280 N. S. SHALER — BROAD VALLEYS OP THE CORDILLERAS 



could have been produced by other than river action. For reasons al- 

 ready given it seems impossible that these channels should have been 

 formed by differential warping, for if such were the case the movement 

 would have had to follow the paths of the much-branched valley ; more- 

 over, the form of the central gorges of these troughs is altogether incon- 

 sistent with the supposition that they are due to such movements. We 

 have thus to believe that the stream beds of this area were, at no geologi- 

 cally distant date, several hundred, perhaps a thousand, feet lower than 

 they are at present. 



The passage from the broad valley of the upper Silver Bow to the 

 narrow gorge by which the waters of the basin find their exit clearly 

 demands explanation. This may possibl}'' be found in the fact that the 

 rocks through which this channel is cut evidently deca}^ rather slowly, 

 and so developed a narrow canyon gorge. It is, however, not unlikely 

 that there has been here, as elsewhere in the Cordilleras, a recent change 

 in the path of escape of the stream by which a new wa}'^ of escape was 

 provided, so that tlie higher portion of the vale was formed or at least 

 outlined with one baselevel of drainage, and the lower at another such 

 level. This is, however, a question of detail that cannot well be here 

 considered. 



In the part of tlie Ruby River valle}^ near Alder creek the evidence 

 obtained from borings, as before noted, together with that gained by an 

 inspection of the surface features of that area, shows conditions of the 

 ancient topography substantial!}^ the same as those found near Butte. 

 The sloping platform of the ancient rocks on the northern side of the 

 vale is evidently continued out for an unknown distance toward the 

 center of the valley. Through it Alder Creek can3'^on extends with an 

 unknown depth. The same appears to be the case with other similar 

 lateral gorges. At various other points in the broad valleys of the Upper 

 Missouri River system chance excavations show a like extension of the 

 slope of bed-rock for some distance out from the mountains toward the 

 streams. None of these sections afford anj'' clear indication of the depth 

 of the central trough, but in all of the instances observed the rate of the 

 decline is so great that the depth in the middle of the trough must be 

 profound. 



The above-noted very limited observations as to the under form of the 

 Cordilleran broad valleys are of themselves clearlj' insufficient to war- 

 rant any general conclusions as to the existence of a like topography in 

 other parts of the region. There are, however, certain groups of facts 

 which enable us, with qualifications to be noted, to extend the conclu- 

 sions, with varying certaint3^ over the whole of this field. First let us 

 note that the valleys in question, though tlie walls of their mountain 



