EXTENT OF rALEOZOTC CONTINENTAL SEAS 309 



began long before the close of the Trenton. On the east and south flanks 

 of Ozarkia the late Black Eiver Kimmswick, a crystalline limestone of 

 earlier date than the Galena dolomite proper, is the last of the Ordo- 

 vician deposits. On these flanks it is immediately succeeded or over- 

 lapped by the Fernvale limestone, which is regarded as one of the first 

 deposits of the Silurian submergence. In Lincoln County, Missouri, 

 where the Fernvale seems absent, the Kimmswick is succeeded by Maquo- 

 keta shale, which, as has been noted, locally succeeds the Fernvale farther 

 south in the valley. 



Bather generally, in the upper Mississippi Valley, the Galena dolomite 

 is succeeded by the Maquoketa shale. That the top of the Galena 

 proper — that is, without the Dubuque limestone — is never younger than 

 the Trenton limestone is, I believe, shown conclusively by its fossil con- 

 tents. That the Maquoketa, which overlies it, is not older than lower 

 Eichmond is proved (1) by the occurrence .of a faunal zone, characterized 

 by CUdophorus neglecta and other small mollusks at its base at Dubuque, 

 while the same zone is found above the Fernvale limestone south of Saint 

 Louis, and (2) by the position of the Fernvale in middle Tennessee, 

 where it overlies the Leipers formation, which is the local representative 

 of the Maysville group of the Cincinnati section. The validity of these 

 correlations can not be questioned. The disastrophic evidence in each 

 case is no less definite than is that of the fossils. 



Clear evidence of this late Ordovician erosion is seen in the bluffs of 

 the Mississippi between Saint Louis and Cairo. Here the fact of base- 

 leveling of the Kimmswick limestone is shown positively by (1) a study 

 of the contact of this limestone and the overlying Fernvale limestone, and 

 (2) the uniform thickness and composition of the Fernvale considered 

 in connection with the wide extent of the overlap of this formation. 



In the vicinity of Thebes, Illinois, and Cape Girardeau, Missouri, the 

 Kimmswick includes at the top some 5 to 30 feet of crystalline limestone, 

 containing Echinosphcerites, Comarocystites, and other peculiar fossils. 

 This bed overlies another, easily recognized by the abundant presence of 

 a large Receptaculites {R. oweni). Every outcrop in this area shows a 

 different horizon at the top of the Kimmswick. That the Kimmswick at 

 Thebes originally included even higher beds than are known there is indi- 

 cated by later Trenton chert, found in broken down pre-Fernvale caverns. 



Going northward from the cape the Kimmswick and Fernvale dip out 

 of sight beneath younger formations. They come to the surface again 

 between Establishment and Plattin creeks on the north limb of the syn- 

 cline, but the outcrops in this area are not favorably situated for detailed 

 study. In the next syncline, which is shallower, these formations are 



