358 



THE AGE OF MAMMALS 



specimens of the following species of Pliocene mammals are supposed to 

 have been derived from the Rattlesnake beds : 



Neohipparion occidentalis Leidy. 



Neohipparion sinclairi Wortman. 



fPlatygonus rex Marsh. 



To these should be added, from specimens in the University of California 

 collection, a horse referred to Pliohippus supremus Leidy, also some remains 

 of rhinoceroses which are specifically indeterminate, a large suilline form, 



Fig. 163. — Miocene exposures near mouth of Rattlesnake Creek, John Day Basin, Oregon. 

 Mascall formation (Middle Miocene) below. Rattlesnake formation (Lower Pliocene) above, 

 separated by an unconformity. Photograph by J. C. Merriam. 



fragmentary remains of a camel smaller than Alticamelus, and portions of 

 a carapace and plastron of a land tortoise (Clemmys hesperia, Hay). 



The Rattlesnake beds are composed of loose gravels, probably repre- 

 senting a fluviatile or flood plain deposition. These gravels, associated with 

 tuffs and rhyolitic lavas, lie upon the up-tilted and eroded edges of the 

 Middle Miocene Mascall Formation (see p. 288). The mammal remains 

 have been obtained both from the tuffs and the gravels. The scattered and 

 broken condition of the bones of one of the horses found in these beds also 

 seems to indicate a long exposure of the remains on a land surface w^hich was 

 being rapidly worked over. The close of the Rattlesnake deposition marks the 

 beginning of a long interval of erosion which may be regarded as the open- 

 ing event of the Quaternary. (Merriam and Sinclair, op. cit., p. 175.) 



