PAPERS ON GEOGRAPHY AND GEOLOGY 353 



THE USE OF THE MICROSCOPE IN THE STUDY 

 OF SUBSURFACE STRATIGRAPHY 



J. E. Lamar, Illinois Geological Survey, Urbana 



For many years paleontology and lithology have been 

 the bases for correlation of sedimentary rocks, but up to 

 comparatively recent years only the grosser character- 

 istics have been made use of for this purpose. It has 

 also been customary to study the cuttings from wells 

 for many years, but again it is only of late, with the 

 stimulation of interest in the field of sedimentation, that 

 a really critical examination of the lithology of cuttings 

 has been at all extensively undertaken. With the closer 

 study of the lithology of rocks has come either contem- 

 poraneously, or as a result thereof, the closer study of 

 the minute or microscopic fossils. 



The microscopic study of sediments, as has been im- 

 plied, may be divided into two general classes, litholog- 

 ical and paleontological. The first concerns itself with 

 the state of aggregation of a rock and the constituent 

 minerals forming the aggregate. That is, it deals with 

 the mineral composition of rocks, the size and shape of 

 the mineral grains, the nature of the cementing mater- 

 ial, and in some cases, with the chemical composition. 

 If the rock is crystalline the nature of this phenomenon 

 is also considered. Various methods are employed to 

 determine these data, but at present the two most com- 

 mon modes of examination consist in the study of rock 

 powders and of thin sections. The first, as the name 

 implies, consists in the examination of the rock either 

 as an artificially produced or natural powder. As the 

 cuttings from wells drilled with a churn drill are com- 

 monly broken up rather finely, this method of study is 

 especially applicable in such cases. 



This detailed study of lithology has been used very 

 satisfactorily in the case of well cuttings for separating 

 one formation from another, as follows: Estimates are 

 made of the percentages of sandstone, shale, and lime- 

 stone in the various samples under a microscope, and 

 these percentages are then plotted as a composite or 

 synthetic log. The formational changes are shown by an 



