BOZEMAN TERTIARIES 469 



many miles, sharply swings around to the northwest and empties 

 into the Snake river near Pocatello town. Another stream, the Bear 

 river, likewise turns squarely upon itself, its upper half running 

 parallel to the lower half but in a diametrically opposite direction. 

 Other streams of the region have equally erratic courses. 



Near Pocatello the Port Neuf river crosses an old stream chan- 

 nel of large size which is now filled high with lava, for the an- 

 cient drainage-ways and valleys of southern Idaho seem to have 

 often flowed with liquid rock as well as water. This old channel 

 apparently extends far to the northward. Southward it reaches the 

 Red Rock Pass, and there coincides with the old river bed which 

 Gilbert interpreted to represent the position of the outlet of Lake 

 Bonneville. That this old channel belonged to a south-flowing, and 

 not a north-running, river of considerable size seems to be clearly 

 indicated by the disposition of the old stream gravels. The peb- 

 bles composing these gravels are not chiefly derived from local rock 

 ledges, but from pre-Cambrian crystallines, such as characterize the 

 Yellowstone Park region. They are numerous, of nearly uniform 

 measurement, and of a size that suggests their removal a distance 

 of about one hundred miles from their parent ledges. 



While, then, there is not much doubt but that the Bozeman Beds 

 extend well into Utah the further problem is their tracing south- 

 ward to their ultimate limit. 



