140 Subsurface Geologic Methods 



being cut through the mineral grains, cement, and pores, give a view of 

 these internal characteristics of the rock and their relationships to one 

 another that cannot be obtained with a binocular microscope. Thin sec- 

 tions have the disadvantage of requiring the use of petrographic micro- 

 scope and a geologist skilled in its operation. Also the preparation of 

 the sections, especially of cuttings that must be cemented together before 

 being sectioned, requires some time and skill. 



INSOLUBLE RESIDUES 



H. A. IRELAND 



An insoluble residue may be defined as the material remaining after 

 rock fragments have been digested in acid. Hydrochloric acid is generally 

 used, but acetic acid is occasionally used if the preservation of delicate 

 fossils or other structures is desired. Residues, such as shale, pyrite, gyp- 

 sura, anhydrite, and glauconite, are not siliceous; therefore, the term, 

 "siliceous residues" cannot be applied correctly. The chief residues are 

 quartz and various types of chert, with chert the most diagnostic for 

 identification and correlation, 



McQueen and Martin in 1931 published methods of preparation, 

 terminology, and practical application of insoluble residues to surface 

 and subsurface correlation and identification of calcareous rocks. The 

 work of Martin is not known so well as that of McQueen although it is a 

 significant contribution. The use of insoluble residues was not wide- 

 spread prior to 1938. After 1940 rapid advances were made with the 

 application of residue work to petroleum geology. The United States 

 Geological Survey and many state surveys now have many publications 

 based wholly or in part on insoluble residue work. Most of the residue 

 work in Texas was developed independently of that of McQueen, and a 

 diversity of nomenclature resulted. In 1946 Ireland called a conference 

 of active workers from the central United States, which resulted in the 

 publication of standardized terminology and a chart, which is published 

 herein in modified form (See table 3.) 



Preparation of Residues 



Types of Samples 



The materials treated for insoluble residues are well cuttings, cores, 

 and outcrop samples. The most desirable outcrop samples are channel 

 samples or a composite mixture of each exposed stratum within a five-foot 

 or other closed interval. Point-to-point correlation is rarely possible, 

 since there is very little probability of sampling exactly the equivalent 

 point some distance away. A six-inch layer outcropping within a five-foot 

 interval will not represent the whole interval, and it cannot be correlated 

 with the equivalent interval a mile away, which may have a six-inch layer 



