920 



SCIENCE. 



[N. S. Vol. IV. No. 103„ 



nental expansion towards the east was far 

 greater in Jurassic time than now, especially in 

 the tropical and South American regions. 

 That lacustral deposits of the alleged nature of 

 Prof Marsh's Atlantosaurus beds of Wyoming 

 and Colorado may have been made upon this 

 Jurassic land mass is not only possible but 

 plausible, but Jurassic deposits at marine sedi- 

 mentation level are undoubtedly missing or un- 

 discovered upon the Atlantic slopes of both 

 American continents. Robt. T. Hill. 



U. S. Geological Suevby. 



Since the above was written, Prof. Marsh 

 has published another contribution upon ' The 

 Jurassic Formation of the Atlantic Coast.'* 

 This contains many statements with which 

 American geologists will differ, and conflicts 

 more or less directly the results of others, who 

 for years have carefully explored and described 

 the Mesozoic formations of this country. 



Prof. Marsh, in his previous papei's, has trans- 

 ferred the Wealden epoch from the base of the 

 Lower Creta,ceous to the top of the Jurassic ; 

 and the tenor of the present article is to re- 

 pudiate the Lower Cretaceous entirely, as is 

 shown in the diagram of the ' Geologic Horizons 

 of Vertebrate Fossils,' and in the many places 

 where he makes the Dakota Formation the 

 base of the Cretaceous. Upon the evidence of 

 plants, vertebrates and mollusks, all other 

 students place the Dakota Formation in _ the 

 middle of our American Upper Cretaceous and 

 at the base only of the upper of the two great 

 series into which the Cretaceous of this country 

 is divided. It is considered also as the time 

 equivalent to the Middle Cretaceous of Europe. 

 Between the Dakota and Jurassic time posi- 

 tions, both in this country and Europe, there 

 are extensive series of sediments representing 

 the great interval of the Lower Cretaceous 

 time. Even Prof. Jules Marcou, who has here 

 alone upheld the Jurassic age of the Lower 

 Cretaceous formations in part, admits that 

 there are extensive Lower Cretaceous beds be- 

 low the Dakota. These Lower Cretaceous beds, 

 to which the Potomac belongs, and not the 

 Dakota, as alleged by Prof. Marsh, are the true 



* This JOUENAL, December 5, 1896, and American 

 Journal of Science, December, 1896. 



base of the Cretaceous system in the Rocky 

 Mountain region, at least in the Trans-Pecos,. 

 Texas and Mexican portions of the Rocky 

 Mountains. Neither is it improbable that they 

 are the true base of the Cretaceous in the 

 Colorado region, if the Atlantosaurus beds are 

 of the Lower Cretaceous series. The latter 

 clearly occupy the stratigraphic position where 

 the Lower Cretaceous beds ought to be, lying 

 beneath the Dakota and above the last deter- 

 minable Jurassic. For years students of Amer- 

 ican stratigraphy have desired to know the re- 

 lationship between these Atlantosaurus beds 

 of Colorado and the nearest allied Potomac- 

 like formations of undoubted Atlantic sedi- 

 mentation. Some have even suspected that they 

 might ultimately prove to be a part of the 

 great basement littoral of the Lower Cretaceous- 

 of the Texas and Potomac regions. In a pre- 

 vious paper I have suggested that there might 

 be stratigraphic relationship between the Trin- 

 ity-like sands at the base of the Tucumcari 

 series of New Mexico and the Atlantosaurus 

 beds of Colorado. An attempt to trace this 

 connection, however, resulted in the conclusion 

 that it would be impossible to prove it, owing 

 to discontinuity of outcrop along the flanks of 

 the Rocky Mountains in New Mexico. Prof. 

 Marsh, however, now assures us that the Atlan- 

 tosaurus beds are the western extension of th& 

 Potomac formation of the Atlantic coast, and 

 hence those who still believe in the Lower Cre- 

 taceous position of the latter, would hereafter 

 be justified by his correlation in mapping the 

 Atlantosaurus beds as Cretaceous. 



An impression is obtained from Prof. Marsh's 

 writings that his deductions are based entirely 

 upon the vertebrates, and that he has not fully 

 considered the correlative testimony of other 

 life forms the species of which occur in greater 

 abundance and have a more world-wide distri- 

 bution than the vertebrates. In discussing the 

 relative merits of plants and vertebrates as 

 stratigraphic criteria, he even states that the 

 attempt to make out the age of formations by the 

 use of fossil plants is too often labor lost. What- 

 ever may heretofore have been the diagnostic 

 value of paleobotany in stratigraphic determi- 

 nation, the recent detailed researches in this 

 country have created for it a position that com- 



