152 



SCIENCE. 



[N. S. Vol. VIII. No. 



fact furnislies a strong argument that the 

 marine Cretaceous belongs to a separate 

 formation from the older fresh-water clays, 

 here regarded as Jurassic. 



Another geologist who has written much 

 about the AVest, but seems to have failed in 

 comprehending the evidence aiforded by 

 the vertebrate fossils from well-marked 

 Jurassic horizons, is C. A. White, and, as 

 his opinion is frequentlj^ quoted, it may be 

 well to correct one of his statements which 

 bears on the question here discussed. In 

 speaking of the Atlantosaurus beds, in 

 1889, he made this statement : 



" If it were not for their dinosaurian 

 faunas their Jurassic age might well be 

 questioned." * 



When this statement was made, more 

 than one hundred species of vertebrate fos- 

 sils besides the Dinosaurs were known from 

 these same Atlantosaurus beds, and among 

 these the Crocodilians, the Testudinates 

 and the various smaller reptiles would have 

 been sufScient to demonstrate the Jurassic 

 age of the strata containing them. More 

 important still, sevei-al hundred specimens 

 of Jurassic mammals had been found, over 

 a score of species were already described 

 and figured, and these alone were sufficient 

 to prove the horizon Jurassic. 



Following these well-known writers, 

 others of less experience in the West have 

 repeated their statements or followed the 

 earlier geologists as to the age of western 

 horizons, and thus tended to continue the 

 confusion where the facts themselves made 

 the whole subject clear. Thus it has come 

 to pass that while the Jurassic formation 

 has been recognized in the Arctic regions 

 of this Continent, and along the Pacific 

 coast, especially through Oregon and Cali- 

 fornia, as well as in Mexico, and likewise 

 in various parts of South America, its de- 

 velopment in the Eocky Mountain region 



* Proceedings of the American Association, To- 

 ronto meeting, p. 213, 1890. 



has received little attention except from 

 those especially engaged in its investigation. 

 It is not strange, then, that those who have 

 not seen how extensive the Jurassic forma- 

 tion is developed in Europe, and have not 

 examined its characteristic exposures in 

 the West, should fail to recognize it on the 

 Atlantic coast where its features at many 

 points are obscure. 



In my paper on this subject last year I 

 endeavored to show that the burden of 

 proof must rest upon those who denied the 

 existence of the Jurassic formation on the 

 Atlantic border. The evidence against it 

 is still based mainly upon fragmentary fos- 

 sil plants, in regard to the nature of which 

 the paleobotanists themselves are not in 

 accord. 



CTCAD HORIZONS. 



I have recorded elsewhere my opinion of 

 the comparative value of different kinds 

 of fossils — vertebrates, invertebrates and 

 plants — as evidence of geological age, and 

 have endeavored to show that plants, as 

 usuallj' preserved and described, are the 

 least valuable witnesses. The evidence of 

 detached fossil leaves and other fragments 

 of foliage that may have been carried hun- 

 dreds of miles by wind and stream, or 

 swept down to the sea-level from the lofty 

 mountains where they grew, should have 

 but little weight in determining the age of 

 the special strata in which they are im- 

 bedded, and failure to recognize this fact 

 has led to many erroneous opinions in re- 

 gard to geological time. There are, how- 

 ever, fossil plants that are more reliable 

 witnesses as to the period in which they 

 lived. Those found on the spot where they 

 grew, with their most characteristic parts 

 preserved, may furnish important evidence 

 as to their own nature and geological age. 

 Characteristic examples are found among 

 the plants of the Coal Measures, in the Cy- 

 cads of Mesozoic strata, and in the fossil for- 

 ests of Tertiary and more recent deposits. 



