128 G. B. Grinnell and E. S. Dana— Tertiary Lake Basin. 



layers of very small water-worn pebbles soldered together into 

 a hard mass, but easily picked out with a knife. Eacii of 

 these layers is about six inches in thickness. Immediately 

 above these strata the Pliocene fossils were found. In seve- 

 ral places fragments of trachyte were noticed in the Pliocene 



Near Camp Baker are a series of upturned ridges of Pots- 

 dam sandstones and limestones at a level very little above 

 that of the Tertiary beds, and doubtless in this region the 

 lake was divided into many arms, which bent around, and 

 extended among, these ridges. 



It is known that in the neighborhood of Fort Shaw, and 

 near Helena, Pliocene deposits exist, and at Fort Ellis and in the 

 valley of the Yellowstone we saw, but were unable to examine, 

 gray sands and marls, which Dr. Hayden refers to the same 

 age. No Miocene beds, however, have been identified at any 

 of these localities. It seems probable that in Pliocene time at 

 least, the Baker Lake may have extended north to the Mis- 

 souri River, and perhaps up that stream to the Three Forks, 

 thus connecting with the lake which existed near Fort Ellis. 



i it would seen 

 edgec 

 and w 



An interesting point in connection with these deposits, is 

 the fact that they are at a much greater elevation than any 

 other beds of the same age now known on the continent. The 

 elevation of the White River and Colorado beds is about 3,000 

 feet, and that of the Oregon basin somewhat less, while that 

 of the deposits near Camp Baker is over 5,000 feet. 



In reference to the relations which this lake basin bears to 

 the Oregon basin and to the White River deposits, nothing can 

 be certainly known without a careful exploration of the whole 

 region and a thorough study of its vertebrate remains. It is 

 by no means impossible that the Baker Lake may have flowed 

 into that at White River by some old river channel, but so 

 little is known of the intervening country that no definite 

 opinion can be pronounced on the subject 



