ABSTRACTS OF PAPERS 29 



an old barrier reef now lying beneath some thirty fathoms of water. Deeper 

 water lies between it and the shore. The critical question is whether the bar- 

 rier reef has been built up from the original sloping sides of the volcanic island 

 by the accumulation of great thicknesses of coral material, or whether the 

 coral structure is a much thinner veneering built on a wave-cut platform. 

 Tutuila seems to declare in favor of the platform. Since the shelf was cut 

 and the barrier reef formed, the island has subsided relative to the sea, drown- 

 ing the corals of the old reefs. More recently still there has been a movement 

 of the strand-line in the opposite direction, as evidenced by a bench ten feet 

 above high tide on nearly all the promontories of the island. A fringing reef 

 is now growing out from the shores just below present scale vel. 



Presented without the use of notes. 

 Disciissed by Dr. T. Wayland Vaughan. 



SOME GEOLOGIC FEATVUES OF THE BEARTOOTH MOUNTAINS, MONTANA 



BY ARTHUR BEVAN ^ 



{Al)Stract) 



This paper is a preliminary report on the salient geologic features of the 

 Beartooth Mountains, which form the front range of the Rocky Mountains in 

 southern Montana and northwestern Wyoming. The most striking physio- 

 graphic features are (1) the abrupt rise of the range from the Great Plains 

 at an elevation of approximately 6,000 feet to an even rim at about 9,000 to 

 9,500 feet; (2) a moderately dissected sub-summit plateau several miles wide, 

 that extends nearly the full length of the range; and (3) distinct remnants 

 of a summit plateau at an elevation of about 12,400 feet. Granite Peak, which, 

 at 12,850 feet, is the highest mountain in Montana, has a nearly flat summit 

 that probably is a similar remnant. 



Nearly all the valleys on the plainsward slope have been severely glaciated 

 during at least two distinct epochs. An extensive ice-cap occupied a consider- 

 able portion of the western slope, from which several lobes passed across the 

 axial divide and down the valleys to the bordering plains on the east. The 

 rocks of the range consist of a pre-Oambrian crystalline core wliich is sur- 

 rounded by an apparently conformable series of sedimentaries that range in 

 age from the Middle Cambrian to the early Eocene (Fort Union). The Silu- 

 rian system only is absent. The columnar sections for northern Wyoming and 

 southern Montana can be harmonized in this range, as the formations of both 

 areas meet along its eastern base. 



The range is structurally an asymmetric anticline that is overturned toward 

 the plains. It is bounded along nearly the entire eastern front by tlie Bear- 

 tooth fault, which is a profound overthrust. This fault is probably the north- 

 ward extension of the Heart Mountain overthrust of northwestern Wyoming. 

 Other faults, about which little is yet known, are present along the western 

 base of the range. 



Eead from manuscript. 



' Introduced by R. T. Chamberlin. 



