228 0. C. Marsh — Footprints of Jurassic Dinosaurs. 



The large footprints here described, however, are from a 

 higher horizon, and one well within the limits of characteristic 

 deposits of Jurassic age. Their position in the series of Meso- 

 zoic strata encircling the Black Hills is indicated in the dia- 

 gram, on page 229. This section is designed to show the succes- 

 sion of the principal geological horizons above the Paleozoic 

 in the Black Hills region. The lowest beds here represented 

 are the red beds, already mentioned, which are mainly fresh- 

 water deposits, and generally are referred to the Triassic 

 formation. 



Next above these comes a series of sandstones, limestones, 

 and shales, containing marine fossils, and named by the writer 

 the Baptanodon beds, from a genus of large swimming reptiles 

 there entombed. This horizon is readily recognized by charac- 

 teristic marine invertebrate fossils, especially Belemnites, first 

 described from this region about forty years ago by Meek,* 

 who recognized the importance of the horizon. Above these 

 marine beds are extensive fresh-water deposits of Jurassic age, 

 which the writer has called the Atlantosaurus beds, from a 

 gigantic Dinosaur specially characteristic of the horizon. It 

 is in this series of deposits on the southwestern border of 

 the Black Hills that the footprints here described were 

 found, and it is a point of much interest that here, too, are 

 entombed remains of large reptiles that probably made the 

 same footprints, as will be shown later in the present article. 



These Atlantosaurus beds, though overlooked by many geolo- 

 gists, have a great development around the margin of the Black 

 Hills, especially along the southern and eastern borders. The 

 bones of gigantic Dinosaurs mark the outcrop of this horizon 

 at various points. The one best known, the writer explored 

 personally in 1889, near Piedmont, South Dakota, and there 

 obtained remains of an enormous Dinosaur, subsequently 

 named Barosaurus.\ During the past season, important parts 

 of the rest of the type skeleton were secured for the Yale 

 Museum, by G. K. Wieland of that University. With these 

 fossils were found remains of a much smaller species, which 

 may be called Barosaurus affinis. 



Above the true Atlantosaurus beds is a series of strata of 

 shales and sandstones, the exact age of which is at present a 

 matter of controversy. In this series, there are two or three 

 layers which contain the remains of Dinosaurian reptiles 

 differing somewhat from those below, but especially from those 

 known in the well-defined Cretaceous strata above. The writer 

 has # now under investigation for the U. S. Geological Survey, 

 various remains of vertebrate fossils from one of these layers. 

 These fossils, with others secured from different localities in 

 the same region, promise to clear up many of the doubtful 

 points now remaining as to the age of these deposits. 



* Proc. Acad. Nat. Sci. Phila., vol. x, pp. 41-59, 1859. 

 f This Journal, vol. xxxix, p. 85, January, 1890. 





