42 GEOLOGICAL FORMATIONS 



regarded as lower Devonian in age — the lower part being re- 

 ferred to the Lower Helderberg. 



As described by Shamard * in the geology of Cape Gir- 

 ardeau county, and also in the geology of Perry county f, his 

 Niagara and Delthyris shales correspond in part to Worthen's 

 Lower Helderberg of the adjoining districts. The upper por- 

 tions of the Clear Creek limestones may finally be placed in 

 the Devonian, but the lower portion ("Lower Helderberg") 

 certainly is Upper Silurian, and probably corresponds, in part, 

 to the so-called Magara of Indiana and elsewhere. 



These limestones attain a maximum thickness of probably 

 400 feet. They are easily distinguished in the eastern parts of 

 Ste. Genevieve, Perry and Cape Girardeau counties, where 

 they form high rural escarpments along the streams of the re- 

 gion. They are rather light-colored, grayish or bluish lime- 

 stones, with considerable chert in layers. Fossils abound in 

 certain beds. 



DEVONIAN. 



GRAND TOWER LIMESTONE. 



In southeastern Missouri, the Devonian rocks are as yet but 

 little understood. They have been subdivided and correlated 

 with the. New York section ; but it is quite manifest that any 

 apparent parallelism is merely coincidental, and does not 

 represent any real relationship. The upper limestones clearly 

 contain the so-called Hamilton fauna of the West, and very 

 properly may be regarded as representing the " Hamilton 

 group " of the adjoining states. The name here used is intended 

 to apply to the Devonian rocks of southeastern Missouri, ex- 

 posed best perhaps in the vicinity of Grand Tower, below those 

 beds containing the fossils of the Western Hamilton. These 

 rocks have been referred chiefly to the Onondaga and Oris- 

 kany by Meek and Worthen and others. The fauna contained 

 seems to differ very considerably from the typical Western 

 Hamilton of other parts of the Mississippi basin. 



*Geol. Sur. MiBBonri, 1855-71, p. 261. 1873. 

 tibid., p. 231. 



