EXPLANATION OF PLATE 19. 

 Microscopic Thin Sections of Flint. 



Fig. 1. White klixt. ' This section shows a ground of chalcedonic particles inter- 

 spersed with numerous irregular areas filled with an outer zone of chalce- 

 donic material and iuterstitially with calcite. The structure may be com- 

 pared with amygdaloids of volcanic rocks. It shows a single shred of 

 ferruginous mica. 

 (Cat. No 59726, U.S.N.l'^ ^ fke County, Illinois. Plate 23 fls-. 5. ) 



Fig. 2. Black flint disk. This slide difl'ers from anything we have had, in that, 

 while it is composed mainly of chalcedonic silica, it has, under the micro- 

 scope, an almost granular aspect, and carries, moreover, a large amount of 

 calcite. There is very little true quartz, the larger granules and crypto- 

 crystalline portions showing the optical properties of chalcedony. The 

 most marked characteristics of the rock is the abundauce of calcite as 

 above noted, and which occurs in the form of aggregate and minute irregu- 

 lar particles as tine as dust, distributed throughout the entire mass of the 

 rock, and also in well-defined rhomboidal crystals. Nothing of organic 

 forms is recognizable. Qualitative test shows the presence of lime, 

 alumina, and iron, as well as silica. / 

 (Cat. No. 15350, U.S.N.M. Cas.s County, Illinois. Dr. J. F. Snyder. Plate 24, fig. 5.) 



Fig. 3. Chert. A dense brownish aggregate of chalcedony and calcite, with many 

 elongated rounded and oval areas now occupied by calcite crystals, but 

 which are suggestive in outline of Fusulina. 

 (Cat. No 26582, U.S.N.M. Kansas.) 



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



