feale.] RESUME MESOZOIC ROCKS CRETACEOUS. 629 



and Western Wyoming into three divisions, the upper and lower ones 

 mainly calcareous and the middle mainly siliceous. 



2. The discovery in the lower group of a new Triassic genus of Ceph- 

 alopods, named by Prof. A. Hvatt, of which three species have been 

 described by Dr. C. A. White. 



3. The association with these Cephalopods, of forms that have here- 

 tofore been considered as Jurassic. 



4. The absence of fossils in the " Eed Beds," or middle group. 



5. In the limestones above the " Eed Beds " the presence of undoubted 

 Jurassic forms, two of which, perhaps, range through the series down 

 into the " Meekoceras Beds," or lower group. 



The latter part of these remarks have been published already in the 

 Bulletin of the Survey, Vol. V, Kb. 1, in almost the same shape as here. 



CRETACEOUS. 



The only group of the Cretaceous from which fossils were obtained was 

 the Fox Hills Group, which at two localities in the Fontenelle Hog- 

 backs yielded the following : 



Ostrea soleniscus. 



Ostrea sp. ? 



Trapezium sp. ? 



Inoceramus si). ? 



The sandstones forming this group are treated of in Chapter III. 



It is probable that the entire Cretaceous series is present beneath the 

 Fox Hills Group, but as we did not find any paleontological proof, I shall 

 content myself by referring the reader to the detailed descriptions 

 already given. 



In the valley of John Day's Eiver, at the top of the Jurassic section, 

 is a quartzite, which has been doubtfully considered as the Dakota sand- 

 stone. Above it is a series of arenaceous beds, some 3,000 feet in thick- 

 ness. We had no good exposure of these beds, but they have been 

 thought to represent the Cretaceous formation, although no organic re- 

 mains were obtained from them. 



A similar set of beds was seen above the Jurassic section on the lake 

 branch of Smith's Fork of Bear Eiver. Here the softness of the strata 

 was the cause of their almost total concealment. 



Above them is a conglomerate, which is succeeded by greenish lam- 

 inated sandstones, which contain lignitie layers. These upper beds have 

 been considered as of Laramie age. 



In the valley of Smith's Fork, south of this locality near the bend, 

 there appears to be no room for the Cretaceous between the Jurassic and 

 the Laramie, and while in the field I was inclined to think there might 

 be a gap between them, caused by subsidence of the surface during the 

 deposition of the Cretaceous. I shall, however, have to leave the con- 

 sideration of the Cretaceous until it is more certainly identified and cor- 

 related with the well-known Cretaceous sections. 



The formations thus referred to the Cretaceous are confined to the 

 eastern half of the district. From the Bear Eiver Eange and the Preuss 

 Range westward they are absent. Whether they were ever present, I 

 cannot say. 



POST-CRETACEOUS. 



It is unnecessary to enter at this place into a discussion as to the 

 origin of the term Post-Cretaceous, as used by Dr. White and the other 



