ART. 21 ORDOVICIAN TRILOBITES ULRICH 79 



with each other we can not say what, if any, differences in age the 

 observed differences in their respective faunal contents may indicate. 



In essentials the Cincinnatian series is much the same on the Cincin- 

 nati axis as in New York. However, both regions exhibit faunal and 

 lithologic details that distinguish the sequence in one from that of 

 the other. Most of these differences have been discussed by Ruede- 

 mann,^" but to appreciate their full significance many as yet unpub- 

 lished facts that have been disclosed by study of this part of the 

 column in Pennsylvania and Virginia must be taken into considera- 

 tion. Obviously the subject is too intricate to warrant anything 

 more than its mere mention on this occasion. The case is similar 

 with respect to the Medinan formations concerning which so much 

 wholly unpublished, or only partly published, information is in 

 hand that adequate discussion of its problems constitutes the mate- 

 rial of another of my uncompleted papers. 



Southern and middle Appalachian region. — It would require at 

 least 10 columns to present in correct and readily understandable 

 manner the variations in sequence of the Ozarkian, Canadian, Ordo- 

 vician, and later formations that are known to occur in the Appa- 

 lachian Valley region from central Pennsylvania to central Alabama. 

 The sections would be in sets of twos and threes taken at intervals 

 across the strike of the valley troughs and two, or better three, sets 

 taken at points along the strike. But, however, interesting and il- 

 luminating such a series of columns would be, my present purpose 

 is particularly concerned only with the oscillations in the valley 

 troughs that are indicated by the distribution of the Chazyan de- 

 posits and faunas. Accordingly this column may be characterized 

 as a rather unsatisfactory composite presentation of the frequently 

 varying sequence of formations in the southern and middle stretches 

 of the Appalachian geosyncline. 



Absence of the Buffalo River series emphasizes the chronologic 

 significance of the break between the Ordovician and Canadian sys- 

 tems. As a rule the value of this hiatus in the valley south of 

 Staunton, Va., is further increased by absence of the Murfreesboro 

 which, as above mentioned, has been observed to wedge in from the 

 west in the eastern part of Lee County, Va. Elsewhere in the valley 

 Ordovician sedimentation usually begins with greatly varying 

 thicknesses of Mosheim limestone, but there are many places in Vir- 

 ginia, Tennessee, and Alabama at which not only the Mosheim but 

 also most or all of the Lenoir and the Holton are missing, so that the 

 Athens shale is in contact with the Canadian. 



The facts just mentioned and the many similar variations that 

 occur at the contact between the Canadian and Ordovician wherever 



»"Ruedemann, Rudolf, The Utica and Lorraine formations of New York, New York 

 State Mus. Bull. No. 258, 1925. 



