APPENDIX. 



Aet. YI. — Notice of a New Genus of Sauropoda and 

 other new Dinosaurs from the Potomac Formation ; by 

 O. C. Marsh. 



The variegated red and gray clays which form so conspic- 

 uous a feature in their outcroppings between Baltimore and 

 "Washington have long been a puzzle to geologists. They have 

 been supposed to be Mesozoic, but as no characteristic fossils 

 had been found at the typical localities, or in the known 

 extensions of the deposits, their true age was uncertain. They 

 are evidently above the red Triassic sandstones, and are sup- 

 posed to pass into clays which extend beneath the Cretaceous 

 marls of Sew Jersey. 



The United States Geological Survey has named these 

 problematic deposits the Potomac formation, and the Director 

 recently requested the writer to institute a special search for 

 vertebrate fossils, to solve, if possible, the question of its age. 

 The field work was intrusted by the writer to Mr. J. B. 

 Hatcher, whose experience in the West has especially fitted 

 him for it. The results of two months' investigation prove that 

 these deposits, so long supposed to be nearly or quite destitute 

 of fossils, contain a rich vertebrate fauna, apparently of Upper 

 Jurassic age, but quite distinct from any hitherto discovered in 

 this country. 



