Chert in many instances has included sand grains, which may be confused 

 with ooliths. Shells are absent, however, and the clear quartz of the said grain 

 may be observed. 



Pseudoolites or shadow oolites resemble oolites and may resemble quartz 

 sand grains. The boundary between the matrix and the oolith is indistinct, 

 however, and the central portion, which cannot be identified as quartz is only 

 a shade lighter or darker than the other portions. Pseudoolites may be ooliths 

 or sand grains that have been resorbed, thus destroying any formerly existing 

 boundaries. 



Dolomolds occur chiefly in chert residues from dolomites, rarely in chert 

 from limestone. Dolomolds are common in shale residues and are present in 

 some pyrite and glauconite residues. Natural dolomclds resulting from weather- 

 ing 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 dissolved 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, whereas 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 The study of insoluble residues is a supple- 



INSOLUBLE RESIDUES ment to and not a substitute for lithologic 



sample examination. The cost of preparing 

 and filing residues and the longer time necessary for the more detailed and 

 careful examination of them are factors that must be considered. The mass 

 characteristics of the major constituents of insoluble residues generally have 

 enough similarity horizontally and vary enough vertically to serve for identifica- 

 tion 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 correlation 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 liberat- 

 ed, concentrated, and exposed by solution of the matrix. Diagnostic material 

 such as foraminifera, some types of chert, dolomolds, disseminated pyrite, fossil 

 replacements, euhedral 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, 



86 



