438 Tarr — Origin of the Chert in the Burlington Limestone. 



the sinaller ones. As growth proceeded, the banding and 

 mottling of the chert was developed, due to carbonaceous 

 materials that are believed to have been included in the chert 

 as it grew. The banding is not so common as the mottling 

 because it was exceptional for the globular masses of colloidal 

 silica to be rolled around by the sea water, thus acquiring the 

 carbonaceous matter. These masses maj have been broken up 

 at times by movements of the sea water, especially after the 

 gel had set, thus furnishing the angular fragments in other 

 masses of chert. As no masses of limestone have ever been 

 observed which contained these circular bands, it is concluded 

 that they do not owe their origin to replacement, but are origi- 

 nal structures of the chert. 



These small gel masses would tend to become spherical and 

 might retain this form up to a certain size. Above this size 

 they would tend to become elliptical (see lig. 5) through their 

 own weight, just as a drop of mercury does. The Unal shape 

 would be due to the pressure of the overlying sediments which 

 would tend to flatten the nodular masses. This is the typical 

 shape of 99 per cent of the nodules seen in the Burlington lime- 

 stone and there are millions of them in it. It is also the character- 

 istic shape of the chert and flint nodules in other formations. 



In this connection it is worth noting that iron, manganese 

 and aluminum oxides often occur in rounded nodular forms 

 which are evidently inheritances of a colloidal state. 



A very common feature of the nodules is the great number 

 of cracks developed in them. (^See figs 8, 9, and 10.) These 

 cracks are without definite orientation and extend down into 

 the chert from a fraction of an inch to thi*ee inches or more 

 and may be half an inch wide. They never pass through the 

 chert. These cracks are filled with limestone that is contin- 

 uous with the limestone surrounding the chert. This vein of 

 limestone was forced into the chert while the limestone was 

 still unconsolidated. Later, apparently while the stylolites 

 were being formed, there was a reopening of some of the cracks 

 and coarsely crystalline yellow or white calcite was deposited in 

 them (see fig. 11). 



These cracks developed during the consolidation of the silica 

 gel. The colloidal silica gel contained large amounts of water 

 which were gradually lost as the gel passed through the opaline 

 state to the finely crystalline chalcedonic state, where it is 

 essentially anhydrous. Much of the water was lost during the 

 early stages of consolidation and this part left no evidence of 

 its former presence in the structural features. During the last 

 stages the results of the shrinkage caused the chert to crack. 

 This was especially true in the large masses which were being 

 flattened out by the weight of the overlying materials, and the 



