E. W. Hilgard — Old Tertiary of the Southwest. 269 



virtue in geometry, this is the inevitable order of succession 

 from below upward. 



But, outside of the State of Mississippi, I can satisfy Dr. 

 Meyer's postulate .of "seeing the Vicksburg rocks actually 

 superimposed upon the Jackson strata." I have seen this in 

 Louisiana, on the Bayou Funne Louis, where I have stood on a 

 ledge of Vicksburg limestone showing a southward dip and 

 containing abundance of Orbitoides, Area Mississippiensis and 

 Pecten Poidsoni, looking down some eighty feet, to northward, 

 upon a level prairie country in which the bones of the Zeu- 

 glodon have been plowed up. 



Without discussing paleontological details for which in the 

 absence of adequate literature and collections I should now 

 have to rely on memory alone, I must remark that I cannot 

 attach much importance to Plagiostoma dumosum as a signifi- 

 cant fossil. Of hundreds of localities examined by me in Mis- 

 sissippi, only two have yielded this shell ; and both belong to a 

 level intermediate between the Jackson and Vicksburg groups. 

 But no Vicksburg locality has failed to furnish what I have 

 been led to consider the decisive mark of the age, viz : Area 

 Mississippiensis ; it is more constant than either the Orbitoides 

 or Pecten Poulsoni, although the latter is rarely absent. For 

 the Jackson age the most constant fossil is the Zeuglodon, bone 

 fragments of which can nearly always be found by diligent 

 search ; and besides, an excellent and constant criterion is the 

 presence of Venericardia planicosta, which has nowhere been 

 found associated with the characteristic Vicksburg fauna. 

 Through this widely diffused and universally recognized shell, 

 as well as through the almost equally constant Gastridium 

 vetustum and Calyptrophorus velatus as common fossils, the Jack- 

 son fauna connects strikingly with the Claiborne and Buhrstone 

 beds ; and I have found this Venericardia in the latter, in almost 

 immediate contact with the Upper Cretaceous rocks of North 

 Mississippi. Upon Dr. Meyer's assumption, the Vicksburg 

 beds, void of both of the above types, would actually be inter- 

 calated between this oldest post-Cretaceous fauna and the 

 Claiborne and Jackson beds. However, his assumption is 

 abundantly and conclusively disproved by the most direct 

 stratigraphical evidence ; which it is to be hoped he himself 

 will undertake to verify before he again ventures to re-classify 

 the Southwestern Tertiary. 



