Subsurface Laboratory Methods 143 



ing may easily cause the overflow and loss of considerable material. A few 

 minutes after the initial application, additional acid may be added, but 

 only experience will tell how much, generally not more than one-third to 

 one-half the capacity of the receptacle. After several hours of digestion 

 the samples should be washed once or twice to remove spent acid, pre- 

 cipitates, and undesirable material. The second application of acid will 

 generally complete the digestion, although one application may be suf- 

 ficient. Small applications of acid will digest samples which obviously are 

 chert, sand, or shale. Incomplete digestion will leave dolomite pellets with 

 rough, jagged surfaces and rounded pellets of limestone. When samples 

 are incompletely dissolved, individual euhedral dolomite rhombs may be 

 a large part of the residue. Final washing should be thorough to remove 

 all traces of acid and prevent scum, caking, or coating on the residues. 



Clay and fine silt are generally decanted in routine work. Little or no 

 work has been done with the fine residues, and their value for correla- 

 tion and identification is yet to be determined. Only outcrop or core sam- 

 ples can be used for study of clay and silt residues, because caving and 

 other contamination of well samples obscure diagnostic features and 

 makes uncertain the identification of indigenous fine clastic material. 



Residues may be dried in an oven, on a hot plate, or on a sand bath. 

 Dry residues are brushed into a pan or funnel for transfer into glass vials, 

 which may be labelled on the cap, cork, or a paper sticker. Permanent 

 storage requires a painted label or glazed surface on the side of the vial, 

 because silverfish enjoy eating the glue from stickers. One-dram vials 

 hold ample residue for study and require very small storage space. Trays, 

 drawers, original vial boxes, or special boxes are suggested methods of 

 storage. 



Description of Residues 



The most common insoluble residues are chert, chalcedony, dissem- 

 inated silica, clastic and crystalline quartz, aluminous matter, and replaced 

 fossils. Anhydrite, gypsum, feldspar, glauconite, hematite, pyrite, fluorite, 

 and sphalerite are the most common minerals, but other insoluble miner- 

 als are found. 



Table 3 is a modified arrangement of the original chart published 

 with the paper on standardized terminology. ^° The terminology is based 

 on description rather than genesis of the residues because genesis of many 

 constituents is unknown, vague, or controversial. Many possible types of 

 residues are given a place in the table, although their existence has not 

 been confirmed. Each term is clear-cut and restrictive, and within certain 

 limits a residue fragment may be pigeon-holed. It should be emphasized 

 that types of residues grade into other types, and, as some specific frag- 

 ments may not be easily placed, workers may place a fragment under a 

 different but related type in the classification. 



'"Ireland, H. A., et al.. Terminology for Insoluble Residues: Am. Assoc. Petroleum Geologists 

 Bull., vol. 31, no. 8, pp. 1479-1490, Aug. 1947. 



