38 GEOLOGICAL FORMATIONS 



SILURIAN. 



The lower Silurian rocks of Missouri are confined entirely 

 to the eastern border of the state in the immediate vicinity of 

 the Mississippi river. In the southeastern part of the district, 

 where the entire series is fully exposed, there is a four- fold, 

 division— a median shale formation with heavily bedded lime- 

 stones above and below. These members are : 



Girardeau limestone. 



Hudson River shales. 



Trenton limestone. 



First Magnesian limestone. 



"first magnesian" limestone. 

 This is usually a buff, heavily bedded dolomitic rock, not 

 very compact, but withstanding well the effects of weather- 

 ing. Its maximum thickness, as determined by Shumard, is 

 over 150 feet. Swallow,* Broadheadf and others considered 

 this formation as belonging to the Calciferous ; while Worthen 

 placed it in the Trenton. In attempting to correlate it with 

 the series of the upper Mississippi river, it seems not likely 

 that it is the representative of the lower Magnesian or Oneota 

 limestone of that region; the Saint Peter sandstone being 

 absent in eastern Missouri, or replaced by limestone as already 

 remarked. 



TRENTON limestone. 



In southeastern Missouri the Trenton limestone as now 

 understood embraces, besides the Trenton as comprehended 

 in earlier reports of the state, the Black Eiver and Birdseye 

 limestones of Shumard. The latter probably more properly 

 represents the lower and less fossiliferous portion of the 

 Trenton of the region, and nowhere can be separated faunally 

 or lithologically from the upper part containing the typical 

 Trenton fauna. 



The lower Trenton ("Black Eiver" and "Birdseye" lime- 

 stone ) is a compact, heavily bedded limerock, often not unlike 

 certain lithographic stones in texture. 



*Geol Sur. Missouri, 1st and 2iid Ann. Rep., p. 114. 1855. 

 tGeol. Sur. Missoari, 1873-74, p. 29. 1874. 



