ORIGIN OF CHERT IN LIMESTONES. 819 



from the surrounding limestone, for in regions in which there are local 

 colonies of silica- secreting organisms there are sure to be similar organisms 

 among the limestone-secreting organisms. 



Therefore I hold that the nodules and minor bands of chert in the 

 limestones are mainly due to segregations by solution and deposition, but 

 that many of the larger and more persistent bands of chert are at places 

 where the silica was originally deposited in large part by the siliceous organ- 

 isms, although the silica of such bands has usually been rearranged. Both 

 original and secondary masses of chert may occur in the same formation. 

 For instance, in the lead and zinc district of southwestern Missouri, in the 

 Carboniferous limestone are one or more large and continuous bands of 

 chert which can be mapped as separate members of a formation, and these 

 are believed to be chiefly original. But in this limestone are innumerable 

 minute bands and lentils, from a fraction of a centimeter to several centi- 

 meters in thickness, and extending from a few centimeters to a few meters, 

 to which the colonization theory can not be fully applied, and which were 

 doubtless largely segregated as above described. Bands of intermediate 

 width are doubtless due in varying degrees to original deposition and to sec- 

 ondary segregation. Where limestone containing chert reaches the surface, it 

 is well known that the amount of the cherty material in the belt of weather- 

 ing is relatively great This is largely due to the more ready solubility of 

 the limestone than of the chert, in consequence of which the percentage of 

 chert is increased. At or near the surface the removal of the limestone 

 may be almost or quite complete, and thus produce a partly coherent or 

 even incoherent belt of almost pure chert along the surface. In many 

 cherty limestone regions, where the soil is residual, one might think that 

 nearly the entire rock was chert, so prominent does this material appear in 

 the soil and in the rock near the surface. 



The silica originally present in a given mass of limestone in consequence 

 of organic precipitation is not the only source of silica for nodules and bands 

 of chert. The silica of the limestone in the belt of cementation may be 

 greatly increased. This may be clone in two ways. First, where the 

 limestones reach the surface, and the segregation above mentioned takes 

 place in the belt of weathering, a part of this silica is being continuously 

 dissolved and transported downward into the belt of cementation This 

 process is especially active where the silica has not become wholly 



