622 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



height of about 7,500 feet. If the present valleys could be filled to the 

 level of the crests of the intervening ridges, the now excessively rugged 

 mountain range would be transformed into a broad plateau. The structure 

 of the rocks composing this plateau would find little, if any, expression in 

 the surface topography. Many of the stratified beds would expose their 

 edges and reveal the fact that they are the truncated bases of folds, and 

 in many instances would stand vertical. In other words, if we accept 

 the ' peneplain idea/ as elaborated by Davis and others, the surface of the 

 plateau would be a plain such as is produced by base-level erosion. Briefly 

 stated, the Cascade mountains as we now know them seem to have been 

 carved from an upraised peneplain. This plain we term the Cascade 

 peneplain, and the plateau may be conveniently designated the Cascade 

 plateau. 



' Rising above the general level of the Cascade Plateau there are two 

 classes of peaks. First, volcanic mountains, of which Glacier Peak (near 

 tbe 48th Parallel of latitude) is the only known representative in the 

 region considered in this paper; and, second, granitic mountains, such as 

 the Wenache mountains and the lofty peaks about Lake Chelan. The 

 volcanic mountains stand on the Cascade Plateau and were formed after 

 the period of base-leveling referred to above, and need not claim further 

 attention at this time. Some of the granite peaks have an elevation of 

 over 9,000 feet, and hence rise some 2,000 feet above the general level of 

 the Cascade Plateau. These are the mountains which, in my opinion, could 

 not have been in existence as topographic elevations at the time the main 

 drainage lines were established. 



' Possibly the granitic mountains referred to are of the nature of 

 monadnocks, or remnants left standing on the Cascade peneplain. If this 

 is true, the river courses which cross them may be explained as an inheri- 

 tance from an earlier time of erosion which preceded the general base- 

 leveling. 



It may also be suggested in this same connection that the Cascade 

 peneplain was developed above the present general summit elevation of the 

 large majority of peaks and ridges now remaining, and has been lowered 

 by erosion, leaving the more resistarlt rocks in the boldest relief. Under 

 this supposition the Cascade Plateau would now have a general surface 

 level of about 10,000 feet, having been raised from near sea level. In 

 favour of this hypothesis it is to be noted that the peaks and ridges of the 

 Cascade mountains are nearly all sharp. No recognizable flat-topped 

 remnants of the original plateau remain in the more elevated portion of 

 the region under review. As soon as a region has been so deeply dissected 

 by streams that the ridges are sharp-crested, any further erosion will tend 

 to a general lowering of their summits, and for a time they will continue 

 to maintain this knife-edge characteristic. For this reason the Cascade 

 Plateau, since being sculptured into a plexus of sharp-crested ridges, may 

 have suffered a general diminution in height, owing to the wasting away 



