Correlations with limestones should involve a different set of conditions. 

 As a general rule limestones are compact or, if porous, contain comparatively 

 large pores and openings. It would be difficult or impossible to grind a thin 

 section of limestone to a thickness allowing the electrons to pass through the 

 specimen and produce an image on the photographic plate. The pores of the 

 limestone are so large as to preclude any precise study of their contour or shape. 

 For these reasons, a possible approach would lie in the study of the residues after 

 the limestone had been dissolved in some suitable solvent. Either the filtrates 

 could be examined for correctable objects, or the residue, which is often largely 

 clay, might lend itself to study in the electron microscope. This latter instance 

 leads into the problem of the study of shales. 



Correlation of Clays 



According to J. Hillier (1946) ". . . particles of various types of clay have 

 probably been subjected to more examination by means of the electron micro- 

 scope than any other type of material." Some clays are composed of grains of 

 about 50 angstroms in thickness and a few angstroms wide. Studies of the nature 

 and correlation of such minute particles in the electron microscope depend on 

 characteristic shapes and not on chemical combinations. 



One of the clays used extensively in laboratories and refineries for filtering 

 is called Attapulgus clay, so named for Attapulgus, Georgia. Chemical analyses 

 of this clay show it to be chiefly montmorillonite, a hydrous aluminum silicate, 

 but the individual microcrystalline masses cannot be identified or resolved under 

 the best light microscope. Figure 8-3 is an electron-microscope picture of this 

 clay, X 20,000, showing an abundance of masses of minute fibers. These fibers 

 are the so-called microcrystalline masses that cannot be identified or resolved 

 under the polarizing microscope. 



Infusorial earth is described as a "siliceous earth made up largely of sili- 

 ceous fragments of Infusoria, used as filling material and as a filtering and ab- 

 sorbing agent." Figure 8-4 is an electron-microscope picture of this material, 

 X 20,000, showing fibers very similar to the Attapulgus clay. The similar shapes 

 of constituent parts of these two materials attest to their similar physical prop- 

 erties. 



Clays might lend themselves to study and correlation in the electron micro- 

 scope in the following ways: 



1. The submicroscopic mineralogy and crystallography of clays might be 

 studied. Minute crystals of rutile have been identified in titanium-rich clays. 

 These crystals were too small to be identified under a light microscope. Detailed 

 studies should bring out several similar instances of submicroscopic mineralogy 

 that could be of value in correlation. 



2. The presence of submicroscopic organic forms too small to be identified 



145 



