TWO OCEAN PLATEAU. 299 



This passes up into assorted basic andesitic breccia, in the lower portions of 

 which are rounded fragments of the sanidine-bearing tuff or breccia. 



In the escarpment of the plateau west of Two Ocean Pass the lower 

 part of the breccia contains many layers that are distinctly waterlaid and 

 assorted, the masses of andesite being more or less rounded. But the 

 upper portion is true breccia, angular and unassorted. On the southern 

 side of Two Ocean Pass the lower part of the plateau mass consists of 

 assorted breccia which in places shows evidence of having been rearranged 

 by water action. Layers of fine sand and gravel and large masses alternate 

 with one another, but the bedding is decidedly irregular. The layers are 

 not of uniform thickness, and are in places cross bedded. This condi- 

 tion continues for about 1,000 feet above the valley, the upper 1,000 feet 

 consisting of true breccia without waterlaid layers. Here, as elsewhere in 

 the breccia, there are evidences of the former existence of localized bodies 

 of water. The matrix is light colored, with angular fragments of all sizes 

 up to those 6 feet in diameter. The andesites vary in character; some 

 are dark, others light; some dense, others vesicular. The3^ carry pheno- 

 crysts of feldspar and pyroxene, with occasional hornblende or olivine. 

 The whole mass has a distinct but irregular bedding, clearly seen at a 

 distance. The surfaces of the layers are rough and irregular. They are 

 generally denser at the bottom of each, so that the top of each weathers 

 more easily and causes the line of bedding to become pronounced in 

 exposures. Where layers differ in color and can be traced for any distance, 

 they are observed to thin out laterally, and are not persistent for long 

 distances. 



Horizontally bedded breccias form the plateau ridges about Jay Creek 

 and those west of the Yellowstone River, and also the mountains at the 

 head of Buffalo Fork of the Snake River, which are the southern extension 

 of this region. Here they have a slight dip to the northeast, They also 

 form the body of Two Ocean Plateau, and are exposed in the latera] 

 canyons cut into it by the tributaries of the Yellowstone. From Atlantic 

 Creek to north of Lynx Creek the petrographical character of the rocks is 

 the same, and there are places where the material has been assorted and 

 rearranged by water action. But the great bulk of the material is purely 

 subaerial ejectamenta. At the head of Lynx Ci^eek, on the continental 

 divide, are remnants of a surficial flow of basalt which extends down the 



