-range constituent can be determined, because the residues will 

 change as the sedimentary environment changes. Obviously correlation using 

 any specific zone of residue types would be less reliable in a basinward or land- 

 ward direction than it would be laterally in a right-angle direction. 



Correlation of individual thin beds may be difficult because of lateral and 

 verticle changes within the sedimentary environment. The subdivision of a 

 thick calcareous section and the inclusion of nondiagnostic thin beds into zones 

 make correlation possible. Identification of the zones is based on such factors 

 as sequence of beds, position in the section, percentage of residue, association 

 of types of residues, and dominant characteristics with chief reliance on dom- 

 inant characteristics. A distinctly significant residue may identify certain zones, 

 although other residue constituents may be present, and even though the 

 diagnostic residue is not the dominant one. An assemblage of residue con- 

 stituents often determines the correlation or identification just as an assemblage 

 of fossils serves for determination. Both microscopic and macroscopic fossils 

 replaced with insoluble material are valuable in some zones. 



Positive identification of some subdivisions is difficult with only a few 

 samples, unless a significant break or change in residue occurs within the interval 

 examined. For example, assume that a limestone 1400 feet thick is divided into 

 6 zones having intervals of 350, 100, 200, 400, 300, and 50 feet. It only ten 5- 

 foot samples were available from zone 1 at the top, it would be difficult or im- 



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