Subsurface Laboratory Methods 179 



with the flat surface on the glass. This operation requires some practice, 

 as it must be accomplished quickly and at a given time. To accomplish 

 the remounting, a glass slide covered with an even, thin layer of balsam 

 is placed on the hot plate and the balsam is cooked within an "instant" of 

 the required consistency. The first mount is then laid upon the new slide 

 with the flat surface of the grains in contact with the newly cooked balsam. 

 It is allowed to remain there until the undissolved balsam melts. (The time 

 required here is short; the new balsam should be correctly cooked upon 

 completion of the operation. One must remove rapidly.) The slides are 

 now removed to an asbestos pad; the upper one (the temporary mount) 

 is pressed down, moved with a circular motion, and gently slid in the direc- 

 tion of the narrowest dimension off" the lower slide. A small quantity 

 of mineral grains may adhere to the first mount, but their loss is not seri- 

 ous as an excess of grains were originally used. For the final step, the 

 slide is allowed to cool. Then it is ground to the required 0.03 millimeter 

 in thickness, and the cover glass is mounted as recommended for an ordi- 

 nary thin section. 



Manual Preparation 



In the foregoing procedure for thin-section preparation, a laboratory 

 with power equipment was assumed. Thin sections are often needed when 

 this equipment is not available. Excellent sections may be prepared manu- 

 ally from rock chips or even small well cuttings by carrying out the grind- 

 ing operations on glass plates. The manual procedure is often to be pre- 

 ferred with friable material, very small chips, or soft rocks such as lime- 

 stones and shales. For base-camp use, a small, compact grinding outfit 

 can be assembled with which a careful worker can make thin sections of 

 any desired rock. 



Thin-Section Study 



A complete petrographic analysis requires the use of every available 

 tool. The use of thin sections is merely one phase in the breakdown of 

 a rock into its component parts. It is, however, an important phase, and 

 usually the choice of the final method of study to be employed in the study 

 of a given rock is determined from thin sections or a combination of thin 

 section, polished surface, and hand specimen. The finer-grained sediments 

 such as shales, siltstones, and mudstones yield comparatively little infor- 

 mation to the investigator in thin section, whereas, the medium-grained 

 clastic rocks offer a fertile field for micro-study. The flow sheet (fig. 74) 

 shows the organization and methods that may be used in a modern petro- 

 ' graphic laboratory. 



Some of the more important data and information that may be ac- 

 quired from thin-section studies of sedimentary rocks are summarized in 

 the paragraphs below. 



