110 Marsh — Jurassic Formation on the Atlantic Coast. 



For example, at the time the above work was published, one 

 of the most fruitful horizons of vertebrate fossils ever discov- 

 ered had been known for several years in the Jurassic of the 

 West. Many tons of gigantic fossil vertebrates had been col- 

 lected from several localities, and the principal forms described 

 and figured, while the illustrations had been reproduced even 

 in text-books. Moreover, the Jurassic horizon in which these 

 and other remains were found had been definitely determined 

 and named the Atlantosaurus beds, and a geological section 

 showing their position and characteristic genera had been pub- 

 lished several times. The fossils thus discovered embraced 

 mammals, birds, reptiles, and fishes, nearly all of well-marked 

 Jurassic types. 



Since 1880, when the statement I have quoted was made, 

 other discoveries have followed in rapid succession, and the 

 Jurassic vertebrate fauna of the West is now known to be a 

 most rich and varied one, far in advance of that from any 

 other part of the world. More than one hundred and fifty 

 species of extinct vertebrates, some of them represented by 

 hundreds of specimens, have been brought to light, and over 

 one hundred of these have already been described, and the more 

 important have been refigured and republished in various 

 parts of the world, including text-books, so that anyone with 

 even an elementary knowledge of the subject can see that they 

 are Jurassic in type. Nevertheless, a number of American 

 geologists whose studies have kept them in other fields still 

 appear to be ignorant of nearly all that has been made known 

 about vertebrate paleontology in this country during the last 

 quarter of a century, and seem to think that the Jurassic 

 formation here is of small importance, and that its area should 

 be restricted rather than enlarged. 



Another of my reviewers was G. K. Gilbert, editor of the 

 work from which I have just quoted. Whether he intended 

 his remarks on my paper to be taken seriously is not clear. 

 Apparently he wished to start an academic discussion on correla- 

 tion, and under the circumstances this would probably have led 

 from the Rocky Mountains to the Mountains of the Moon, one 

 of his latest fields of investigation. If he is really in doubt 

 about the methods of correlation of vertebrate fossils, he can 

 perhaps find the information he needs in text-books. 



First of all, however, I must question the accuracy of some 

 of the statements in his review of my paper. One of these is 

 as follows : 



" Through a comparison of vertebrates from .the Potomac forma- 

 tion with vertebrates from other formations he has inferred the 

 Jurassic age of the Potomac ; but he gives no hint of the charac- 

 ter of his evidence or the course of his reasoning."* 



* Science, vol. iv, p. 876, 1896. 



