354 HATCHER — MARINE AND NON-MARINE FORMATIONS. [April 7, 



the Iguanodont group of the Predentata are represented by smaller 

 and less specialized forms. The dinosaurian fauna of the Atlanto- 

 saurus beds as we now know it is certainly more nearly allied to 

 that of the Kimmeridge and the underlying Oxfordian than to the 

 Wealden or the Purbeck, and there is little doubt that the true 

 Atlantosaurus beds, those lying below the Lakota of Darton, are of 

 Upper Jurassic age. 



Throughout the northern area of their distribution the Atlanto- 

 saurus beds are conformably underlaid by a series of marine deposits 

 variously known as the Baptanodon beds, the Shirley beds, the 

 Sundance formation, etc., and universally referred to the Upper 

 Jurassic. That these beds are of Upper Jurassic age has not been 

 questioned and is abundantly confirmed by both their invertebrate 

 and vertebrate faunas. About the Black Hills in South Dakota, the 

 Big Horn Mountains in northern Wyoming, and at other places 

 these marine beds attain a considerable thickness, 400 to 500 feet, 

 while the overlying Atlantosaurus beds of fresh water origin show a 

 more restricted development in these regions than they attain 

 farther to the southward. As we proceed southward, however, the 

 marine deposits diminish in thickness and disappear altogether near 

 the boundaiy line between the States of Colorado and Wyoming. 

 Farther south the Atlantosaurus beds attain to a maximum thickness 

 of perhaps 500 feet and rest directly but unconformably upon the 

 " Red Beds," now usually considered in their upper members at 

 least as of Triassic age, but formerly referred to as the Jura-Trias. 

 Still farther south in New Mexico, Lee has found conditions which 

 lead him to believe that these fresh water beds are continuous with 

 marine beds of Lower Cretaceous age pertaining to the Comanche 

 series. Though the evidence is at present not at all conclusive and 

 nothing is known concerning the character of the vertebrate fauna 

 of the fresh water beds at this locality beyond the fact that they 

 contain dinosaurian remains, yet it is quite possible that the upper 

 members of this series, the Lakota of Darton, may pass eastward 

 into true marine deposits of Lower Cretaceous age. 



To the eastward the Dakota sandstones, at the summit of the 

 series of fresh water and aeolian deposits under discussion, overlap 

 the underlying Lakota sandstones and Atlantosaurus beds, and in 

 eastern South Dakota, Nebraska and Kansas they rest unconform- 

 ably upon strata pertaining to the Carboniferous or Permo-Carboni- 

 ferous. 



