Maech 12, 1897.] 



SCIENCE. 



421 



the most complete demonstration that could 

 be made of the essential diiference between 

 the Older and the Newer Potomac, and all 

 the proof that should be necessary to es- 

 tablish my fundamental thesis that, while 

 the former must lie very near the base of 

 the Lower Cretaceous and may even extend 

 somewhat into the Upper Jurassic, the lat- 

 ter must be correlated with the extreme 

 Upper members in the European series of 

 Lower Cretaceous deposits. 



So far as I am concerned, I have no inter- 

 est whatever in the mere question of 

 names, for example, as to whether the 

 Wealden should be called Cretaceous or 

 Jurassic, and I have done what I could to 

 show that the Older Potomac was laid 

 down under conditions verj' similar to those 

 of the AVealden of England and that, in all 

 probability, the process of deposition of 

 portions of both at least was going on at the 

 same time. If Professor Marsh, through- 

 out his papers, had substituted the term 

 Wealden for ' Jurassic ' it is doubtful 

 whether they would have given rise to any 

 discussion, so far as the Maryland beds 

 containing the vertebrate remains are 

 concerned. But he has chosen to employ 

 the term Jurassic without qualification, and 

 there are indications that he does not mean 

 to correlate the Potomac formation with 

 the Wealden, but regards portions of it at 

 least as Oolite. In his last paper* he says : 

 "It cannot, of course, be positively asserted 

 at present that the entire series now known 

 as Potomac is all Jurassic, or represents the 

 whole Jurassic. The Lias appears to be 

 wanting, and some of the upper strata may 

 possiblj^ prove to belong to the Dakota." 

 This would give the Potomac formation an 

 enormous extension, viz., from the base of 

 the Oolite to the Upper Cretaceous. The 

 less than twelve hundred feet that it has 

 been possible thus far to measure in the 



*Amer. Jouru. Soi., 4th Ser., Vol. II., December, 

 1896, p. 436. 



Potomac formation * would seem to be an 

 exceedingly thin stratum to represent such, 

 a period, even after allowing for any amount 

 of contemporary erosion. 



Professor Marsh says that it is a i-eproach 

 to science that the Jurassic has not been 

 discovered in the eastern part of the conti- 

 nent. This may be true, provided it exists, 

 but if it does not exist the finding of it 

 would be a still greater reproach to science. 

 His section would seem to indicate that he 

 regards the Dakota group as forming the 

 lowest member of the Cretaceous. This has 

 never been maintained by any geologist. It 

 is true that it was claimed for many years 

 that it represented the lowest Cretaceous in 

 America, but those who made this claim 

 assumed the absence of the Lower Creta- 

 ceous in any part of this country. Pro- 

 fessor Marsh's assumption, if that is what 

 he means,t would carrj' with it some'pecu- 

 liar consequences; it would make the beds 

 that are now known to underlie the Dakota 

 group (the Comanche series, the Kootanie, 

 the Shasta group, and the Queen Charlotte 

 Island group, as well as the Potomac for- 

 mation) all Jurassic. A number of these, 

 especially those of Texas and the Pacific 

 coast, are marine deposits and contain 

 abundant invertebrate remains, fully es- 

 tablishing their Lower Cretaceous age. But 



*15th Ann. Kept. U. S. Geol. Sur%'., p. 339. 



t Since this was written I have had an interview 

 ivith Professor Marsh and was glad to learn that he 

 disclaims such an interpretation of his section. He 

 maintains that the explanation on p. 144 of the 16th 

 Annual Report United States Geological Survey was 

 intended to prevent this impression from being gained 

 and called my attention to the following words, and 

 espeoiallj' to those in italics: "This diagram repre- 

 sents the principal geological horizons of vertebrate 

 fossils in North America, as determined by the writer." 

 To have justified such an interpretation his diagram 

 should have embraced no formations from which ver- 

 tebrate fossils had not been determined by him. A 

 glance at the diagram, however, shows that there are 

 two groups opposite which he has indicated no verte- 

 brate remains, and one of these unfortunately is the 

 Dakota group. 



