Marsh— Jurassic formation on the Atlantic Coast. 433 



Art. LXII. — The Jurassic Formation on the Atlantic Coast ^^ 

 bj O. C. Marsh. 



The absence of all Jurassic strata in the eastern part of 

 the United States has been generally regarded as a settled point 

 in geology for half a century or more. The reason for this 

 vacancy has also been one of the problems geologists have had 

 to deal with, since the formations above and below are well 

 represented. Until a comparatively modern date, this sup- 

 posed absence of Jurassic deposits was thought to be true, also, 

 for the rest of this country. I well remember the parting 

 advice given me by an eminent professor of geology with 

 whom I studied in Germany.f " The first thing you should 

 do on your return to America is, — look for the Jurassic forma- 

 tion. I am sure it is there, full of fossils." This advice I 

 followed, and on my first visit to the Rocky Mountains, in 

 1868, I found this formation near Lake Como, Wyoming, well 

 developed, and containing an abundance of typical fossils. As 

 this locality is now a famous one, I have brought here a col- 

 ored drawing that shows the characteristic variegated strata of 

 the Como Bluff, from which so many remains of Jui'assic verte- 

 brates have been taken during my long explorations there. 



The base of this section is a red sandstone, apparently of 

 Triassic age. Next above are Jurassic marine beds, with many 

 invertebrate fossils and a few remains of reptiles. Over these 

 beds is a series of peculiar, highly colored clays of fresh-water 

 origin and considerable thickness, rich in vertebrate fossils. 

 Crowning all is the characteristic Dakota sandstone, generally 

 considered of Cretaceous age. The position of this series of 

 strata in the geological scale is shown in the section below, 

 which represents especially the succession of vertebrate life in 

 the West during Mesozoic and Cenozoic time. 



The Baptanodon Beds. 



The same marine beds that constitute the base of the Como 

 Jurassic series. Meek had previously identified near the Black 

 Hills, by means of invertebrate fossils (Proc. Acad. Nat. Sci., 

 Phila., vol. X, pp. 41-59, 1859). I found these deposits again in 

 1870, near the Green River in Utah, and since then at various 

 other points. These strata I have named the Baptanodon beds, 

 from a genus of large swimming reptiles entombed in them. 



* Abstract of Communication made to the National Academy of Sciences, New 

 York meeting, November 18, 1896. 



I Ferdinand Roemer, whose researches here had already added much to our 

 knowledge of the geology and paleontology of this country. 



