36 THE DEVONIAN OF MISSOURI. 



Conocardium ohioense Meek and Paracyclas elliptica Hall 

 lingered over from the Mineola and did not migrate to the Iowa 

 seas during Callaway time. The western immigrants of the 

 Cedar Valley fauna did not reach the Callaway area and the 

 Callaway is more closely related to the eastern Devonian than 

 the Cedar Valley is. The Callaway seas were, probably con- 

 temporaneous with the Cedar Valley seas and the two formations 

 can be definitely correlated. 



It is remarkable that the Callaway has none of the standard 

 list of Hamilton species as given by Williams 1 while it contains 

 •many species that are present in eastern Hamilton formations. 

 The central Missouri sea was connected with the eastern United 

 States seas during Mineola and most of the Callaway species 

 survived from the Mineola or developed from the Mineola 

 species. The possible immigrants from the northwest by way 

 of Iowa are Spirifer asper Hall, Cranaena iowensis (Calvin), 

 and Schizophoria striatula (Schlotheim). 



It seems probable that central Missouri was at the south 

 end of a bay that extended southward from Iowa and that eastern 

 Missouri was a land barrier which kept out eastern immigrants. 



The Callaway is late Hamilton in age but may have been 

 also, in part, contemporaneous with lower Tully. 



THE SNYDER CREEK SHALE 



Historical. — In Callaway and Montgomery counties the 

 Callaway limestone is overlain by a shale and limestone forma- 

 tion ranging up to 63 feet in thickness. Gallaher first called 

 attention to the formation in 1900 as follows: "Following im- 

 mediately after the Hamilton limestone are the Snyder Creek 

 Shales, another Devonian deposit which is limited almost ex- 

 clusively to Callaway County. Plate 43 shows several important 

 rocks, in their natural order of occurrence, but poorly developed." 2 

 The plate 43 is a photograph of the shales at their type locality 

 on Craghead Creek. 



Mr. D. K. Greger, who was Gallaher's assistant, probably 

 called his attention to the shales. Keyes mentions the Snyder 

 Creek shales in several places in his articles on Missouri .geology, 

 and in 1909 Greger 3 gave the following brief description of them: 

 "Structurally the Craghead Creek shale is readily divisible into 

 three members; the lower portion made up of dark blue and 



•U. S. G. S., Bull. 210, p. 60. 

 •Missouri Geol. Surv., vol. 13. p. 153. 

 •Am. Jour. Sci., vol. 27, p. 375. 



