354 ILLINOIS STATE ACADEMY OF SCIENCE 



abrupt increase or decrease of one or more of the three 

 major sediments, the sandstone, limestone, or shale. 



The second method of study, that of thin sectioning, 

 gives more exact details than the first, and in the case 

 of consolidated rocks, such as limestones and sandstones 

 or unpulverized samples, is often the more satisfactory. 

 It necessitates the making of a thin section which in- 

 volves the expenditure of some little time but affords 

 a detailed knowledge of the crystalline structure and 

 mineralogical character of the specimen examined. 



In the study of the paleontology of sediments with a 

 microscope, the procedure consists in the examination of 

 the rock as small fragments or thin sections. If the rock 

 to be studied is a shale, it is soaked in water and washed 

 under a strong stream of water until the fine shale par- 

 ticles have been removed and only the larger and heavier 

 fragments of calcareous or siliceous material remain. 

 This residue contains whatever minutia or small fossils 

 the original shale contained. It is dried, sieved to var- 

 ious sizes, and examined under the binocular microscope. 



Limestones, particularly shaly limestones, are treated 

 in a somewhat similar fashion. They are soaked in 

 water, then very carefully granulated with particular 

 care that none of the finer chips be lost. The resulting 

 fragments and powder are dried, sieved, and also ex- 

 amined under the binocular microscope. 



As previously stated, this method is satisfactory with 

 shales or shaly limestones, or even granular limestones, 

 but its use with dense, fine-grained limestones is rather 

 doubtfully successful. In studying this last kind of rock 

 it may be necessary to resort to thin sections and from 

 them determine, if possible, what diagnostic life forms 

 the rock contains. If the fossils in the limestones are 

 silicified, however, very satisfactory results can be ob- 

 tained in some cases by breaking the specimen into small 

 pieces and allowing them to etch in dilute hydrochloric 

 acid. The etching removes the calcium carbonate and 

 leaves behind whatever siliceous or argillaceous material 

 was contained in the rock. In this residue the silicified 

 minutia are found if present. 



