44:2 C. A. White — Lower Cretaceous of the Southwest. 



The paleontological contrast between the Upper and Lower 

 Cretaceous is also great, as has just been intimated. With the 

 exception of the Laramie formation at the top of the Upper 

 Cretaceous series, and the Dakota at its base, all the formations 

 of that series are characterized by marine fossils, and the south- 

 ern equivalent of the Dakota formation is also of marine 

 origin, as is shown by -the character of its fossils. But while 

 the marine faunas of the respective formations which compose 

 the Upper Cretaceous are so related to one another by identity 

 of a portion of their specific and generic forms as to indicate 

 that no complete chronological break occurred between any of 

 them, such faunal relationship between the Upper and Lower 

 Cretaceous, so far as is now known, does not exist. That is, 

 not only are no known species common to both divisions, but 

 many of the genera and some of the families of mollusks 

 found in the lower, are not known in the upper, division. Be- 

 sides the lithological and paleontological contrast between the 

 two divisions, which has just been noticed as indicating their 

 separateness, they have been found in some places to be plainly 

 unconformable. 



The extent of the chronological hiatus between the Upper 

 and Lower Cretaceous we have at present no satisfactory 

 means of determining, because the Cretaceous record for our 

 continent, so far as it is now known, is much broken below the 

 horizon of the Dakota Group ; because most of the known sub- 

 divisions below that horizon are geographically widely separated 

 from one another and all of them have not yet been thoroughly 

 studied ; and also because the European record is not an adequate 

 standard in this case. For the latter cause, we cannot say with 

 confidence that the Comanche series really represents any one 

 of the divisions of the European Cretaceous from the Gault to 

 the Lower ISfeocomian inclusive. Furthermore, while it is not 

 at present improbable that this series is equivalent with the 

 Queen Charlotte Island and Kootanie formations, as has been sug- 

 gested by Dr. G. M. Dawson,* we have yet no direct evidence 

 of it ; that is, there is no known stratigraphical continuity be- 

 tween those northwestern formations and the Comanche series, 

 and no fossils have been found common to both. The pre- 

 sumptive evidence of their equivalency is therefore apparently 

 confined to the position which each is known to hold beneath 

 acknowledged Upper Cretaceous strata and to certain probably 

 contemporaneous displacements of the lower series which have 

 occurred in both regions. 



The unconformity between the Lower and Upper Cretaceous 

 of the Southwest, together with the faunal break between 

 them, which have already been mentioned, make it evident 



* This Journal, vol. xxxviii, p. 122. 



