Tarr — Origin of the Chert in the Burlington Limestone. 421 



tervals along planes in the beds (see fig. 7). This lack of a 

 definite relationship to the bedding planes should be noted. A 

 layer of nodules may occur in the central part of a thick mas- 

 sive bed of limestone which shows no evidence of a bedding 

 plane along the plane of the chert even after weathering. The 

 vertical interval between the planes of chert varies consider- 

 ably ; it may be from a few inches to mauy feet ; but, what- 

 ever the distance, the planes of chert are always parallel to each 



Fig. 6. 



Fig. 6. Chert nodules, Burlington limestone, Columbia, Mo. 



other and to the bedding planes. The horizontal interval be- 

 tween the different nodules appears to be more uniform, as 

 there seems to be a rather definite spacing of the nodules. 

 This spacing of the planes of chert without reference to the 

 bedding planes of the limestone and yet always parallel to them 

 has a very important bearing upon the origin of the chert. 



The shape of the chert nodules appears to be the result of 

 flattening. The elliptical and the lenticular shapes are sugges- 

 tive of this, as though the elliptical shape might be due to the 

 spreading of the soft, globular masses of gel under their own 

 weight. The bands frequently seen in the chert are further 

 evidence of flattening. The great majority of these bands are 

 horizontal and approximately parallel to the bedding planes. 

 Those which have the same general shape as the exterior of 

 the nodule always have the bands broadened at the ends of the 



