A PECULIAR OOLITE FROM BETHLEHEM, PENNSYLVANIA. 



By Edgar T. Wherry, 



Assistant Curator, Division of Mineralogy and Petrology. 



The material here described occm-s in Lerch's quarry, on the west 

 side of Monocacy Creek, one-quarter mile north of the northwest 

 corner of the borough of Bethlehem, and a mile and a half norlh of 

 Lehigh River, near the center of the AUentown quadi-angle, in North- 

 ampton County, Pennsylvania. The rock quarried is a magnesian 

 limestone belonging to the "AUentown" formation, which represents 

 the Upper Cambrian (Ozarkian) in this region. 



A variety of geologic phenomena is exhibited at the locality. In 

 the southern portion of the quarry the beds are thrown into a fine 

 anticline, the north limb of which is cut by a vertical fault marked 

 by brecciation and shearing of the rock. In the northern part the 

 beds are steeply upturned, and show slickensides where they have 

 slipped over one another. Tension cracks filled with secondary 

 dolomite and quartz crystals are numerous, and a miniature cave con- 

 taining tiny stalactites and beautiful twinned calcite crystals was at 

 one time opened in the course of quarrying operations. Well-devel- 

 oped colonies of several types of cryptozoon, splendid ripple-marked 

 surfaces, and strata crowded with oolite grains are also to be seen at 

 various horizons. 



In these oolitic beds the separate grains, or ooids, are usually from 

 1 to 1^ mm. in diameter, but occasionally attain 5 mm. They are 

 normally spherical in shape, although elongated or irregular lumpy 

 forms are not micommon, especially among the larger ones. The 

 color of the grams is usually a dark gray, and they stand out dis- 

 tmctly against the paler tint of the enclosing rock. 



The details of their structure can best be seen when sections are 

 exammed under a low-power microscope. They sometunes show a 

 distmct concentric an-angement of lighter and darker layers with 

 weU-rounded grains of quartz or calcite as nuclei, but in most cases 

 recrystallization has obliterated aU traces of both nucleus and con- 

 centric rings. A carbonaceous pigment is usuaUy visible, and minute 

 irregular or subangular masses of limonite, probably pseudomorphous 

 after pyrite, are dotted here and there, being especially abundant 

 near the surfaces of the grams. 



Proceedings U. S. National Museum, Vol. 49-No. 2102. 



153 



