tl:4tl: Tarr — Origin of the Chert in the Bicrling I on Limestone. 



there is an active circulation in limestones, there solvent action 

 is gi'eatest. So why shonld we think that such solutions will 

 deposit silica ? Cox, Dean, and Gottsclialk "^ have recently 

 shown that colloidal silica, which is the usual form in which 

 silica is transported, is precipitated rapidly by calcium carbon- 

 ate in the presence of carbon dioxide. As limestone dissolves 

 when there is an abundance of CO^ in the solution, there would 

 be a rapid deposition of any silica present along the solution 

 cavities. Such, howevei', is not the case. Throughout this 

 area there is no evidence of any deposits of silica in tissures, 

 joints or bedding planes, whatever their position may be. 

 There is removal of limestone, but no silica is deposited in the 

 opening so produced. The reactions in the limestone are 

 dominantly those of solution and not deposition. 



The position of the chert in the beds of limestone, its occur- 

 rence along planes in the absence of a localizing agent, and the 

 impermeability of the limestone are evidence against the 

 replacement theory, unless the replacement takes place on the 

 sea bottom. 



(h) J^o adequate souroe of the silica. — In order to have 

 replacement occur in a rock, silica must be derived from the 

 rock itself or be introduced into it from outside sources. In 

 the case of the Burlington formation the limestone has been 

 shown to contain but very little silica and that was in the form 

 of quartz grains. Embedded in the limestone as these quartz 

 grains are, they are less available than even silica in sandstone 

 would be. Other limestone and dolomites which were studied 

 prove that the silica could not have come from the beds them- 

 selves. In this case an outside source must be sought. What 

 that source might be is not suggested by the advocates of the 

 replacement theory, apparently because no source is evident. 

 Even if a source were available the difficulties of taking the 

 silica into solution and holding it there are great. Most 

 ground waters are either carbonate or saline and thus are weak 

 electrolytes and do not contain much colloidal silica. The 

 exceptions to this are certain hot sodium chloride waters.f 

 From the ground waters themselves there is little evidence 

 favoring the view that they are actively transporting colloidal 

 silica in more than very small quantities. In carbonate rocks 

 the transportation difficulties are even greater, as in the pi'esence 

 of CO2 in the water calcite rapidly causes the precipitation of 

 silica, as shown by Cox, Dean, and Gottschalk. This might be 

 taken as evidence favoring the formation of chert, but it must 

 not be forgotten that the active circulation is along only the 



*Cox. G. H., Dean, R. S., and Gottschalk, V. H., Bulletin 2, vol. iii, pp. 

 9-12, 1916, Mining Exp. Sta., Mo. School of Mines, Rolla, Mo. 

 f Lindgren, W., Mineral Deposits, pp. 50-53, 



