426 Tarv — Origin of the Chert in the Burlington Limestone. 



as it must have been formed before the erosion which produced 

 the unconformit}' between the beds. 



Further, the chert found in these conglomerates and soki- 

 tion cavities exhibits every feature seen in the chert embedded 

 in the limestone around the cavities or under the conglomer- 

 ates. It is no more altered and shows no evidence of having 

 increased ii> size since the erosion interval between the Penn- 

 sylvanian and the Mississippian, or since Cambrian or Ordo- 

 viciau times in the older occurrences. Specimens from the 

 conglomerate could not be distinguished from those taken from 

 the fresh limestone. 



It will thus be seen that the fixing of the time of formation 

 of the chert in the Mississippian period greatly shortens the 

 time available for ground waters to produce the chert nodules, 

 since all the aggregation of tlie silica must have occurred pre- 

 vious to the erosion in pre-Pennsylvanian times. But what is 

 also very strong evidence against the theory of origin by ground 

 waters is the fact that since Mississippian times there has heen 

 no growth or addition of material to the chert nodules^ as is 

 shown by the absolute similarity of the chert found in the 

 limestone, the conglomerates, and the solution cavities. Cer- 

 tainly ground waters have had ample opportunity to do more 

 work of this character in the long interval of time since the 

 Pennsylvanian than they had during the time the Mississippian 

 formations were accumulating. Therefore, if the chert is 

 foi-med by circulating waters, why have the nodules not grown ? 

 It cannot be claimed that the ground waters wei'e less efficient 

 solvents of silica at that time than at the present. Accurate 

 and acceptable statements of the character of the solutions 

 which were able to dissolve the silica from a rock in prefer- 

 ence to dissolving the more easily soluble calcium carbonate 

 are not generally recognized, though under the theory of the 

 ground water origin of chert such solutions must have existed. 

 In part, the ground waters were doubtless what Lindgren 

 designates as the *' calcium carbonate waters in sedimentary 

 rocks," because the dominant circulation would be in limestone 

 and shales ; and, in part, sulphate waters from the Pennsylva- 

 nian shales. The Pennsylvanian shales were deposited above 

 the Burlington limestone and contain considerable pyrjte that 

 would make possible the liberation of the sulphate radical. 

 The following analyses show this to be true. Two spring 

 waters from Pennsylvanian rocks near Columbia, Missouri 

 show a great preponderance of the sulphate radical. Two 

 spring waters from the Burlington limestone are dominantly 

 calcium carbonate waters. 



It is not probable that there has ever been much of a change 

 in the character of the ground w^aters in this region, because 



