University of California. 



[Vol. 2. 



the gravels on the south side of the East Fork valley prove to be 

 the uppermost beds of the Mascall formation, the era of deposition 

 would appear to be concluded in a manner similar to that in which 

 the John Day ended. 



RATTLESNAKE FORMATION. 



Throughout the length of the Mascall formation exposures in the 

 valley of the East Fork, they are seen everywhere to have been 

 capped at one time by a series of later deposits which have some- 

 times been confused with them. To these beds the name Rattle- 

 snake formation is applied by the writer, the typical occurrence 

 being on Rattlesnake Creek about one mile west of Cottonwood. 

 The Rattlesnake has been referred to by Wortman and Matthew* as 

 the "gravels" at the top of the Cottonwood section. Possibly 

 Cope also refers to them in an indirect way, though the writer is 

 not able to determine certainly just what is meant by his statement. 



Stratigraphy. — In Fig. 2, PI. 8, and Fig. i, p. 307, the typical 

 exposures of the Rattlesnake show it resting unconformably upon 

 the Mascall. It is usually inclined only slightly, having a southerly 

 dip of about five degrees. At one locality on Birch Creek where a 

 section of the Rattlesnake was carefully examined, it was found to 

 comprise 30-40 feet of coarse basal gravels, above this about 25 

 feet of soft brown tuff, and capping this about 30 feet of rhyolite 

 At other localities more than 100 feet of gravel have been seen 

 upon the rhyolite. The basal gravel beds show a thickness of 200 

 feet or more in other localities. They are frequently very coarse 

 and contain many pebbles evidently derived from the Columbia lava. 



In the Birch Creek section the base of the rhyolite flow is 

 filled with angular pebbles of Columbia lava. A stratum about one 

 foot thick at the bottom is largely made up of pebbles, and above 

 that, at least half way up to the top of the flow, pebbles are found 

 scattered through the mass. The rhyolite exhibits everywhere a 

 tendency to split up into large irregular columns about twenty-five 

 feet across. At several localities near the mouth of Beach Creek, 

 a radial or boquet-like columnar structure is beautifully developed 



*W. D. Matthew, Bull. Am. Mus., V. 12, p. 23. 



