EXPLANATION OF PLATE 21. 



Microscopic Thin Sections of Flint and Other Rocks. 



Fig. 1. Oolitic CHERT. This is made up of rounded concretionary masses of chalce- 

 douic silica held together by an interstitial cement, whicli is largely quartz 

 in a finely granular condition, but in part chalcedony. The oolitic forma 

 are rendered very impure by iuclosures of dust-like particles and black, 

 opaque particles of iron ore, while the interstitial material is compara- 

 tively colorless. 



(Cape May, New Jersey.) 



Fig. 2. Gray chert. This section shows a mass of irregular rounded oval, greatly 

 elongated and sometimes angular, areas with curvilinear outlines, of a 

 dirty-brownish color, and which are sometimes wholly without action on 

 polarized light and sometimes show the cryptocrystalline structure char- 

 teristic of chalcedony. These areas are interspersed with silica in the form 

 of colorless chalcedony and granular quartz. 



(Cat. No. 71607, U.S.N.M. Clark or Lewis Covinty, Missouri. Plate 23, tig. G.) 



Fig. 3. Quartzite. An indurated siliceous sandstone, consisting of well-rounded 

 grains of colorless quartz bound into a compact mass by a secondary dispo- 

 sition of interstitial silica. This secondary silica has so oriented itself 

 with regard to the original sand grains as to convert the rock into ao 

 aggregate of imperfectly outlined quartz crystals, of which the original 

 sand grains form the nuclei. 



(Cat. No. 26268. U.S.N.M. Potsdam, New York.) 



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



