JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Voh. 9 MARCH 4, 19 19 No. 5 



PETROLOGY. — Microscopic examination of clays. R. E. Som- 

 ERS, Cornell University. (Communicated by David White.) 



Most of the clays referred to in this report were examined 

 microscopically in order to determine the minerals contained in 

 them, and in addition thin sections of some of the burned sam- 

 ples were studied so far as time would permit in order to obtain 

 some idea of the changes that took place in burning. 



The investigation is not to be regarded as an exhaustive one, 

 as much still remains to be done along this line, but so far as it 

 went, it is thought that the results are of interest. 



Table i gives the minerals that were noted in the different 

 specimens examined, and also their approximate abundance. 



IDENTIFICATION AND CHARACTER OF MINERALS 



Quartz. — When in medium or coarse grains, quartz can be 

 readily discriminated by means of its index of refraction, low in- 

 terference color, and lack of cleavage. There are no other min- 

 erals in clays which resemble it under these conditions. When, 

 however, it is in small grains, it greatly resembles kaolinite. Its 

 index of refraction is then of little assistance, the interference 

 color is lowered to about that of the kaolinite, and its determina- 

 tion has to depend upon its more angular shape and its more 

 rapid extinction, or "quicker wink." Such grains are generally 

 too small to make use of the difference in optical character. 



While quartz was noted in many of the clays examined, it 

 was particularly abundant in the residual ones. Most of the 

 Wilcox clays from the Embay ment area showed but little. 



Kaolinite. — This can be distinguished by its orthodox charac- 



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