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



The mottling and banding in the nodules are readily 

 explained by the original deposition theory but difficult to 

 explain by the replacement theory unless they are (which they 

 are not) structural features of the limestone, for replacement, 

 molecule by molecule, does not shift materials about and cause 

 them to become banded or mottled. 



{d) Fossils in the chert. — The well-preserved internal struc- 

 tures of the calcareous fossils in the chert (brachiopods, entire 

 blastoids and corals) show that they have been protected by 

 some medium. These structures are lacking in the fossils in 

 the limestone, in fact, the blastoids are rarely found in the 

 limestone at all. The fossils in the chert do not show evi- 

 dences of having been crushed, broken and deformed like 

 those in the limestone do. According to the replacement 

 theory this should have been the case, for not all the replace- 

 ment occurred before the recrystallization of the limestone or 

 the deformation movements of the region, in fact replacement 

 should be going on now more actively than in past times. The 

 great numbers of calcareous fossils in the chert (few of them 

 having been siliciiied) point to some mode of protection. That 

 protection was provided by the dense silica gel after it became 

 firm. 



Fossils comprise the most of the calcareous material in the 

 chert. If replacement has occurred why were not the fossils 

 replaced ? The size of the particles could not have been a 

 factor because many very small fossil remains are found in the 

 chert. The axial canal of a crinoid stem which was filled with 

 silica, though all other parts remained calcareous, was found, 

 which shows that the fossils were simply surrounded by the 

 silica and not replaced. 



Thus the fossils in the chert are thought to be original con- 

 stituents of the chert, the organic remains having fallen into 

 the soft colloidal gel on the sea bottom and been completely 

 covered by the addition of more material on the exterior. In 

 this way they were protected and preserved. 



{e) Weathering of the chert. — The chert shows a weathered 

 zone on the exterior which is from a fraction of an inch to two 

 inches or more in thickness. This zone consists of a white, 

 porous material similar to the tripoli of Newton County, 

 Missouri. It is coarser, however, and more porous than the 

 tripoli, and although both have been interpreted as having the 

 same origin the evidence is not yet conclusive. 



This weathered zone has essentially the same chemical com- 

 position as the unaltered material (see analyses). The calcium 

 carbonate is lower, as a rule, and it contains slightly more water. 

 The microscope shows that it consists of grains of chalcedony 

 and quartz which are smaller than those in the dense unaltered 



