Ziegler — Siliceous Oolites of Central Pennsylvania. 123 



The outer rim and also the nucleus are made up of quartz, 

 showing the same oolitic structure (tig. 12). The oolites are 

 irregular in shape and size, and are made up of concentric 

 layers of fibrous chalcedony about a paler colored irregularly 

 shaped nuclei. In thin sections the color of the oolites is a 

 very pale grayish and translucent yellow, the fibrous structure 

 being indistinct, faintly visible only under the crossed nicols. 



Fig. 11. 



Fig. 12. 



Fig. 13. 



Fig. 11. Oolitic limestone, in which, the oolitic chert concretions occur, 

 x 40. 



Fig. 12. Outside boundary of the chert nodules, showing the replacement 

 by quartz. x 29. 



Fig. 13. Inner crystalline ring, showing partially replaced quartz oolites 

 in large calcite crystal, x 50. 



The outside rim of the separate oolites is clear white, transpar- 

 ent, and well-defined fibrous chalcedony. The cementing 



