PEALE.] GEOLOGY LIGNITIC GROUP. 153 



saunan, the organic remains favor the conclusion that it (Bitter Creek 

 series) is. Tertiary." Professor Lesquereux, from the study of the flora 

 as we have already seen, refers the beds to the Eocene period. Cope, 

 in his conclusion, (Keport United States Geological Survey, 1873, p. 447), 

 says there is, then, no alternative but to accept the result that a Tertiary 

 flora was contemporaneous with a Cretaceous fauna, establishing an 

 uninterrupted- succession of life across what is generally regarded as 

 one of the greatest breaks of geologic time. 



I insert here a letter from Prof. Theodore Gill, on the value of the 

 D'mosaurian remains, as proof of the Cretaceous age of the strata in 

 which they are found : 



Smithsonian Institution, 



Washington, January 9, 1876. 



Deak Sir: Your letter, requesting a copy of my commuuication to the meeting of 

 the National Academy of Sciences respecting the values of fossils as indices of forma- 

 tions, has duly come to hand. 



My remarks were not published otherwise than in brief newspaper reports. They 

 were elicited by the controvers;\' then prevailing respecting the^age of the lignite-beds 

 in which the Dinosaurian remains, to which you allude, were found. The substance of 

 my criticisms was that we should not be too much influenced in our views as to the 

 age of any group by one or two including fossils ; and I brought up a number of cases 

 to show how mistaken we should be if we allowed ourselves to be too much influenced 

 by what was known of the paleontology of any limited country. Among other in- 

 stances, I especially alluded to the fact that for a long time the fish, genus Ceraiodns, , 

 was regarded an indication of a not later than Triassic age of any including forma- 

 tion ; and showed that while such is the case for the northern hemisphere, so far as 

 yet has been observed, we still have living Ceratodi in the southern hemisphere, not 

 long ago discovered. I proceeded then to show the close relation between the species 

 of this group, which lived in Triassic Europe, and the species which live in modern 

 Australia. I further indicated that the remains of Agaihaumas, found in the lignite- 

 beds, were not sufficiently characteristic to enable us to determine the exact relation- 

 ship of the genus, and that it was apparently not, at least nearly, allied to any of the 

 previously recognized forms of the order. The conclusion was drawn that inasmuch 

 as the same typical structure could persist and had persisted, with shifting geographi- 

 cal relations, through such long ages as had Ceratodus, that in the case of the Dinosmtri- 

 ans, where the affinities were less intimate, there could be no a priori reason why they 

 might not have had a similar history. It was in fact assuming the premises in dispute 

 to assert that because the remains of the Dinosaurian were found in a given horizon 

 they must necessarily indicate Mesozoic age, notwithstanding the other associated fos- 

 sils. I finally urged that in this case, where the evidence was conflicting, we would 

 have to wait for further proof from other quarters, and especially for the tracing of 

 definite horizons toward, and co-ordination with, those above and below the beds in 

 dispute. 



I adduced a number of other facts bearing on the subject ; but this will be sufficient 

 to show the drift of my argument and the evidence of the facts set forth. 



Hoping that this epitome will answer your purpose, I remain, yours trulv, 



THEO. GILL. 



Dr. A. C. Peaxe. 



Cope also says,* " The appearance of mammalia and sudden disap- 

 pearance of the large Mesozoic types of reptiles may be regarded as 

 evidence of migration, and not of creation. It is to be remembered that 

 the smaller types of lizards and tortoises continue, like the crocodiles, 

 from Mesozoic to Tertiary time without extraordinary modification of 

 structure. It is the Dinosauria which disappeared from the land, driven 

 out and killed by the more active and intelligent mammal : herbivorous 

 reptiles like Agathaumus and Cionodon would have little chance of suc- 

 cessful competition with beasts like the well-armed Bathmodon and 

 Metalo2)hodon." If the smaller types persisted, why not the larger? 

 The only reason he gives is that they would have little chance of suc- 

 cessful competition with the well-armed mammals. Why should this not 



*Eeport United States Geological Survey, 1873, p. 442. 



