Subsurface Laboratory Methods 183 



Thin-Section Correlation 



Consolidated sedimentary rocks are often sufficiently individualistic 

 enough that a careful megascopic inspection is adequate for correlation 

 purposes. On the other hand, in a thick, monotonous series of shales, of 

 limestones, of sandstones, or of combinations of these, megascopic identi- 

 fication and correlation are frequently uncertain or impossible. Identifica- 

 tion and differentiation of a specific horizon may be accomplished by a 

 microscopic study of thin sections. Possible microscopic evidences that 

 may be used include textures, structures, cementing minerals, detrital 

 material, degree of crystallization, amount of recrystallization, finer or- 

 ganic forms, and other morphological criteria. The photomicrographs of 

 figure 75 illustrate a few of the microscopic features of four different sand- 

 stones. 



Vug and Opening Studies 



Normal-sized thin sections are limited in area. Ordinarily, in petro- 

 graphic work the slides used are 45 x 25 millimeters, and the finished rock 

 section rarely covers more than two-thirds of this area. For vug and open- 

 ing studies, larger areas are required. The writers have made and used to 

 advantage thin sections mounted not only on a 2 x 2-in. kodachrome, slide- 

 cover glasses, but also on standard 3^ x 4-in. lantern, slide-cover glasses. 

 These sections are usually somewhat thicker than 0.03 millimeter, but they 

 may be projected in an ordinary slide projector equipped with polaroids. 

 If the sections are cut in parallel series, a fair picture of the size and con- 

 tinuity of "vugation" is presented. 



Storage of Sections 



Problems of storage of specimens and sections for future reference 

 continuously plague the research worker. Each man has his own solution. 

 One space-saving device is to mount the thin section, heavy-mineral con- 

 centrate, and a chip sample from the same specimen on a three-inch bio- 

 logical glass slide rather than on the conventional petrographic slide. 



Summary 



Thin sections are not the answer to all sedimentary problems, but they 

 disclose the internal view of a rock from which the research man may add 

 to his present knowledge of the rock; and at the same time, they aid in 

 determining his future mode of attack. 



