182 The Petrography and Genesis of Sediments 



have been exposed to atmospheric weathering, and at that time probably 

 lost a part of its feldspars by decomposition. Generally the feldspars are 

 about 10% of the light portion. The high percentage (25%) in sample 

 10 is probably due to derivation of the material from nearby. 



The observations on mica also agree in a general way with Collet's con- 

 clusions in that mica is not abundant in the samples with primary glau- 

 conite, while in very micaceous samples primary glauconite does not 

 occur. But this may be due mainly to the fact that the glauconitic 

 sands are usually coarser and in such coarse sediments mica is generally 

 more scarce. More advanced decomposition of the mica in glauconitic 

 samples was not noticed. 



