778 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



the Fort Union, penetrated by the Medora well below the lowest lignite outcropping in the field, 

 the total thickness is 1,720 feet. 



Overlying the heavy sandstone wliich forms the sumnait of Sentinel Butte and constitutes 

 the topmost member of the Fort Union formation, there are about 40 feet of calcareous clay 

 and limestone, as shown in the Sentinel Butte section. These beds are merely the remains of a 

 formation which doubtless at one time covered a large area in this region. Strata which have 

 yielded Oligocene vertebrates and which occupy a similar horizon immediately over the massive 

 sandstones at the top of the Fort Union occur in Chalk Butte, 70 miles farther southeast. Other 

 similar buttes in northwestern South Dakota and southeastern Montana have likewise been 

 referred to the Ohgocene. 



On the basis of mammalian remains, Osborn ^^ correlates a portion of the Fort 

 Union with the Torrejon formation of New Mexico. In the San Juan Basin of 

 northwestern New Mexico the Torrejon is conformably underlain by the Puerco, 

 which carries a basal Eocene fauna. 



3>-M 10. OLYMPIC PENINSTTLA AND PTJGET SOUND REGION, WASHINGTON. 



Arnold ^^^ describes Eocene deposits on the north coast of the Olympic Peninsula 

 as follows : 



The oldest formation of definitely known age on the Olympic Peninsula is a 1,200-foot 

 series of black basalt and greenish basalt tuffs and tuffaceous sands found in the vicinity of 

 Port Crescent and here designated the Crescent formation. It comprises the region immediately 

 west of Crescent Bay and a prominent ridge extending eastward from the latter to Freshwater 

 Bay. Venericardia pldnicosta Lamarck, Turritella uvasana Conrad, and other characteristic 

 fossils found in the tuff indicate the Eocene age of the series and its general contemporaneity 

 with the Tejon of California. 



The basalt occurs in two thick sheets, an upper and a lower, each of which may represent 

 several surface flows. Between the two basalt sheets and intimately associated with the top 

 of the lower is a series of roughly bedded fossUiferous tuffs. * .* * In the region of Crescent 

 Bay the lower basalt has an exposed thickness of 200 feet, while the tuffs and upper basalt 

 sheet each show approximately the same. The Freshwater Bay section gives basalt and coarse 

 massive basalt tuff 600 feet, thin-bedded green tuff 355 feet, and black vesicular basalt 200 feet. 

 The base of the Crescent formation is not exposed, so that the subjacent rocks are unknown. 

 The overlying sediments consist of coarse conglomerates separated from the basalt by an 

 erosion interval. Faults define the contact between the Crescent formation and the Clallam 

 formation (Ohgocene-Miocene) adjacent. 



These basalts and tuffs are the only rocks of igneous origin found along the whole length 

 of the northern shore of the peninsula. Taking into consideration the volcanic activity which 

 prevailed during the Eocene in the Cascade Range, only a comparatively short distance away, 

 this single and rather limited occurrence of eruptives seems rather remarkable. The paucity 

 of igneous rocks, however, may possibly be accounted for, at least along the northern coastal 

 border of the Olympics, by the fact that formations younger than the basalt are the only ones 

 exposed, and it is possible that some of these newer rocks are underlain by the Eocene basalt 

 series. 



The Puget group comprises many thousand feet of estuarine and marsh deposits, 

 which are extensively coal bearing. Willis ^^^ says : 



The coal-bearing rocks of the Puget Sound basin have been designated the Puget formation. 

 They are prevailingly sandstones of variable composition, texture, and color, thinly interbedded, 

 and frequently cross stratified. Their composition varies from that of a typical arkose, consisting 

 of slightly washed granitic minerals, to siliceous clay. Beds of conglomerates or concentrated 



