150 Subsurface Geologic Methods 



present in some pyrite and glauconite residues. Natural dolomolds result- 

 ing from weathering are common on certain types of outcrop samples. In 

 dolomite the dolomolds are assumed to be the impressions from dissolved 

 dolomite rhombs, but in shale the cavities are likely to be a result of dis- 

 solved interstitial calcite. Disseminated abundant fine dolomite or calcite 

 crystals in chert, silt, or shale will leave a very finely porous residue, too 

 fine to be observed except under high magnifications. The residue of a 

 sample with large quantities of dolomite rhombs will have an intersecting 

 lacework of fragile skeletal dolomolds, while a sample with a few rhombs 

 will leave scattered dolomolds in the insoluble matrix. Dolomolds may be 

 large or small, but generally all in any one fragment will be essentially the 

 same size. 



Use of Insoluble Residues 



The study of insoluble residues is a supplement to and not a substitute 

 for lithologic sample examination. The cost of preparing and filing resi- 

 dues and the longer time necessary for the more detailed and careful exam- 

 ination of them are factors that must be considered. The mass character- 

 istics of the major constituents of insoluble residues generally have enough 

 similarity horizontally and vary enough vertically to serve for identification 

 and correlation of lithologic units within a thick section of calcareous rock. 



Lithologic similarities of thick sections of nonfossiliferous calcareous 

 rocks prevent their subdivision into thinner zones for more-detailed corre- 

 lation and identification and structural mapping. Insoluble constituents 

 having diagnostic characteristics may be obscured by the volume of the 

 fragments in a lithologic sample and by being embedded in a solid 

 matrix. These constituents are liberated, concentrated, and exposed by 

 solution of the matrix. Diagnostic material such as Foraminifera, some 

 types of chert, dolomolds, disseminated pyrite, fossil replacements, euhe- 

 dral crystals, mineral or clastic inclusions, and silt aggregates are not 

 observed or recognized until they become residues. 



Residues reflect clastic conditions, sea-bottom environment, current 

 action, and adjacent land-mass conditions, which 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 lithologic 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 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; 



