Tarr — Origin of the Chert in the Burlington Limestone. 425 



dates the erosional interval between the Mississippian and 

 Pennsjlvanian periods. It is not possible to fix the age any 

 closer 'than this. That the chert is older than the stjlolites is 

 evident from the fact that the stylolites pass around the chert 

 nodules. Though it does not indicate anything as to the age 

 of the Burlington chert, it is important to note that at the base 

 of some of the lower Paleozoic formations there is occasionally 

 a conglomerate which contains sharply angular fragments of 



Fig. 11. 



Fig. 11 (nearly natural size). Sketch of a crack in the chert (2) filled 

 with limestone (1) and later after partial opening, with calcite (3). Such 

 cracks always die out downward. 



fresh chert derived from formations below. An example of 

 this is the Joachim formation just over the Jefferson City 

 formation (Ordovician) in Boone County, Missouri. 



Another proof that the age of the chert is greater than that 

 of the formation above it is found in the cave deposits in the 

 Potosi dolomite (Upper Cambrian) in Washington County, 

 Missouri. The materials in the cave deposits are quartz and 

 chert. The latter is in angular fragments sometimes as large 

 as one inch in length. These fragments were derived from the 

 adjacent rocks and hence are of Cambrian age. During all 

 the time since then, they have not been altered. 



These various occurrences indicate that the period during 

 which the chert was formed was very short in all formations, 



Am. Jour. Sci. — Fourth Series, Vol. XLIV, No. 264. — December, 1917. 

 30 



