288 C. R. KEYES DEVONIAN INTERVAL IN MISSOURI 



except the very base, prior to the recent efforts to collect them from the 

 higher horizons, made by Mr Rowley and myself. The main body of 

 the Louisiana limestone is almost devoid of organic remains. With this 

 understanding, the fauna of the Eureka shale, so far as it is now known, 

 may be made the correlative of that at the basal layers of the Louisiana 

 limestone. I had already come to this conclusion independently and 

 some time before Professor Williams made his trip to Arkansas. The 

 only difference between us on this point is that I reached my own con- 

 clusions along somewhat different lines. 



Both Professors Williams and Weller have unintentionally somewhat 

 strained the facts in their identifications of species by comparing them 

 only with stratigraphically higher forms. Comparison with the lower 

 and older species gives very different results. Nearly all of the forms 

 enumerated by the authors mentioned, besides many other species that 

 were collected, were carefully compared more than a dozen years ago, 

 during the progress of the Missouri geological survey in the southwestern 

 part of the state. The deductions then reached were to the effect that 

 the closest affinities of the various organic remains represented were to 

 be found in the earlier forms rather than in the later. It appears beyond 

 all doubt that some of the species listed b}> r the authors referred to do not 

 occur so low as the horizon of the Eureka shale. 



Williams places the Sy lam ore sandstone, a part of the Black shale 

 (Eureka shale in part), and certain green shales which occur in northern 

 Arkansas, in the Devonian. He considers these to represent all the dep- 

 osition that took place during that time. 



The rounded and highly polished " pebbles " which are so widely dis- 

 tributed through the black and green shales I have regarded as chiefly 

 coprolitic in character. In certain layers they occur abundantly with fish 

 teeth and fish bones. Many of them are highly phosphatic in nature. 



While the basal limits of the Devonian in the northern part of the 

 Mississippi valley have long been definitely located, the nether delimita- 

 tion in the Arkansas region has, until very recently, remained undeter- 

 mined. Professor Williams has shown beyond all reasonable doubt that 

 the great Saint Clair limestone comprises in reality two distinct faunas — 

 a lower one that is Ordovician and an upper one which is Silurian. The 

 latter is what is generally known as the Niagara fauna, and limits the 

 Devonian downward. 



Character of the Transition Zone 



There is a zone of indeterminate extent that, so far as its faunal fea- 

 tures go, is often claimed as being undetermined as to geological age. 



