18 INTRODUCTORY. 



traces, and these can be unraveled. It was a period of slow uplifting, reach- 

 ing a great amount in the aggregate; and it was also a period of stupendous 

 erosion. The uplifting was, however, unequal. The comparatively even 

 floor of the old lake was deformed by broad gentle swells rising a little 

 higher than the general platform. In consequence of their greater altitudes, 

 these upswellings at once became objects of special attack by the denuding 

 agents, and were wasted more rapidly than the lower regions around them. 

 Here were formed centres, or short axes, from which erosion proceeded 

 radially outward, and the strata rising very gently toward these centres, or 

 axes, from all directions, were bevelled off. As erosion progressed, so also 

 did the local upliftings, thus maintaining the maximum erosion at the same 

 localities. 



It is a most significant fact that the brunt of erosion throughout 

 the Plateau Country is directed against the edges of the strata and not 

 against the surfaces. This is directly traceable to the fact that the strata 

 are nearly horizontal, the dips rarely exceeding four or five degrees, and 

 even then only where a great monoclinal flexure occurs. The rains wash 

 and disintegTate most rapidly where the slopes are steepest, and where the 

 strata are flat the steepest slopes are the valley sides and chasm walls. 

 Thus the battering of time is here directed against the scarps and falls but 

 lightly on the terrepleins. 



Ordinarily, the local uplifts have one diameter longer than the others, 

 and we may call the greatest the major axis. The strata dissolved away 

 in all directions from this axis, and after the lapse of long periods the 

 newest or uppermost stratum encircled the centre of erosion at a great dis- 

 tance from it, the next group below encircled it a little nearer, and so on. 

 This has been the history of each of the subdivisions of the central part of 

 the Plateau Country. Upon the western and northern sides of the Colorado 

 five of these centres are now easily discerned. By far the largest and 

 probabl} 7 the oldest is around the Grand Canon; a second lies east of the 

 Kaiparowits Plateau; a third is found about 50 miles south-southwest of 

 the junction of the Grand and Green; the fourth is the Henry Mountains, 

 and the fifth is what is known as the San Rafael Swell, lying between the 



