FLORA OK THK CHEYENNE SANDSTONE OF KANSAS. 



201 



Les(|iierc'ux licscrihod is tho same us that 

 described by Richardson '- in 1915 as the 

 Pur<jat()ire formation and referred to the top 

 of the Lower Cretaceous. This formation has 

 also frequently been called "Lower Dakota." 

 Below this, in the type section at Morrison and 

 within the Morrison formation (" Athnito- 

 saurus beds") as originally described, there is 

 about 100 feet of friable sandstone and shale 

 containino; traces of a IU)ra similar to that 

 found in tho overlying sandstone. This flora 

 has been discussed by Knowlton," who quite 

 rightly concludes that it is Upper Cretaceous. 



I am not concerned in this paper with the 

 taxonomic proposals regarding what shall be 

 the stratigraphic limits of the Dakota, but 

 solely with the general relations and their bear- 

 ing on the geologic history of the region and 

 the boundary between Lower and Lfpper 

 Cretaceous. 



It has been customary for geologists, par- 

 ticularly those who had a leaning toward 

 philosophy, to postulate a rhythm of positive 

 and negative movements of the strand b}^ 

 which the boundaries of the different systems 

 could readily be determined. There may be 

 some ])hysical basis for this conception, but it 

 should l)e recalled that all series of changes can 

 be considered rhythmic, with some elasticity 

 in the apjilication of criteria, and I am one of 

 those reactionaries who believe that, however 

 imperfect the scheme as devised for the region 

 first and longest studied, namely, Europe, the 

 classic names and approximate limits of the 

 systems should be adhered to; for, after all, the 

 best classifications, whether of geologic time 

 or of formations, igneous rocks, or organisms, 

 are those which are most easily understood and 

 used. 



Time is continuous, time boundaries are 

 always subjective, and the time-honored 

 terms Permian or Triassic or "Lower Carbon- 

 iferous" or Lower Cretaceous are to me as 

 essential to clear thinking and the interchange 

 of geologic ideas among nations as the minutes, 

 hours, and days of the current time scheme, 

 however illogical these may seem in sidereal 

 astronomy. 



According to the customary American 

 scheme the Lower Cretaceous should be con- 



" Richardson, G. B., U. S. Geol. Survey Geol. Atlas, Castle Rock folio 



(No. 198), 1915. 

 " Knowlton, V. II., .\m. Jour. Sci., 41h scr., vol. 49, pp. 189-194, 1920. 



sidered to have ended witii the withdrawal of 

 the Lower Cretaceous sea and the U|)per 

 Cretaceous to have begun with the initial 

 transgression of the U|)per Cretaceous sea. 

 Where the interval between these two events 

 was long, with continental deposition, much 

 confusion ami difi'erence of interpretation re- 

 sults. A classic instance of such differences 

 is the controversj' over the boundary I)etween 

 the Cretaceous and Tertiarj^ in the (ireat 

 Plains and Rocky Mountain region of North 

 America, which the Tertiary sea was so incon- 

 siderate as not to invade. If geology at its 

 inception had concerned itself chiefly with 

 continental deposits and IjiikI f)lants and ani- 

 mals and had ignored marine formations and 

 life the situation would be exactly reversed, 

 and th(> marine sediments would probably be 

 those in dispute. 



On none of the continents, so far as I can 

 discover, did the sea complete a cycle of inva- 

 sion and withdrawal of what might be called 

 the first magnitude during the Lower Creta- 

 ceous epoch. In the Atlantic (^oastal Phiin 

 no marine Upper Cretaceous deposits earlier 

 than the European Turonian are known ex- 

 cept in the Texas area, where marine forma- 

 tions representing a part but not all of the 

 Lower Cretaceous of Europe advance halt- 

 ingly from the south. The oldest of these 

 formations is the Trinit}', which in my judg- 

 ment is nowhere as old as the Neocomian of 

 Europe. This is followed by the Fredericks- 

 burg group, which llill called Neocomian but 

 which contains a younger fauna. If one dis- 

 regards Bose's correlations of the Mexican 

 Cretaceous on the ground that Mexico is too 

 remote from the north Texas-Kansas area, 

 Whitney's studies of the fauna of the Buda 

 limestone not only clearly show its Cenomanian 

 age but also show that it is' late Cenomanian. 

 Similarly the fauna of the Georgetown lime- 

 stone is Cenomanian. (Whitney has refrained 

 thus far from making any intercontinental 

 correlations.) It is a striking confirmation 

 of this correlation that the Buda limestone 

 near Austin and hence in the region of more 

 continuous marine conditions than farther 

 north should be immediately overlain i)y the 

 Turonian Eagle Ford formation. Tiie |)i()l)!cm 

 of working out the interfingering of forma- 

 tions between north and central Texas is 

 largelj^ a problem of invcrtcljrate paleontology 



