EXPLANATION OF PLATE 16. 

 Microscopic thin Sections of Flint. 



Fig. 1. Flint.' An extremely i5ne-grained aggregate of clialcedonic ])article8. The 

 structure is cryptocrystalline, so fiuc that the optical properties of the 

 individual particles can not be determined. Tliroughout this crypto- 

 crystalline base or groundmass are scattered numerous small colorless 

 polarizing particles and occasional segregation areas of the chalcedonic 

 material in a coarser or more granular condition. Beyond this, the 

 microscope sliovvs only minute amorphous yellowish and black particles 

 which are presumably ferruginous and carbonaceous matter. Organic 

 remains (spouge, spicula, and diatoms) were not specially sought for, but 

 we find an occasional form in outline suggestive of a chalcedonic cast of 

 the shell of a foraminifera. Section nearly colorless. 

 (Cat. No. 139130. U.S.N.M. Brandon, England. Plate 24, fig. 7.) 



Fig. 2. Flint. Substantially the same as Cat. No. 139130, with the exception that 

 the section shows a greater number of the spherical areas of radiating 

 particles of chalcedonic quartz. No forms observed that can be identified 

 with certainty as foraminifera. 

 (Cat. No. 139112, U.S.N.M. Grimes Graves, England. Plate 23, tig. 1.) 



Fig. 3. Flint. For all the microscope discloses, this might be a section from speci- 

 men Cat. No. 139112, from Grimes Graves. V'his specimen was found by 

 the author in a prehistoric workshop at Dorchester, Dorsetshire, England, 

 and came from one of the neighboring Hint mines. 



(Cat. No. 99866, U.S.N.M. Dorchester, England. Plate 23, flg. 7.) 



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



