Tarr — Origin of the Chert in the Burlington Limestone. 417 



which is usually white and which is a weathered portion of 

 the chert. This zone varies in thickness but is usually about an 

 inch thick. Slides were made of the fresh and the altered por- 

 tions of the same nodule in order to determine if possible what 

 changes had taken place in the formation of this outer zone. 

 The only difference observed was in the granularity of the 

 chalcedony. The entire weathered zone was a mass of gran- 

 ular chalcedony (see fig. 2). The various fossils were readily 



Fm. 3. 



Fig. 4. 



Fig. 3(x40). Banded chert, Burlington limestone. A mosaic of quartz 

 and chalcedony. Dark bands due to carbonaceous material. 



Fig. 4 /x 40). Fresh chert, Burlington limestone. Coarsely crystalline 

 quartz and chalcedony. Large black and white areas are quartz, radial areas 

 chalcedony. 



seen and many grains of calcite were scattered through the zone, 

 showing that calcium carbonate has probably been removed by 

 leaching. The grains are very small, ranging from -006 to 

 •0013'"'" in diameter. 



Some of the slides show the banding whicli is a megascopic 

 feature of the chert nodules (see fig. 3). The bands were found 

 to be due to minute black specks of what appeared to be or- 

 ganic matter but whicii may be pyrite in part. The black 

 specks were also seen in the lighter colored portions of tlie 

 chert but were not so numerous as in the darker bands. 



Other materials observed in the chert were grains of pyrite, 

 usually small and never exhibiting crystal faces ; a few grains 



