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



placed, and still others were unaltered. In all instances of 

 alteration the calcite was replaced bj quartz. The most 

 coarsely crystalline areas and the largest grains of quartz are 

 in the sih'cified fossils. Scattered through the grains of quartz 

 there are many very small grains of calcite, these grains being 

 the unreplaced residue of the original fossil. These calcite 

 grains are so minnte that they do not show any cleavage planes 

 or other crystalline characteristics, except the interference tints. 



Fig. 1. 



Fig. 2. 



Fig. 1 (x40). Fresh chert, Burlington limestone, Columbia. Mo. Shows 

 fine mosaic of quartz and chalcedony ; the larger areas of the latter show the 

 interference cross. 



Fig. 2 (x 40). Altered chert, Burlington limestone. Shows fine mosaic of 

 chalcedony in the altered outer portion of a chert nodule. 



"When a fossil is only partially replaced the quartz crystals 

 form a border around the calcite. This border may occupy 

 fully one-fourth of the area of the fossil. 



The dense white chert consists almost entirely of a mosaic of 

 chalcedony and quartz. The grains which compose this mo- 

 saic are very minute. Probably not more than one per cent 

 of the chert is amorphous silica. The amorphous silica remains 

 dark during a complete rotation of the stage of the microscope 

 between crossed nicols. Since it may show an interference 

 cross at times, it may be more abundant than is stated. By re- 

 flected light it has a pale milky-white luster. 



Much of the chert in the Burlino-ton has an outer zone 



