EXPLANATION OF PLATE 17. 

 Microscopic Thin Sections of Flint. 



Fig. 1. Flint.' Iu every way essentially similar to Cat. No. 139130. The segrega- 

 tion areas sliow the chalcedonic particles more distinctly in the . fan- 

 shaped or radiating forms characteristic of the mineral. 

 (Cat. No. 101U57, U.S.N.M. Havelse, Denmark. Plate 24, fig. 9.) 



Fig. 2. Flixt. Slightly less uniform in structure than Cat. No. 99866, but other- 

 wise essentially the same. These gregations of coarser particles are in 

 the form of irregular strings and' spots, rather than iu oval areas as in the 

 other specimens. 

 (Cat. jSTo. 1U0138, TJ.S.N.M. Camp Barbet, Mouy, Meudon, France. Plate 2;i, tigs. 9, 10.) 



Fig. 3. Scrapers. Grouudmass of this rock is essentially similar to that of speci- 

 mens Cat. Nos. 139130, 101057, from Brandon, England, and Havelse, Den- 

 mark, respectively. An occasional grain of quartz may be distinguishable, 

 but the only difference of note is a large amount of black amorphous 

 impurities with which the rock is injected. The chalcedonic forms noted 

 in Cat. No. 139130 as suggestive of foramiuifera are more abundant and so 

 plainly defined as to leave no doubt regarding their nature. 

 (Cat. No. 100259, U.S.N.M. Spiennes, Belgium. Plate 23, tig. 3.) 



1 Mineralogical descriptions by Dr. (1. P. Merrill, T'. S. National Museum. 



