Ziegler — Siliceous Oolites of Central Pennsylvania. 121 



Figure No. 5 shows a thin-section taken at the contact line 

 between the calcareous and siliceous oolite. A study of the 

 thin-section reveals the fact that the oolites, which are calca- 

 reous, in all cases show a distinct nucleus of one or more small 

 rounded quartz grains, while the siliceous oolites only rarely 

 have the nucleus preserved, but usually show its replacement 

 by fibrous chalcedony or a very fine aggregate of chert. This 

 would seem to indicate that silica-laden waters are here replac- 

 ing a calcareous oolite. 



An analysis of an oolite specimen of this type gave E. H. 

 Barbour the following result : * 



Calcareous Oolite. Siliceous Oolite. 



Si0 2 3-70$ 56-50^ 



Al .O. 



Fe,0 3 f ^ 



CaC0 3 88-71 16-84 



Mo-C0 3 8-09 260 



H 2 ._ 12-54 



Spec, gravity 2-654 2-688 



Type Four. — In addition to the above types of occurrence 

 we have one at least 1200' above the highest definite siliceous 

 horizon found in place. Several excellent sections of the inter- 

 mediate strata are available, but no oolitic chert was observed. 

 In this horizon the oolites occurred in chert concretions, while 

 in the others definite layers are the rule (fig. 15). They are 

 fresh, occurring in a very recent road-cut along Spring Creek 

 about two miles south of Belle Fonte. They are made up of two 

 separate layers, an outer dense cherty and an inner calcareous one, 

 and in several also show a chert nucleus. The inner ring is com- 

 posed of a coarse crystalline material showing separate faces up 

 to one-half inch in diameter, which are penetrated by a fine 

 dotted-like intergrowth strongly suggesting graphic structure. 

 Under the microscope the outer ring, the nucleus, and the dot- 

 ted intergrowth prove to be quartz, while the crystalline matrix 

 of the latter is coarsely granular calcite showing well-defined 

 twinning striatums. 



The limestone is oolitic (fig. 11). Distinct nuclei could be 

 discerned, but it was impossible to determine their exact char- 

 acter, except that they were predominately calcite with traces 

 of silica apparently crushed together. There is no uniformity 

 in their shape ; circular, elliptical, both straight and curved rod 

 like, and more irregular types were present. The oolitic 

 structure is probably syngenetic with the formation of the 

 * This Journal (3), vol. xl, page 246. 



