Tarr — Origin of the Chert in the BurlirKjton Limestone. 445 



larger divisional planes and if there is deposition it should 

 occur there. This is not the case, for the chert is most often 

 found in other places than along the divisional planes. 



(c) Adverse evidence of the striccture of the nodules. — The 

 elliptical form of the nodules is assumed bj many to mean that 

 silica was deposited around a nucleus and, then, since the con- 

 cretions were thought to occur always along bedding planes, 

 that material was added to the sides of the nodules, thus giving 

 them the elliptical shape. Since the nodules occur indepen- 

 dent of such planes this argument has no weight. 



If replacem.ent has occurred, either the structural features 

 of the replaced limestone should be preserved, or the limestone 

 should be displaced hj the growth of the concretion. Careful 

 examination of thousands of specimens has failed to reveal a 

 single one with the structural features of the limestone pre- 

 served. Neither has a single nodule showing displacement of 

 the beds been found. It was thought that one such occurrence 

 was found in the Jeiferson City dolomite (Ordovdcian), but 

 careful study led to the conclusion that the displacement was 

 apparent only, being really due to the deposition of the over- 

 lying sediments around the mass of colloidal silica. 



The occurrence of angular brecciated fragments of banded 

 chert in the chert nodules is evidence against replacement, for 

 no solution could deposit a small, sharply angular, banded area 

 of chert within a larger mass. These fragments are not replaced 

 limestone, for no such structural feature was ever found in the 

 limestone. This feature is undoubtedly original. 



The cracks in the chert were not developed in hard, solid 

 chert, for if a deformative movement were to fracture such a 

 rock, the crack would certainly extend through the nodule as 

 the later fractures have done. However, the limestone-filled 

 cracks which had developed in the chert did not extend 

 through them and are not necessarily in the same position on 

 the upper and lower sides. 



Furthermore, the fact that the limestone in the cracks is 

 continuous with that outside the chert nodule proves that the 

 cracks developed before the hmestone was consolidated. This 

 being the case, the views that the chert is of recent (geologi- 

 cally speaking) origin are incorrect. 



It is impossible to believe that any ground-water solution 

 could replace a mass of limestone and leave a few long narrow 

 areas as cracks, so this view can be dismissed. The cracks are 

 undoubted proof of the early formation of the chert and of its 

 deposition as an original mass of colloidal silica. 



The cracks on the interior, which have drusy linings and 

 occasional crystals of sphalerite, pyrite, or chalcopyrite, tell the 

 sa'ne story as do those in the outer portions. 



