Subsurface Laboratory Methods 151 



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 exam- 

 ination of samples, but only the study of residues would show the small 

 changes that might be useful in a detailed subdivision of beds. Correla- 

 tion using residues of secondary origin could only be used locally or as 

 far as the effect of the modifying conditions could be traced. 



Correlations for distances greater than fifty 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 landward direction than laterally in a direction at 

 right angles. 



Correlation of individual thin beds may be difficult because of lateral 

 and vertical changes of the sedimentary-environment time. The subdi- 

 vision 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, per- 

 centage of residue, association of types of residues, and dominant charac- 

 teristics with chief reliance on dominant 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 constituents often deter- 

 mines the correlation or identification just as an assemblage of fossils 

 serves for determination. Both microscopic and macroscopic fossils re- 

 placed 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 1,400 feet 

 thick is divided into six zones having intervals of 350, 100, 200, 400, 300, 

 and 50 feet. If only ten 5-foot samples were available from zone 1 at the 

 top, it would be difficult or impossible to identify their positions in the 

 zone, although the zone itself could be identified. If the samples over- 

 lapped into zone 2, then the boundary could be recognized, and it could 

 be stated than 300 feet or more of zone 1 was absent. 



Many cherts are alike in color and texture, and similar cherts in 

 two different zones would prevent identification, unless an associated 

 residue was diagnostic or a zone boundary was passed. The similarity 

 of the brown, smooth chert in the Lower Devonian and the Upper Ordo- 

 vician in west Texas has been mentioned previously. If a chert in zone 1 

 was similar to a chert in zone 4 in the section postulated in the last para- 

 graph, the two zones could easily be confused. If the set of ten samples 



