PENNSYLVANIA SILICEOUS OOLITES 



763 



He assigned this cloudy appearance to impurities taken up by the sand 

 grain in the process of the deposition of the silica about it. No form or' 

 composititon of these impurities was mentioned. An examination with 

 a very high power of the microscope, however, shows them to be minute, 

 microscopic, rhombohedral crystals of calcite contained within the silica. 



Figure 1. — Diagrammatic Reproductions of oolitic Structure 



Showing how the four identical oolites of A, when cut by the plane of a section as 

 Indicated by the dotted line, may give rise to four apparently different types of oolites 

 of somewhat different size. This probably explains the greater part of the apparent 

 variation in the State College type of siliceous oolites. 



The outer zone of secondary enlargement is always apparently clear, but 

 may contain a few of these minute rhombohedral crystals of calcite. 



Around the original or secondarily enlarged nucleus, and forming the 

 greater part of the mass of the oolite, is siliceous material either in gran- 



