124 Ziegler — Siliceous Oolites of Central Pennsylvania. 



materia] is brownish yellow, weakly translucent, and fibrous 

 quartz. Occasionally small vugs are left whose sides are 

 roughly parallel to the sides of the separate oolites and which 

 have been filled with beautifully colored fibrous chalcedony. 

 The outside margin of the rim is a ragged, irregular line. 

 About an average distance of 02 mi " from the outer edge there 

 is a well defined row of quartz oolites. Two or three tenths 

 of a millimeter on the outer side of this row the oolitic struc- 

 ture of the limestone becomes apparent. Close about the con- 

 cretion the limestone shows evidences of recrystallization and 

 hence has probablj' lost its oolitic structure on this account. 



The inner crystalline ring (fig. 13) of the concretion proves 

 to be made up of large well-defined calcite crystals showing 



Fig. 14. 



Fig, 14. Oolitic chert concretions, with inner crystalline ring (A), and 

 chert nucleus (B). 



sharp twinning lines, which are penetrated by a dense inter- 

 growth of quartz grains. Many of the quartz grains of this 

 inner ring have a nucleus enlarged by thinly fibrous chalce- 

 dony. On account of the dense intergrowing structure of the 

 fibers, the quartz acts as an isotropic mineral, hence is uniformly 

 black throughout a whole revolution of the stage between 

 crossed nicols. In fact, at first, this complete extinction led to 

 the assumption that all the quartz had the same optical orienta- 

 tion. Closer observation disproved this assumption. The 

 separate grains are rounded in general appearance, with their 

 boundaries with the cementing calcite curiously pitted into 

 small angular embayments, which are invaded by the calcite. 

 The latter is in large well-defined grains and over areas up to 

 one and one-half centimeters in diameter has the same optical 

 orientation. It was estimated that one grain of the calcite had 

 from 100 to 270 quartz grains enclosed in it. At both the 

 boundaries with the outer chert layer and the inner nucleus 

 the calcite presents an irregular front with many small points 

 and peninsulas extending out into the quartz. These are 

 always confined to the cementing material. The manner in 

 which the calcite extends into the quartz rims and envelops 

 the separate oolites suggests the spreading of a flood through a 

 district covered with many small rounded elevations. 



The Origin of the Oolite. — Study of the field-occurrence of 



