EXPLANATION OF PLATE 18. 



Microscopic Thin Sections of Flint. 



Fig. 1. Flint.' This specimen difters from Cat. No. 100259, or Plate 17, fig. 3, first, in 

 coarseness of texture, and second, in showing an abundant sprinkling of 

 crystalline granules of ([uartz. The slide is made up of irregularly oval 

 areas of chalcedonic particles, sometimes rendered almost opaque by fer- 

 ruginous and carbonaceous impurities, the interstices being occupied by 

 the material dift'ering only in degree of purity, the carbonaceous matter 

 being confined mainly to the oval areas, the appearance being as though 

 the interstitial deposit was made subse(iuently and under more favorable 

 conditions (as regards purity). 

 (Cat. Xo. 99008, U .S.X.M. Grand Tres.signy (Indre-et-Loire), France. Plate 24, fig. 8.) 



Fig. .2. PiXK ILIXT. This, like those from Europe, is mainly a compact crypto- 

 crystalline mass of chalcedonic silica, with segregation areas of the same 

 material showing the characteristic spherulitic and fan-shaped arrange- 

 ment of the particles. In a few instances the slide shows small areas of 

 granular crystalline quartz. The rock is injected with iron oxide sufficient 

 to give it a reddish or yellowish tinge, and the foraminifera remains noted 

 in the European specimens are quite lacking. 

 (Cat. Xo. 98344, U.S.N.M. Flint Ridge, Licking County, Ohio. Plate 24, tig. G.) 



Fig. 3. Black flint. This slide differs from the last (Cat. No. 98344) mainly in being 

 of finer texture and in carrying au abundance of muddy and opaque car- 

 bonaceous matter which is not distributed uniformly through the mass of 

 the rock, but occurs rather in blotches and streaks. The slide shows 

 further numerous irregular sharply angular areas with curvilinear out- 

 lines so filled with impurities as to be of a dirty-brown color, and which 

 are wholly without action on polarized light, indicative of silica in an 

 opalescent form. There are numerous elongated cylindrical bodies which 

 are without action on polarized light, which are suggestive of something 

 of an organic nature. 

 (Cat. Xo. 98344a, U.S.X.M. Flint Ridge, Licking County, Ohio. Plate 24, fig. 4.) 



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



