278 N. S. SHALER BROAD VALLEYS OF THE CORDILLERAS 



one time overspread all of the valley of that stream. Not only on either 

 side of the channel lie the dissected fragments of an extensive mesa of 

 basalt, but there are the stocks of minor cones within a radius of 5 miles 

 which were evidentl}^ in eruption long after the older clay-like material 

 had been much dissected by the torrents that traverse it. These condi- 

 tions make it reasonable to suppose that the clays above referred to have 

 been derived from the decomposition of volcanic ash accumulated when 

 it fell or washed into position by the rains which commonly attend 

 eruptions. 



It appears clear that these ash beds were accumulated during a vol- 

 canic outbreak which followed the construction of the main benches of 

 the valle^^ They evidently do not extend beneath the general surface 

 of the benches at least in those parts of the area adjacent to the moun- 

 tains, though they are probably to be found in the central part of the 

 troughs. The deposit which lies beneath Alder creek has been traced by 

 borings almost out to Ruby river, a distance of 3 miles from the moun- 

 tain walls. On either side it appears to come into contact with, but not 

 to pass under, the material of the benches. It thus appears likely that 

 the ash must have been laid down on the general surface of the country 

 and afterward swept into the channel of the creek. It is certain that 

 none of it is noAv visible on the levels above the stream bed, and that it 

 is covered by a thick layer of debris com})rised of the rocks which make 

 up the mass of the neighboring mountains. 



The fact that at Alder creek the rock channel has been so deeply filled 

 that at 7 miles up from its junction with Ruby river its present level is 

 over 100 feet above its original bed, and that an even deeper infilling 

 has taken place in the Silver 13ow valley near the head of a small branch 

 of the river, shows clearly that ver}^ great clianges have taken place in 

 relatively modern times in the details of the form of the valleys of the 

 Upper Missouri district. I now propose to consider what are the real 

 shapes of these vales beneath the masses of detritus with which they 

 have become thus occluded. I have already noted the fact that the 

 valley deposits, from the lack of natural sections, most effectively mask 

 the shape of the bed-rock outlines. At only one point, namely, in the 

 city of Butte, have I been able to find any artificial cuts through the 

 upper parts of the slopes that reach the bed rock. In that town, how- 

 ever, man}^ openings, especially those for a system of sewerage, have 

 disclosed facts of much interest, which, taken with other bits of infor- 

 mation, enable us to construct a tolerable picture at least locally true of 

 the hidden bed-rock floors of these troughs. 



It is first to be noted that the detrital slope on which the city of Butte 

 is built extends from near the top of a degraded mountain to Silver Bow 



