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



The Eocene is apparently wanting in the Black Hills region, 

 and the Miocene at many points rests directly on Cretaceous 

 strata. The Miocene is represented by three great horizons, 

 each marked by an abundance of mammalian remains. The 

 Brontotherium beds are at the base, the Oreodon beds next 

 higher, while the Protoceras beds crown the series. The Plio- 

 cene is also well developed, as shown in the section, with the 

 Pliohippus beds below and the Equus beds above. Over all, 

 the Quaternary and recent deposits are present, marked by an 

 interesting and characteristic fauna. 



Below the series here given, the geological structure of the 

 Black Hills, as well known, is briefly as follows : The central 

 mass is Archean. Outside of this are Silurian strata with the 

 Potsdam sandstone at the base, and limestones of upper Silu- 

 rian age above. The Devonian is apparently wanting, and the 

 Paleozoic ends with Carboniferous strata, mainly limestones. 



The age of the Atlantosaurns beds has long been demon- 

 strated to be upper Jurassic, by the conclusive evidence of 

 vertebrate fossils there entombed. Testimony from other 

 kinds of fossils has not been wanting, and many new facts are 

 coming to light. One noteworthy instance may be fitly 

 recorded here. Remains of the gigantic Dinosaur Barosaurus 

 are characteristic of a definite layer in the Atlantosaurus beds 

 on the eastern side of the Black Hills. Just above this layer, 

 at various localities, there is a thin seam of arenaceous shale 

 filled with remains of minute Ostracoda, and also containing a 

 few fishes. As this seam has a definite position, its fossil con- 

 tents became important in determining its exact age. Among 

 the fish remains, a perfect tooth was readily recognized as 

 Hyhodus polyp ~rion, Ag., a characteristic Jurassic fossil, found 

 in the Dogger, at Stonesfield, England. 



A small piece of this shale containing the ostracods was 

 sent to Prof. T. Rupert Jones of London, the highest author- 

 ity on the subject, and in a letter to the writer, dated February 

 9, 1899, he reports that in the specimen sent, three species 

 were represented, namely, Cypridea punctata, Forbes, Meta- 

 cypris Bradyi, Jones, M. Whitei, Jones, all characteristic of 

 the Purbeck. These new facts need no comment. It was 

 already known that two of these species, identified by the same 

 author, occur in the same horizon of the Atlantosaurus beds in 

 Colorado, about four hundred miles further south. 



The general characters of the large footprints here described 

 are well shown in those represented on Plate Y, one-sixth 

 natural size. In figure 1, page 231, are given the outlines of 

 three other tracks, one-tenth natural size. These are from the 

 same locality and horizon as those on Plate Y. They are all 

 reverse impressions, or natural casts, of the surface immedi- 

 ately overthe true footprints, which were made in a soft mud 

 that has itself not been preserved sufficiently to retain the 

 imprints originally made in it. 



