Tai'V — Origin of the Chert in the Bu7'lingto7i Limestone. 443 



tion of the calcareous muds which formed the limestone. This 

 silica is believed to have come from the land, probably during 

 periods of peneplanation, and then to have been dispersed by 

 the currents in the sea. Precipitation occurred by means of 

 electrolytes in the sea vrater, and the silica gel assumed a glob- 

 ular form on the sea bottom, w^iich form passed, in large part, 

 into an elliptical shape through the weight of the gel itself and 

 later on account of pressure from the beds above. The distribu- 

 tion of the chert in the formation and its relationship to the 

 fossils are interpreted to favor this view. Absence of siliceous 

 organisms in both chert and limestone is also evidence for the 

 theory. 



8. Evidence Against the Replacement Theory. 



Since the view that chert owes its origin to the replacement 

 of limestone by silica is so prevalent, the evidence which the 

 writer has collected against that theory will be given. This 

 evidence will be given under the following heads : (1) the posi- 

 tion of the chert in the limestone ; (2) no adequate source of 

 the silica ; (3) structure of the nodules ; (4) the fossils in the 

 chert, and (5) the weathering of chert. 



(a) Position of the chert in the limestone. — Chert occurs 

 in all parts of the beds of limestone. It may be along the 

 bedding planes (not very common) or anywhere in a bed, 

 but always it is along a plane, a fact which is recorded in many 

 places in the literature concerning chert. This occurrence in 

 any part of a bed or formation without reference to structural 

 features which might induce deposition along a given plane is 

 a fact that has not been suitably explained by the replacement 

 theory. There is nothing in the chemical character of the 

 rocks that will account for this method of distribution of the 

 chert. 



In this same connection there is the question of the permea- 

 bility of the limestone. If replacement is to occur the solutions 

 bearing silica must enter the rock in some way. Water does 

 not circulate through limestone except along the divisional 

 planes, and when the chert nodules are at a distance from such 

 planes this distribution should be considered as evidence that 

 the chert was not formed by replacement of the limestone. In 

 most dense iine-grained limestone the porosity is less than one- 

 half of one per cent, hence the permeability would be much less. 

 Such spaces are primarily capillary, and it is not probable that 

 the circulation of solutions in capillary spaces would ever 

 replace a large mass of limestone with silica. 



The belief that solutions are especially active as depositional 

 agents along bedding and joint planes in limestone can be 

 viewed in the opposite light, viz., that they are more active as 

 solvents along these planes. It is well known that wherever 



