464 STUDIES FOR STUDENTS 



distinct and that the presence of Aturia ziczac determines these 



lower beds to be Eocene or Oligocene. The overlying shales 



and sandstones do not contain this fossil and are regarded as 



Miocene. In his " Correlation tables of Tertiary formations : 



data to 1895" Dall places the Aturia beds in Lower Oligocene, 



-Astoria shales in Upper Oligocene, and Astoria sandstones in 



JMiocene. 1 



UNFOSSILIFEROUS FORMATIONS 



THE SPHINX CONGLOMERATE FORMATION 



Sphinx conglomerate is the name applied by Peale 2 to a 

 group of nonfossiliferous beds covering an area of about two 

 square miles, but having a thickness of 2000 to 3000 feet. The 

 formation occurs in the Madison Mountain range, Montana. 

 The beds consist of "reddish sandstones and coarse conglomer- 

 ates of limestone pebbles and bowlders cemented with a reddish 

 sand." They are horizontal and stratified. They are described 

 and mapped as Eocene. 



THE PINON CONGLOMERATE FORMATION 



Weed describes 3 briefly, under the name Pinon conglomer- 

 ate, certain beds which occur in the southern part of the Yel- 

 lowstone National Park. He says they consist of a series of 

 conglomerate beds with local intercalations of sandstone, the 

 formation resting unconformably upon the upturned Laramie 

 (Cretaceous)." No fossils are mentioned and they are pre- 

 sumably nonfossiliferous. They are described and mapped as 

 Eocene. 



THE SAN MIGUEL FORMATION 



The San Miguel formation was named by Purington 4 and 

 referred by him to the Eocene "because of the great uncon- 

 formity at its base and because it underlies the volcanic complex, 

 which is thought to be of Eocene age in the portions here devel- 

 oped." It occurs near Telluride and Silverton, Col., and rests 



'Eighteenth Ann. Rep. U. S. Geol. Surv., 1895-6, Part II, pp. 327-348. 



2 Geol. Atlas of U. S„ Three Forks Folio, Mont., 1896. 



3 Geol. Atlas of U. S., Yellowstone National Park Folio, 1896. 



4 Geol. Atlas of U. S., Telluride Folio, Col., 1899. 



