184 C. K. KEYES — A DEPOSITIONAL MEASURE OF UNCONFORMITY 



the Missouri-Kansas province, witli which it is proper!}^ comi)ared — the 

 indications are that the age of the strata 3delding it is not of the " Upper 

 Coal Measures " at all, but of the " lower division " — that is, of the Des 

 Moines series of the more northern localities. 



All of the Arkansas species, with very few exceptions, are, in Iowa, 

 Missouri, and Kansas, the most widely distributed forms. Most of them 

 range from the base to the top of the Des Moines series, and continue 

 upward. In the lower series the marine beds are almost wholly absent, 

 only a few thin limestones being present in the whole succession. Never- 

 theless, the same species, which are found in Arkansas, occur abundantly 

 not onl}' in the thin limestone layers, which are rarely more than a few 

 inches in thickness, but also in the calcareous shales, and in less num- 

 bers even in the bituminous shales. 



Of the corals listed from Arkansas onl}^ one form has a range that is 

 unusually " high " in the northern succession ; all of the others start 

 almost from the very base of the series. The crinoids and brj^ozoans 

 are all common in the Lower Coal Measures. All fourteen s))ecies of 

 brachiopods are of very frequent occurrence in the Lower Coal division, 

 many commencing down in the Mississippian. One possible exception 

 is Terebratula bovidens, which at present appears to be 'absent from some 

 of the lower Des Moines beds. Of the lamellibranchs, all twent3^-two 

 species are the most characteristic forms in the ver}' base of the Missou- 

 rian ; one-half of the number are found lower down, and no less than 

 seven are tyi)ical Des Moines forms, in fact having an optimum habitat 

 not in the marine beds, but in the bituminous shales. The seven species 

 of glossophora that are enumerated are the most abundant forms of the 

 Lower Coal Measures throughout the northern district, and they are 

 preeminently the characteristic fossils of the black shales everywhere. 

 Among the ten cephalopods named, no less than five are of common 

 occurrence in the Des Moines beds, and not infrequently they are found 

 in the black shales; the other five, so far as known, range low in the 

 Missourian. 



If the fiiunal evidence as recently presented is to be relied on at all, it 

 would appear that there are no grounds f©r believing that there are 

 necessarily present in the deposits of the Arkansas Valley region any 

 strata higher than perhaps the middle of the Missourian series of Mis- 

 souri and Kansas. 



The fossils listed by Drake * as coming from the Cavaniol group would 

 hardly be considered as coming from the horizons farther north, called 

 the Upper Coal Measures (Missourian series). The species are all very 



* Proc. Am. Philos. Soc, vol. xxxvi, 1898, p. 393. 



