ilO Taw — Origin of the Chert in the Burlington Limestone. 



egress for the solutions. If there are no beddnig planes to 

 guide the solutions, how can the distribution ol' the chert along 

 a plane be explain* d ? If there are solutions in the lime^.one 

 in other places thau along the bedding planes (which seems 

 unlikely), it is not possible that thej could develop the nodules 

 and masses of chert along a plane without a physical guide. 

 Furthermore, there is nothing of significance in the chemical 

 composition of the limestone which could cause this type of 

 distribution. Though it has been reported that all gradations 

 of chert into limestone and dolomite can be found, the writer 

 by careful examination has found that the contact of the chert 

 with the limestone is usually sharp. However, this would not 

 necessarily be the case if the silica were an original deposit, 

 for there is no reason to believe that the deposition of silica 

 should cease abruptly. Kather there should be more or less 

 of a gradation, and the definiteness of the line of contact is, 

 therefore, a remarkable feature. It is evidence of the control 

 that the larger mass of silica has over that in the soft muds 

 around it. The outer parts of the nodules are usually richest 

 in fossils or calcite, a thing to be expected. 



Silicified limestone, on the other hand, shows gradations 

 which are easil}^ detected. 



The relationship of the chert to the limestone is best explained 

 by the theory of their contemporaneous deposition. 



5. Fossils and the Silica. 



The relationship of the fossils to the chert is significant. 

 Though not all of the chert is fossiliferous this is the usual 

 mode of occurrence. The fossils are scattered throughout the 

 chert, or they may be confined to the outer part, or to bands 

 running through the nodules. This outer zone is from one to 

 five inches wide. Some fossils occur in the lower parts of the 

 chert nodules, but most of them are found in the upper parts. 

 All the fossils found in the Burlington limestone may be 

 obtained from the chert. Crinoid stems are the most numer- 

 ous, just as they are in the limestone. 



The numerous fossils in the chert are composed of calcite, 

 only a small percentage of the total number being silicified. 

 In the microscopic sections studied the outer part of the cal- 

 cite of some of the fossils was slightly silicified, but this is not 

 the rule, for the thousands of casts of fossils examined in the 

 weathered chert show" that the fine markings of the fossils are 

 perfectly preserved in the silica. Often the spii*aiia of the 

 brachiopods and the septa of the horn corals are perfectly pre- 

 served in the chert by being silicified or by being coated with 

 silica. In many instances, w^hen the fragment of the crinoid 



