Tarr — Origin of the Chert in the Burlington Limestone. 447 



chert. The chalcedony appears to be growing at the expense 

 of the quartz. Water may find access to the interior through 

 joints and the same tj'pe of alteration takes place as on the 

 exterior. 



These changes show that the nodules, instead of increasing 

 in size at the present time as they should be doing if they are 

 due to circulating ground waters, are actually being attacked 

 by the ground water where it can come in contact with them. 

 However, the weathering of chert appears to take place very 

 slowly, for chert on the slope and in the streams shows no evi- 

 dences of alteration, while that in the conglomerate at the base 

 of the Pennsylvanian and Ordovician is still fresh. The cal- 

 careous materials are slowly removed when the chert is exposed 

 to the weather. This leaves the fossiliferous fragments of the 

 chert very porous. 



Shepard* has called attention to the fact that when a piece 

 of chert is broken, the two pieces will not fit together exactly. 

 This is due to the very brittle character of the chert. It has 

 been involved in the movements that have occurred in the 

 region and the stress to which it was subjected produced incipi- 

 ent cracks and strains. The chert when broken along these 

 cracks springs back into its original unstrained shape. Tem- 

 perature changes are very effective in the disaggregation 

 of the chert. As a result the talus consists of small, sharp, 

 angular fragments of chert and larger pieces of limestone. On 

 the face of a bluff the chert weathers out faster, forming 

 re-entrants. 



There is no evidence to support the view of Ulrich,f Lee,:}: 

 and others that the chert in the dolomites of S.E. Missouri is 

 a product of weathering. Lee states that coarse chert float 

 leads up to apparently non-siliceous dolomite, but that the 

 weathered surface can be seen on close inspection to contain 

 various shapeless forms of chert. These pieces are assumed to 

 be in process of growth at present. He states further that 

 " freshly cut surfaces of such beds in quarries ^nd road cuts do 

 not show chert." Yet Lee (page 13, of his report) gives a 215- 

 foot section of the Gasconade formation with chert distributed 

 throughout most of the formation, and on page 12 states that 

 " the lower beds of the Gasconade penetrated by the drill holes 

 are denser and contain more dense bluish chert than the upper 

 part of the section." Evidently chert does not occur in the 

 beds below the surface and certainly all this chert is not of the 

 type which occurs in beds. In short, Lee furnishes sufficient 



*Shepard, E, M., Geology of Greene County, Mo., Geol. Surv., vol. xii, 

 p. 111. 



fBain. A. F. and Ulricb, E. O., " Copper Deposits of Mo.," U. S. Geol. 

 Surv., Bull. 267, p. 29. 



:{:Lee, W., Mo. Bureau of Geol. and Mines, vol. xii, pp. 15-19, 89-40. 



