Tarr — Origin of the Chert in the Burlington Limestone. 415 



without bands of any kind, some nodules show bands, which 

 are usually horizontal and but rarely concentric. 



The nodules frequently contain cavities which are almost 

 always lined with quartz crystals. Fossils in the chert, espe- 

 cially brachiopods, blastoids and horn corals, are often lined 

 with such crystals. The spiralia of brachiopods are often pre- 

 served and covered with quartz crystals. The major part of 

 the nodule is made up of the massive white or mottled chert 

 but small masses of ci-ystals may be included in the interior 

 and very rarely there are concentric bands for a few inches 

 around these masses. Some nodules consist of fragments of 

 chert with more or less concentric banding. These have the 

 appearance of having been formed, then broken up and re- 

 cemented with silica. 



The exterior is often bleached, soft, and porous, due to 

 alteration. This shell, often an inch or more in thickness, is 

 similar to the tripoli of southwestern Missouri. Such leached 

 zones are generally lacking in the chert of the Ordovician and 

 Cambrian formations in Missouri. 



C. MICROSCOPIC DESCEIPTION. 



The microscope shows the chert to be composed of opaline 

 or amorphous silica, chalcedony, and quartz. The amount of 

 amorphous silica in the Burlington chert is small. The chert 

 consists mainly of a granular mosaic of chalcedony and quartz 

 (see figs. 1, 2, and 4). Scattered throughout the slide are cir- 

 cular areas, from -013 to -026"'™ in diameter, of very fine-grained 

 chalcedony and quartz, the grains ranging from -0006 to •0013"'"' 

 in diameter. These circular areas may possibly be interpreted 

 as representing the original form of the colloidal silica. Such 

 globular forms have been described by many other authors 

 (see especially, Jukes-Brown and Hill, W. Hinde and J. A. 

 Howe), but have usually been regarded as the tests of foramini- 

 fera, although in some instances writers say that they are 

 uncertain that such is the case. It would seem that these 

 globules, which in some instances are amorphous silica, might 

 show the structure of the original precipitate. 



Spherulites are rare in the chert and those present are not 

 perfect. They appear to occupy the areas between the fossils. 

 The dark cross is very indistinct and the spherulites are small. 



Both non-fossiliferous and fossiliferous chert were studied. 

 They were examined especially for remains of originally silice- 

 ous fossils, but none were found. In the fossiliferous speci- 

 mens studied, crinoid stems, fragments of brachiopods, and 

 bryozoans were fairly abundant. Some of the fossils were 

 practically replaced by silica, others were only partially re- 



