MOOE, STUDY OF THE MORRISON FORMATION 151 



placed the formation in the upper Jurassic, being closer to the Purbeekian 

 than to the Oxfordian in age. Logan (1900, 5) correlated the Morrison 

 of the Freezeout Hills region with the Wealden of England. Riggs 

 (1901, 4) described the Morrison or McElmo of the Grand Eiver Valley 

 as Jurassic. Loomis (1901, 6) noted the resemblance of the Morrison 

 mammalian fauna to the fauna of the Purbeck beds of England, and on 

 this ground assigned the formation to the Jurassic. Hatcher (1903, 4) 

 correlated the lower beds of the Morrison at Caiion City with the Sun- 

 dance beds in Wyoming. He considered the cycads as pointing to the 

 Jurassic age of the deposits, and the dinosaurs as agreeing most closely 

 with those of the Middle Oolite series of Europe. He concluded that 

 there is undoubtedly Jurassic represented in the formation, but that it 

 was quite probable that some of the formation might be of Lower Cre- 

 taceous age. Williston (1905, 4) gave strong evidence from the verte- 

 brate fauna for Comanchean age of the Morrison. Lull (1911, 8) dis- 

 cussed the fauna of the Arundel formation and considered it to be Lower 

 Cretaceous or Comanchean in age. 



In view of the great differences of opinion concerning the age of the 

 Morrison it is important to review the evidence at present available on 

 this subject. The principal evidence from the stratigraphic relations is 

 here summarized. 



The youngest beds upon which the Morrison rests are the sandstones 

 of the ITnkpapa formation in the Black Hills area. (It is possible that 

 the Exeter sandstone in ISTew Mexico may be equivalent in age to the 

 Unkpapa.) Below the Hnkpapa, which is thin, lies the Sundance for- 

 mation. Over wide areas the Morrison lies on th* Sundance directly. 

 The Sundance, according to Stanton (1909, 9), belongs to the lower part 

 of the upper Jurassic. The Jurassic sea retreated, therefore, consider- 

 ably before the close of the Jurassic period, although it is possible that 

 post-Sundance beds were laid down and eroded before the deposition of 

 the Morrison. The Morrison over wide areas cannot be older than 

 middle or late upper Jurassic in age. It is probable that some of the 

 lower beds in the southwestern areas where the Sundanc^e is absent may 

 be slightly older than the oldest Morrison beds which directly overlie the 

 Sundance. In the Black Hills area there is a time interval between the 

 Sundance and the base of the Morrison which is represented by the 

 Unkpapa sandstone. There is often a sharp contact between the Morri- 

 son and Sundance formations in the Wyoming areas. This indicates that 

 the Morrison in these areas may be considerably younger than the 

 Sundance. 



The oldest beds overlying the Morrison, the age of which is definitely 



