EXPLANATION OF PLATES 779 



Plate 27. — Micro-sections of oolitic Structures 



FiGUBE 1. — Calcareous oolites from the Cambro-Ordovician limestones north of 

 Bethlehem, Pennsylvania. 

 The recrystallization is complete. About one-half of each oolite 

 is dark, the remainder pure white. No explanation of this 

 can yet be offered. X 15. 



Figure 2. — Section from the margin of an oolitic chert nodule from the Upper 

 Beekmantown, Belief onte, Pennsylvania. 

 Nicols crossed. X 15. This shoves numerous rhombic crystals of 

 dolomite in the siliceous oolites and the granular dolomitic 

 ground-mass. 



Figures 3 and 4. — ^Another section similar to that shown in figure 2. 



Figure 3, with plain light ; figure 4, with crossed nicols. These 

 figures show the zones of rhombohedral crystals around the 

 oolites. 



Figures 5 and 6. — Siliceous oolites from the Cambro-Ordovician beds north of 

 Bethlehem. 



Figure 6 is from a section of the same specimen as figure 5, but 

 cut perpendicular to that section. This shows the flattening 

 which these oolites have suffered under dynamic action. 



The sections illustrated by figures 1, 5, and 6 were loaned to the 

 writer by Mr. F. J. Keeley, Curator of the Vaux Mineral Col- 

 lection in the Academy of Natural Sciences, Philadelphia. 



