fore or after tlie deposition of the Loup Fork deposits; but, juiising 

 from ttie character of tlie strata in the middle valley of the Ruby 

 river, I think the basaltic flow that forms the plateau between this 

 river and Black Tail Deer creek overlies the Loup Fork. West of 

 Dillon and south of Argeuta are gray cliffs of what is probably Loup 

 Fork, capped with gray lava in layers a few inches thick, showing 

 many successive flows. The plateau divide between the Ruby and 

 Madison rivers is capped by basalt several hundred feet thick (See 

 Three Forks Atlas Sheet T.T. S. Greol. Sur.). Around Virginia City this 

 lava is seen to overlie the lake beds the tops of which reach an alti- 

 tude of ()200 feet. Over this the lava cap reaches an altitude of 7000 

 feet; but farther south near Old Baldy the height is about 8400 feet. 

 IT, seems likely as will be seen later that at one time the waters of 

 the lake extended over this divide into the upper Madison valley lake 

 thus making one vast irregular lake occupying the valley of the uppei- 

 Missouri above the region north of Helena, and all the tributaries flow- 

 ing in above that place. 



Usually the Loup Fork beds are nearly horizontal liut the White 

 River beds are often inclined at a considerable angle sometimes as 

 high as thirty-live degrees. It is evident that during the White River 

 period the valleys occupied nearly the same position as at present, 

 that in these valleys sediment was deposited to a considerable thick- 

 ness; the lakes were then drained and a large part of the sediment 

 removed, leaving benches and bluffs in more protected places as at the 

 present time. The difainage again being intercepted in Loup Fork 

 times, lakes were formed once more and new deposits formed over the 

 old; then the lakes were again drained and erosion i-emoved a great 

 part of these deposits. It must not be forgotten, however, that some 

 of the larger valleys were probably not evenly fllled and the centers 

 were left as depressions, the bulk of the deposits being deposited 

 nearer shore. 



While the White River deposits were being eroded the lake in 

 the Smith river valley was formed. This was probably dry land again 

 l»efore the later Loup Fork lake w^as established. Scott has ascer- 

 tained by a study of the vertebrate fossils from the Smith river region 

 that there are here two distinct horizons; and my collection seems at 

 present to indicate that there Avere at least two in the other valleys, 

 both distinct from those on Smith river. 



Though these Miocene deposits are geologically modern, vet they 

 must represent, humanly speaking, a vast lapse of time. Peale esti- 

 mates the thickness in the (xallatin and Madison valleys to be about 

 2000 feet or more. The area of drainage was small, probably not larger 

 than the whole area of deposition. The streams were short, and it 

 evidentlv took a vast amount of time for the small deltas to push 



