Marsh — Jurassic formation on the Atlantic Coast. 435 



The main physical features of the Jurassic strata in the 

 West, especially the variegated fresh-water deposits, are so 

 striking that, once seen, they will not soon be forgotten. As 

 these physical characters may be used as one means of readily 

 identifying this horizon, I have brought here, besides the colored 

 drawing of the Como section in Wyoming, two others illustrat- 

 ing sections in Colorado. One is from Morrison, near Denver, 

 and the other one hundred miles further south, near Canon 

 City, both representing, in the Atlantosaurus beds, localities 

 famous for the vertebrate fossils they have furnished. I know 

 of no other geological horizon in the West marked by such 

 striking and characteristic physical features. 



The Pleiirocoelus Beds. 



In the East, the strata most resembling the Atlantosaurus 

 beds in physical characters are the Potomac clays and sands 

 so conspicuous between Washington and Baltimore, and 

 known to extend, also, both to the north and south. Although 

 fifteen hundred miles to the eastward, tliese Maryland strata 

 so strongly recalled those I had explored at the base of the 

 Rocky Mountains, I felt reasonably sure, even before I had 

 examined them, that this series w^ould turn out to be essentially 

 the same age as the Atlantosaurus beds of the West. This 

 proved to be the case. Although the Potomac beds have been 

 generally regarded as Cretaceous, I can now safely say that the 

 vertebrate fossils I have secured from them, especially the 

 Sauropoda^ demonstrate their Jurassic age beyond reasonable 

 doubt. I stated this conclusion in my first description of 

 Potomac fossils, and it is now fully confirmed by more recent 

 discoveries.^ 



The fact that the Sauropoda of the Potomac beds are all of 

 diminutive size, in comparison with the western forms, is a 

 point of some importance in estimating the age of the strata 

 that contain them. It is a rule almost without exception, that 

 the earlier members of an order of ancient vertebrate animals 

 are small, while the last survivors before extinction are the 

 largest. The gigantic forms of every such group left no suc- 

 cessors. Hence, the small PleurocodidcB of the East may 

 possibly be the ancestors of the huge western Atlantosauridoe, 

 but can hardly be their descendants. The other vertebrate 

 fossils from the Potomac of Maryland, although fragmentary, 

 all appear to be Jurassic in type. 



* This Journal, vol. xxxv, p. 90, 1888. See also, Sixteenth Annual Report, 

 U. S. Geol. Survey, Part I, p. 183, 1896. 



