Tarr — Origin of the Chert in the Burlington Limestone. 439 



soft calcareous muds from above forced into the cracks. When 

 crystallization eventually occurred in the limestone the material 

 in the veins crystallized alon^j^ with the main niass. 



The shrinkage of the gel through a loss of water developed 

 small cracks on the interior of the nodule, into which some silica 

 was carried and eventually crystallized as adrusy surface of 

 quartz crystals. The slow loss of water from these internal 

 cracks permitted the qnartz to grow into large crystals. Rarely, 

 various sulphides were deposited in these shrinkage cracks. 

 Such cracks are usually found in the small, nearly round 

 nodules. 



When several nodules w^ere formed close together they united 

 to form a larger mass which shows a lobed circular outline 

 from the flattening of the several spherical aggregates. 



When an excessive amount of silica was added to the sea for 

 a time a more or less continuous bed of chert was produced. 

 This would occur when the rate of removal of silica from the 

 land exceeded that of the calcareous materials, and during 

 periods of peneplanation or base leveling. 



{d) Associated minerals. — Colloids have strong absorption 

 properties and since salts of the ordinary metals can freely pass 

 into or through colloids the presence of sulphides in the result- 

 ing chert would be expected if the metallic ions were in the 

 sea water. This is found to be true, as pyrite, sphalerite, chab 

 copyrite, and rarely galena occur in the chert nodules, usually 

 in the cracks and in the interior of fossils. These minerals are 

 generally well crystallized, which is a feature characteristic of 

 minerals crystallizing under such conditions. 



Jf. Relationship of Chert to the Enclosing Rock. 



Everywhere in the Burlington limestone the chert occurs in 

 nodules, elongated masses, and beds. The planes along which 

 it occurs are at irregular intervals because of the variation in 

 the rate at which the material was added to the water and the 

 slowness of concentration. It may or may not appear along 

 the bedding planes. When present in large amounts the 

 numerous nodules and masses in the massive limestone beds 

 are found along a common plane; that is not always a bedding 

 plane. This absence of bedding planes is partly due to the 

 rapid variation in the size of the matm*ials deposited and largely 

 to the absence of tine silts in the areas of limestone deposition. 

 This absence of silty material is evidence favoring the view 

 that low-lying lands were adjacent to the Mississippian sea. 



The occurrence of the chert along a plane which is not a 

 bedding plane should be noted, as those who think that chert 

 is secondary hold that the bedding planes furnish places of 



