264 Dr. C. Keyes — Isostasy in the Rocky Mountains. 



instead of continuing to rise, or at least remain stationary, proceeds 

 to sink. Indeed, the area may have begun to sink long before it 

 reached base-level. At any rate depression is active until the old 

 plain attains an horizon more than two miles beneath the surface 

 of the ocean. Where once lofty mountains pierced the sky, marine 

 sediments 10,000 feet in thickness accumulate. In spite of the 

 removal of its former great load the net result of the regional vertical 

 movement is notably negative. 



Neither does the insinking of the earth's crust in this region 

 appear to have taken place joari 'passu with extensive transference 

 of sediments. Cretacic depression manifestly initiates itself long 

 prior to the beginning of the sedimental loading, perhaps before 

 the mountains were completely demolished. Then, curiously 

 enough, so soon as the enormous loading is finished the region is 

 uplifted again into lofty ranges, as towering,- perhaps, as any which 

 appear before or since. Instead of the sequence of events satisfying 

 the isostatic equations, the very reverse proves true all through. 

 Thus is regional unloading succeeded by crustal downsinking, 

 and erogenic upraising is accomplished under maximum load. 

 - Surely some orograjDhic force other than the isostatic one is at work. 

 An especially noteworthy feature concerning the Jurassic 

 revolution in the Southern Rocky Mountain region is that it is not 

 an isolated or unconnected catastrophe. An identical sequence of 

 events is repeated a little later in Laramie time, and again in 

 Miocene time. To-day the same process is going on for a fourth time. 

 During the epoch when Laramian sedimentation was proceeding 

 around the ancestral Rockies, the latter were themselves razed to the 

 level of the sea. Under a prodigious load of sediments, more than 

 two miles in thickness, the tract is uplifted a distance of between 

 four and five miles; and that before it is appreciably unloaded. 

 Again it is worn down to the ocean level. With the lightening of 

 load on account of erosion there appears to be no recognizable 

 uplifting. After base-levelling, the area instead of rising sinks, 

 until the peneplain is buried under miles of Tertiaries. 



In the Miocene period, for a third successive time, are the Rocky 

 Mountains reared to lofty heights, only to be reduced to a low-lying 

 plain. Recently, notable uplifting is repeated. The rejuvenated 

 streams are now about completing their canyon-cutting. On the 

 flanks the winds are levelling and lowering the surface. Soon the 

 mountains will be turned into plain. Are they destined to a new 

 engulfment and a new sinking a mile or two or more beneath the 

 waters of the sea ? Experience of the past for three successive 

 times proclaims the affirmative. 



In all four orogenic cycles the procedure is the same. Removal 

 of load is not accompanied by further upraising. Rolling of oceanic 

 waters over the tract precedes regional loading. Maximum areal 

 loading is followed by great uplifting. Every stage is met with 

 phenomena diametrically opposed to that which isostasy logically 



