FLORA OF THE CHEYENNE SANDSTONE OF KANSAS. 



201 



Lesquereux described is the same as that 

 described by Richardson '- in 1915 as the 

 Purgatoire 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 ('' Atlanto- 

 saurus beds") as originally described, there is 

 about 100 feet of friable sandstone and shale 

 containing traces of a flora similar to that 

 found in the overlying sandstone. This flora 

 has been discussed by Kntiwlton,'^ 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 LTpper 

 Cretaceous. 



It has been customary for geologists, par- 

 ticularly those who had a leaning toM^ard 

 philosophy, to postulate a rhythm of positive 

 and negative movements of the strand by 

 which the boundaries of the different systems 

 could readily be determined. There may be 

 some physical basis for this conception, but it 

 should be recalled that all series of changes can 

 be considered rhythmic, with some elasticity 

 in the application 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 tune 

 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. 



Accojding to the customary American 

 scheme the Lower Cretaceous should be con- 



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

 (No. 1981, 1915. 

 " Knowlton, F. H., Am. Jour. Sei., 4th ser., vol. 49, pp. 189-194, 1920. 



32333°— 22 16 



sidered to have ended with the withdrawal of 

 the Lower Cretaceous sea and the I'pper 

 Cretaceous to have begun with the initial 

 transgression of the Upper Cretaceous sea. 

 Wliere the interval between these two events 

 was long, with continental deposition, much 

 confusion and difference of interpretation re- 

 sults. A classic instance of such differences 

 is the controversy over the boundary between 

 the Cretaceous and Tertiary in the Great 

 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 cliiefly with 

 continental deposits and land plants and ani- 

 mals and had ignored marine formations and 

 life the situation would be exactly reversed, 

 and the 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 &eta- 

 ceous epoch. In the Atlantic Coastal Plain 

 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 Trinity, which in my judg- 

 ment is nowhere as old as the Neocomian of 

 Europe. This is followed by the Fredericks- 

 burg group, which Hill 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, 

 Wliitney's studies of the fauna of the Buda 

 limestone not only cleaiiy show its Cenomanian 

 age but also show that it is late Cenomanian. 

 Similaily the fauna of the Georgetown lime- 

 stone is Cenomanian. (Wiiitney 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 by the 

 Turonian Eagle Ford formation. The problem 

 of working out the interfingering of forma- 

 tions between north and central Texas is 

 largely a problem of invertebrate paleontology 



