114 



University of California. 



[Vol. 3. 



reached several hundred feet.* The eroded surface was then 

 flooded with enormous quantities of basaltic lava. Flow after 

 flow followed until their total thickness became, in the limits of 

 the John Day Basin, as much as 1500 feet. 



These basalt floods, which seem to have inundated the most 

 or the whole of the Blue Mountain Range,! constitute what is 

 perhaps the greatest lava field extant and what is certainly the 

 most important formation of the plains of eastern Oregon and 

 Washington and western Idaho. 



One or more thin flows of basaltic rock are intercalated with 

 the lowest beds of the formation above the main lava series. 



The Mascall Formation. — This formation, which overlies the 

 great basalt series without notable unconformity, is preserved 

 within the John Day Basin only in a monoclinal trough extend- 

 ing along the southern end of the basin from Spanish Gulch to 

 Canon City. 



In their general field characters, the Mascall beds show some 

 resemblance to the John Day, though they differ in the more 

 frequent occurrence of fine bedding, and occasionally are cross- 

 bedded. They seem to be mainly composed of tuff's and ashes, 

 but comprise some material of organic, origin and probably some 

 sandstone and conglomerate. They are also in general charac- 

 terized by light colors, gray or cream, in contrast to the 

 variegated tints of the John Day. Dr. Merriam estimates the 

 thickness of the formation at from 800 to 1000 feet, and the age 

 as probably upper Miocene. 



The Rattlesnake Formation. — This lies in almost horizontal 

 attitude upon the tilted and truncated Mascall beds in the 

 elongated area mentioned above. Dr. Merriam named this 

 formation and considered it of Pliocene age. It appears to be of 

 fluviatile origin in large part, and comprises a large amount of 

 coarse gravel and sandstone, together with fine material that 

 may be tuffaceous. Somewhere near the middle of the section 

 there occurs a widely spread sheet of light colored pumiceous 

 tuff, overlaid by a glassy gray rhyolite. 



*Angular as well as erosional unconformity is noted by Dr. Merriam, op. cif., 

 p. 299. 



t Merriam, op. cit., p. 303. 



