74 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 7t> 



column is taken from the Mississippi and Ohio Valleys and the Ordo- 

 vician part from the southern Appalachian Valley and central 

 Tennessee except the Buffalo River series, which is best developed in 

 northern Arkansas. As will be observed, only the Chazyan part of 

 the column is divided into units of formational rank, this being the 

 part that is mainly concerned with questions discussed in this paper. 

 Besides, and however detailed the correlations of American Ordo- 

 vician and Silurian formations may be, we can as yet do no better 

 in correlating European formations of these periods than to suggest 

 more or less indefinitely located positions for them in one or another 

 of our series or groups. 



Though this column is called a time scale it should not be as- 

 sumed that even its OrJovician part accounts for all of the time 

 included in this period. In fact it accounts only for those subordi- 

 nate parts of the accesible depositional record that was laid down in 

 American epicontinental basins; and of these only those whose 

 sequential relations have been established. As most if not all of the 

 named minor units of the scale are separated from each other by 

 stratigraphic breaks of undetermined time significance it follows 

 that these depositionally unrecorded intervals, at least, are not ac- 

 counted for. Doubtless some of these intervals are represented, 

 probably only in part, by deposits in other areas of the North Ameri- 

 can Continent, but these could not be used in constructing the scale 

 because their relations to those found in the southern Appalachian, 

 Ohio, and Mississippi Valleys are insufficiently understood; and we 

 know even less about the correlation of the European and American 

 Ordovician deposits. Finally, as suggested previously (p. 54), there 

 may have been times when the continents on both sides of the 

 Atlantic were so elevated above sea level that the basins in which 

 Ordovician marine deposits are now accessible were completely 

 drained. Obviously such times also are not accounted for. It fol- 

 lows, then, the " generalized time scale " of the chart is incomplete 

 to these several extents and pretends to be nothing more than a 

 temporary standard for comparison. 



Perhaps I should call attention also to the fact that the correla- 

 tions with formations in European countries differ considerably from 

 those given in a similar table published by me only three years ago.^' 

 However, the changes occur mainly in the lower two-thirds of what 

 I think should be included in the Ordovician system, which, as many 

 know, I define differently from the original and even yet prevailing 

 conception of that term. Briefly stated, my definition of the Ordo- 

 vician system is based primarily on diastrophic criteria that in my 



^ Relative values of criteria used in drawing the Ordovician-Silurian boundary : Geol. 

 See. America Bull., vol. 37, p. 329, 1926. 



