between the Appalachian and the Rocky Mountains. The beds receiving the 

 most attention have been Upper Cambrian and Lower Ordovician, but Silurian, 

 Devonian, and Mississippian beds have been extensively studied. 



The space allotted here would be inadequate to give worth-while descrip- 

 tions of the subdivisions of the thick sections of calcareous rocks in the various 

 parts of the United States. Anyone concerned would profit more to confer with 

 workers familiar with local areas and sections. 



Insoluble residues for paleontological studies have not been as widely used 

 as they could be. Much very valuable information may be obtained regarding 

 environments, population distribution, and morphology (Ireland, 1956). Ob- 

 viously the method is limited to arenaceous foraminifera, phosphatic structures, 

 siliceous radiolarians, diatoms, sponge spicules, and other forms whose parts 

 are insoluble in hydrochloric or acetic acid. The residue technique is most 

 fruitful with micropaleontology. Most microscopic forms which are too small 

 to see when embedded, must be liberated for observation. Most forms found in 

 the residues are clean, unweathered, unbroken, and otherwise undamaged. The 

 loss of specimens is minor, and their recovery is simple when contrasted to the 

 treatment necessary to recover forms from shale or clays. Many new forms and 

 previously unknown evolutionary changes have been discovered from study of 

 the insoluble residues of calcareous beds. 



The extensive and successful application of residues to petroleum geology 

 proves the value of residue studies, but few petroleum geologists have published 

 results. The Insoluble Residue Library of Midland, Texas, is financed and 

 operated by a dozen or more companies, which employ specialists for residue 

 examination or subscribe to a special service furnished by the Midland Residue 

 Research Laboratory. The Missouri Geological Survey uses insoluble residues 

 as a standard procedure for the correlation of formations older than Pennsyl- 

 vanian; its collection of residue samples is probably the largest and the finest 

 in the world. The state geological surveys of Illinois, Kansas, Missouri, 

 Oklahoma, and Texas and the United States Geological Survey have utilized the 

 study of residues as a regular part of their programs for subsurface work. 



PLOTTING RESIDUE DATA The symbols and overprints given in Figure 

 AND DESCRIPTIONS 5-3 are recommended for standardized use. 



They are essentially those used originally by 

 the Missouri Geological Survey; but certain modifications, combinations, and 

 additions make it conform to the standardized terminology. The symbols now 

 used by the Missouri Geological Survey may be found in Grohskopf and Mc- 

 Cracken (1949), and a typical log published by McCracken (1955) show a few 

 modifications of the earlier published set. Color was used formerly by the writer 



89 



