Dean — Formation of Missouri Cherts. 411 



Art. XXVI. — The Formation of Missouri Cherts; 

 by Reginald S. Dean. 



A recent paper by Tarr 1 on the cherts of Missouri has 

 made it seem that the views on this subject which have 

 been developed during- the last few years at the Missouri 

 School of Mines might be profitably presented. A pre- 

 liminary report was made in Bulletin No. 2, Mis- 

 souri Mining Experiment Station, November, 1916, and 

 although our work is not yet complete certain ideas have 

 been developed which seem to be in advance of those 

 previously presented. 



The general occurrence and form of the chert nodules 

 have been well described by Tarr. There are however 

 a number of characteristic occurrences and forms which 

 have a definite bearing on the origin which have not been 

 sufficiently emphasized. Chert nodules occurring in a 

 limestone formation which has small shale layers will 

 nearly always have a greater number of nodules in the 

 shale than in the surrounding limestone and there is 

 usually a segregation of the chert along the bedding 

 planes. In these cases the beds around the nodule are 

 bowed out showing that the nodule has grown since the 

 beds were consolidated. In the case of the Jefferson 

 City formation the amount of the bowing of the beds from 

 this cause is often sufficiently great to induce small faults 

 of six inches to one foot in displacement. Although the 

 general form of the nodules is that of a smooth ellipsoid 

 a large number of the smaller nodules have protrusions 

 which indicate that during formation the feeding of the 

 nodules was not equal from all sides. The cherts are in 

 general cryptocrystalline and compact but some show 

 openings at the center which are filled with a tripili. 



A fundamental question is the relative age of the chert 

 and the surrounding rock. The bowing of the beds pre- 

 viously referred to seems to be irrefutable evidence in 

 favor of the epigenetic origin of the chert since the beds 

 could not have been distorted by the growing chert had 

 they not been formed at the time; further evidence is 

 found in the uneven growth of the chert mentioned. Tarr 

 argues that since chert nodules are found in limestone at 

 some distance from openings of any sort that the chert 



1 This Journal, 44, 409, 1917. 



