EXPLANATION OF PLATE 20. 



Microscopic thin Sections of Flint and other Rock. 



Fig. 1. Black flint.' Essentially the same combiuation as Cat. No. 15350, from 

 Illinois. 



(■^. X. Plates 19, fig. 2 ; 24, fig. 5.) 



Fig. 2. Argillite. Schistose aggregate of quartz particles and much undetermin- 

 able gray matter which might readily pass for partially metamorphosed 

 argillitic material of a sedimantary rock. 

 (Cat.No. lliDOlO, U.S.N.M. District of Columbia, vicinity of Chain Bridge. Plate 24, fig. 3.) 



Fig. 3. Argillite ? The groundmass of this rock is made up of a gray material show- 

 ing between a crossed nicol. No distinct crystalline forms, but breaking up 

 as the stages revolve into irregular areas polarizing faintly in light and t 

 dark colors. The properties are too obscure to be of determinative value. . 

 Throughout this groundmass are scattered numerous irregular areas of I 

 quartz, of feldspars which have crystallized in situ, and small shredaof I 

 greenish mica. I am unable to satisfy myself regarding the petrographio 

 nature of the rock, and can only suggest that it may be an argillaceous 

 sedimentary which has undergone a certain amount of dynamic meta- 

 morphism. 



(Cat. No. 99269, U.S.N.M. Trenton, New Jersey.) 



' Mineralogical descriptions by Dr. G. P. Merrill, U. S. National Museum. 



