178 REPORT UNITED STATES GEOLOGICAL SURVEY. 



15. Placenticcras lenHci&are Owen sp. 



16. Fish vertebrae. 



From the valley of Little Thompson Creek I continued southeast- 

 wardly to the valley of the Saint Vrains, meeting with no other expos- 

 xues of strata until I reached that valley. Proceeding down the valley 

 upon its northern side I observed a few slight exposures of friable sand- 

 stones in the valley side, and although I obtained no fossils from them 

 I regard them as belonging to the Fox Hills Group. 



Beaching the confluence of the Saint Vrains with the South Platte 

 Biver, I found considerable exposures of the uppermost strata of the 

 Fox Hills Group. For the distance of a mile or more, these strata, hav- 

 ing a visible thickness of about GO feet, are exposed in the form of a 

 more or less precipitous bluff with a talus of debris at its base. These 

 strata are composed of soft, sandy, and partially argillaceous, bluish, and 

 grayish material below, capped with sandstones above, the layers of 

 which vary in condition from hard and somewhat massive to soft and lam- 

 inated. The principal or only layer containing invertebrate fossils was 

 found near the top of the section, and only two or three feet in thick- 

 ness. Some eight or ten feet above this, in softer sandstone layers, that 

 peculiar fucoid, Halymenites, was found in abundance. Above this is a 

 few feet of still softer sandy material that merges above into the debris 

 of the plains. 



As will be seen by the following lists of fossils collected here, these 

 strata are without doubt exactly equivalent with the uppermost layers 

 found in the valley of the Cache a la Poudre, and are doubtless the high- 

 est Fox Hills strata that exist in this region ; they are also without 

 doubt the highest marine Ceretaceous strata that are yet known in North 

 America.* Between the top of the section at the mouth of the Saint 

 Vrains and the higher adjacent land surface there is sufficient thickness 

 of material to make it evident that the lower strata of the Laramie Group 

 exist there, but they were not recognized with certainty. However, for 

 reasons that I shall state farther on, I think it not improbable that the 

 bed containing the Halymenites belongs to the Laramie Group. If so, 

 there is no perceptible plane of demarkation between the Fox Hills and 

 Laramie Groups, at least at this point. 



LIST OP POSSILS COLLECTED AT THE MOUTH OF THE SAINT VRAINS 



RIVER, COLORADO. 



1. Halymenites major Lesquereux. 



2. Pteria Haydeni Hall & Meek. 



3. Nucula planimarginata Meek & Hayden. 



4. Tancredia americana Meek & Hayden. 



5. Cardium speciosum Meek & Hayden. 



6. Protocardia subquadrata Meek & Hayden. 



7. Tellina scitula Meek & Hayden. 



8. Tellina equilateralis Meek & Hayden. 



9. Mactra (Cymbophora) warrenana Meek & Hayden. 



10. Mactra (Cymbophora) alta Meek & Hayden. 



11. Pholadomya f 



12. Pachymya herseyi White. 



13. Hentalium gracile Hall & Meek. 



14. Gyliehna scitula Meek & Hayden. 



15. Action woosteri White. 



*Whetber the Laramie Group is of Cretaceous age or not will be briefly discussed 

 on following pages. 



